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Author Topic: Victorian Boombox Mk III. A brand new start.  (Read 3537 times)
J. Wilhelm
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« Reply #25 on: April 09, 2020, 12:12:05 am »

So a good number of changes have been made since the last time I posted. This project is far more difficult than it looks. At issue is the apparent performance of the waveguide, which while being good and a match for the bass reflex speaker, simply refuses to go below the cutoff of of a typical bass reflex enclosure. During this period, i have assumed 2 things: 1. The elbow enclosure of the driver is acoustically "dirty" and wastes acoustic energy by way of having too many surfaces and corners reflecting and absorbing sound. 2. The tuning of the horn simply doesn't match the fundamental frequency of the driver. And so my modifications follow that train of thought

Not in order of execution, but I began by smoothing the "elbow enclosure." I added ramps whenever there was excess volume with a right angle corner that should not be there. One wide "ramp" and one corner reflector took care of that. I also added perforations located as close as possible to the walls and each other to make sure that the elbow makes a single waveguide with the SoundBeam™and does not "partition" the waves. Lastly, I added triangular reflectors to cover all the corners around the driver, thus making the elbow's cavity as spherical as possible around the driver.

During the experiments I found that these improvements eliminated the "flatness" of the sound in the mid range of frequencies, and up to the lower high frequencies. So I was very happy about that. The speaker doesn't sound muddled anymore, but that didn't help increase the low bass performance, which has remained on par with bass reflex systems above 80 or so Hz.











These modifications above happened at the same time as I radically changed the length of the horn in the SoundBeam, because I didn't know what effect smoothing the inside of the horn would, have. I hoped that bass would increase due to fewer losses. I did use the frequency analyzer and a tone generator, and that revealed both a shortcoming and an advantage of the horn. The frequency cutoff below 80 Hz is too fast compared to the bass reflex system - this should not happen at all, it's supposed to be the other way around. On the other hand, the frequency response of the horn is flat as a board with a brief spike around 132 Hz, at least from 83 Hz to over 15 kHz, it's one of the best frequency response profiles I've seen for speakers. That blue Sony driver is very impressive for a traditional full range cone.

So at least i know the speakers will sound decent. I didn't make garbage. But I've not arrived at the end of this, by a long shot. Otherwise I'll just glue the two Sony enclosures together and declare "I'm done" *face-palm*

The Sony driver will audibly reproduce sound down to about 54 Hz or so (!), but much to my chagrin, the driver in the bass reflex system will produce the same sound about at about 5 dB higher than the horn below 83 Hz. Above that they're a good match. What's the secret sauce of the Sony enclosure? Test after test, I could not find the resonance of the horn, hoping that I'd have that "aha" moment, when i realized the horn is resonating at an unexpected frequency. No such luck.  My conclusion was that my estimate for the resonance frequency of the driver was plain wrong. So I consulted articles on the subject of waveguide horn design, and one article suggested that simply copying the Helmholtz resonance frequency of the equivalent ported bass reflex enclosure should be a good starting point to determine the length of the waveguide horn. In other words, let Sony tell me what the resonance is. And that's when things got interesting.

Now I realize I should have cracked open my engineering notes (difficult to do as they're in deep storage) or at least consulted a book, because my first assumption was that the bass reflex enclosure was a simple Helmholtz resonator with a hole opening equal to the area of the two ports of the Sony box. Not true. But at least, this error led me to figure a way to lengthen the horn to match what I thought was a resonance of 63 Hz, based in the length and area of the two ports in the Sony enclosure. Mind you, the Sony enclosure doesn't even have batting inside. It's just a plain particle board hollow box, with two 3 cm diameter pipes, 11 cm long, built into the molded plastic face.

Nevertheless, i pushed forward with an idea to lengthen the horn. I thought that by adding some inserts to the narrow part of the horn, i could in effect fold the horn again. Basically the horn is folded about one axis first, but you can lengthen the horn by folding again along the perpendicular axis. You just need to add some separators,shaped to divide the waveguide. My idea also was to fix the pesky problem of having the "ports" be facing in opposite directions, a consequence of the horn geometry. But if you fold the horns a second time, you can fold the outlets to one side or the other. In this case, the ports are now two slots on one side of the cabinet, and the cabinet could have the slots facing up or down to the ground (for the bass).


Inserts for long horn configuration create a new fold for the horn


Inserts for short horn configuration. A ramp an some spacers to plug holes from long horn setup


Mixed mode, one side short and one long, to compare


Close-up of the long horn outlet. Sound comes out from slots on one side for both channels


I built the "long horn" configuration you see in the pictures above expecting to see resonance at 63 Hz.

When - again-- I failed to get the sound I expected (though I resolved the sound "flatness" issue as i show in the first pictures), I just bolted into another direction. I had seen that 132 Hz peak in my spectrum analyzer before, so i thought "I've already punched a whole bunch of holes on the beam, so what do i have to lose? " Maybe the resonance was 132 Hz, even if no equation predicts it. Both the Sony enclosure and mine show the same 132 Hz peak, so that must be the resonance of the Sony driver, yes? So again I opened the box and designed a new set of Inserts to *shorten *the horn for a frequency of 132 Hz. In one night I had made the inserts and some spacers to plug the holes i made for the "long horn" setup.

Welp. Same thing. This time the frequency response of the horn was really poor in the range below 100 Hz, and about as muddied as before elsewhere in the spectrum. Sooo, no. Matching 132 Hz is not the answer. It makes things worse. That must be the first harmonic or something along those lines... Aaargh! I was running in circles.

So back to the drawing board. I should have done the calculations, but by sheer laziness I found a scholarly article on the tuning of a Nigerian wood gong, which is basically a Helmholtz Resonator with two necks, like an Ocarina it has a resonance cavity and multiple openings. The Sony box has two ports, an my initial mistake is that you can't add the areas of the openings to get the resonance of 63Hz. And my original 83 Hz (based on tapping the speaker) estimate could be wrong too (who knows?). How far could I get from the real answer? I know better than to go above 100 Hz. 132 Hz was NOT the right direction.

For those of you who know about parallel and series impedance in electric circuits (mathematically analogue to acoustic channels), you know that the impedance made from two parallel components is equal to the inverse of the sum of the iinverseof the individual impedances of each component. Thus the simple algebraic equation for the Helmholtz resonance doesn't apply. The answer will vaguely resemble the Helmholtz resonance equation but will not be the same. Which brings me to the present calculation.., and I realize how much stuff I've forgotten about phasor diagrams and even complex numbers and using Euler's equation... I promise i will derive the expression below by myself, from scratch, as punishment for my laziness.

Impedance model of a Helmholtz Resonator with 2 openings
Zc is the impedance of the cavity of volume V
Zp is the impedance of each pipe
Zr is the end point correction for the pipes (radiative loss)
t is the length of the pipes (ports) assuming they're the same length
Sh is the cross sectional area of each pipe (both the same)




The resonant frequency of a two-necked Helmholtz resonator with two pipes with flanged piped end corrections, is... 57.9 Hz. Not so far from the 63 Hz I had wrongly estimated, fortunately. And fortunately, the horn geometry is "wide enough" to support resonance in the range 55 Hz to 79 Hz in the "long horn" configuration (of you do ray tracing you'll see that range in resonant pathways) .

But I still don't know what is going on. I hate to think that this is as far as I can go. The speakers sound nice in the long horn configuration, but I'm still in the dark about resonance. As far as I am concerned, the driver alone resonates at 83 Hz, with an apparent strong harmonic at 132 Hz. Tuning the horn to 132 Hz produces horrible results, and the Sony "magic sauce" port bass reflex, made from a thin unassuming hollow box, resonates at 58 Hz according to theory and shows the same mysterious peak at 132 Hz.

I'll resume testing tonight
J "befuddled" Wilhelm
« Last Edit: April 09, 2020, 01:31:51 am by J. Wilhelm » Logged

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« Reply #26 on: April 09, 2020, 04:25:26 am »

Today's test. Im using the long horn configuration. Still no identifiable resonance. I used three tools. A frequency generator, white noise and a spectrum analyzer.

This time I filled only the horn "elbow" enclosure, from the "top of the line" lightly packing with the green batting, and did hear a marginal improvement in the low frequency response, compared to tightly packing, or compared to no batting, so I decided to see what the spectrum analyzer has to say about it.

The frequency response is remarkably similar between the two speakers, and yet, the bass reflex seems to "do a lot more" in the lower spectrum, when you put your hand or your ear next to the ports, even if the spectrum analyzer says otherwise (see plots). At least I know more air is being moved by the Sony speaker ports, but somehow the bass in the horn seems to be coming from the loaded driver, not the port. You can hardly feel any motion of air nextto the port of the horn. Theory states that when the waveguide horn is resonating most of the sound should be coming from the port. This is, what I think I haven't achieved yet.

Response to white noise
Waveguide was lightly packed with batting about 11% of length from speaker
This is a freeze frame of a real time average, that's why the plot is jagged.

Front position 3 ft away.



Side position 1 ft away



« Last Edit: April 09, 2020, 05:57:21 am by J. Wilhelm » Logged
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« Reply #27 on: April 18, 2020, 06:18:36 am »

I've been slow to make progress on the project these last few days. But I did manage to get some extra information on what went wrong.

I decided to try an alternative setup, to try to quantify any losses incurred by the "elbow enclosure." Resonance aside, I suspected that sound bouncing off walls and possibly resonating inside the elbow might account for the 5 dB loss below 83 Hz. The simplest way to do this is is to find a way to mount the speaker directly into the horn, with no elbow.


As a temporary measure, I found a rubber pipe fitting meant to adapt toilet discharge pipes to 4 inch PVC waste pipes. The inner diameter of the wide part of the adapter is wide enough to insert the Sony driver and the narrow portion of the adapter can be inserted into the mouth of the waveguide.

Then I used a sine wave sweep to compare the frequency response of the horn and the Sony bass port reflex speaker (l have not taken stills from those plots, my apologies). What I found is that at 53, 63, 73 and ending at 83 Hz, I consistently measured a 2-3 dB higher response with the setup shown above as opposed to the wooden elbow enclosure. The length of the resonator will be wrong, but I'm not looking at resonance (which I don't have anyway) over that wide of a bandwidth. I'm also ignoring losses, due to the fact that the rubber adapter is soft and a poor reflector of sound, and that I have a few holes in the corners I had to plug with whatever I had at hand.

So let's just say that I found that the energy recovered by the new setup should be higher than 2.5 dB, just by placing the driver in line with the waveguide, even with the lossy rubber adapter.

The implication is not surprising at all. I need to change the design of the elbow. I expected losses from the elbow enclosure, but what is notable is that so much of the loss is just from the shape of the elbow enclosure itself, because the elbow is quite rigid and dense as it is, so the only thing that can be wrong is the shape. It could be that the elbow is in fact acting like a separate Helmholtz resonator and affecting the transmission of sound into the horn waveguide. The "neck" of the elbow is narrow enough to increase impedance and create a secondary resonance cavity. Any reverberation insidethe  elbow will eventually reflect into the horn, but by then the walls have absorbed some energy. In other words the elbow is an energy trap.

So there is no choice. Regardless of resonance, I have to simplify, smooth and straighten the path of sound from the driver to the horn. I don't think I have to forgo a 90 degree bend in the wave but I think that bend should be as smooth as possible, with walls - again - as rigid as possible to reflect and avoid absorbing the sound before it arrives at the horn.

~

The other study I did was to consider the volume of the horn. In theory, the horn's volume should not matter for resonance, but in practice makers recommend preserving the volume of the horn at roughly that of an equivalent acoustic suspension speaker, or in the case of a bass reflex system, the resonance should be the same as an equivalent Helmholtz Resonator. So it's not a bad idea to start comparing physical dimensions and try to preserve similar parameters, such as air mass in the enclosure.

I estimate the volume of the tapered and folded horn proper to be roughly 50 % of the volume of the Sony speaker enclosure (neglecting volume of small reflectors inside), because the volume of the pine boards needs to be subtracted, and the tapering horn only occupies half of the SoundBeam™ wooden enclosure.

Neglecting the volume taken by small internal reflectors, I estimate that if I add a rectangular prism of volume, equal to the internal area of the SoundBeam™ wooden enclosure, I should be able to match the volume of the Sony enclosure just by adding 26. 0 cm of length to one side of the SoundBeam™. That's not too much of a problem, because as it stands now the side length of the elbow enclosure already is 25 cm, so I wouldn't be affecting the overall size of the boombox by much. It's just that I have to discard all the work that I did on the elbow enclosures, or find a way to reuse that wood in a creative way.

The correction in the horn length gives me a ¼ wavelength resonance of 59. 01Hz for the horn. When I re-measure the Sony Bass reflex box carefully (which I did to compare volumes) the Helmholtz resonance of the Sony enclosure is 59.05 Hz, a perfect match of both air volume and resonance frequencies! That was entirely serendipitous. Let's see if that works.

So the idea now is to simply extend the sides of the SoundBeam and place the drivers in such a way that they are fully inserted into the waveguide, as opposed to having  some sort of a separate enclosure with a "neck." I think this was a major issue which I chose to neglect at first, not realizing how severe the losses would be.

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« Reply #28 on: April 18, 2020, 07:40:44 am »

You have a Spec An??? In your home???!!! Jeez, I worked in electronics for 35 years typically from  LF to microwave (40GHz) and the only equipment I ever had in my house (and still have) is a DVM (currently a Fluke 179) and a soldering station  (and a bunch of parts, of course). Thinking about getting a used VariAC, tho'...
So what  brand of spec an you got?
« Last Edit: April 18, 2020, 07:42:15 am by Deimos » Logged

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« Reply #29 on: April 18, 2020, 08:22:54 am »

 Smiley
You have a Spec An??? In your home???!!! Jeez, I worked in electronics for 35 years typically from  LF to microwave (40GHz) and the only equipment I ever had in my house (and still have) is a DVM (currently a Fluke 179) and a soldering station  (and a bunch of parts, of course). Thinking about getting a used VariAC, tho'...
So what  brand of spec an you got?

Hahaha!  Grin No not a professional tabletop instrument! Though that'd be awesome if I did! You need to check out the kind of apps you now have available for your Linux PC and Android / Apple smartphone nowadays!  Grin

Example of Google Play Store Search :Spectrum Analyzer Apps

These are the tools I used  Grin

"Sound Analyzer Free" by nobapp
https://play.google.com/store/apps/details?id=jp.nokubi.nobapp.soundanalyzer.free

"Frequency Sound Generator" by LuxDeLux
https://play.google.com/store/apps/details?id=com.luxdelux.frequencygenerator

White Noise Generator by Hipxel
https://play.google.com/store/apps/details?id=com.hipxel.white.noise.generator.free

It's nothing really accurate, but gives you an idea of what you're doing! Especially when comparing one sound source to another, you only care about the relative difference (in this case using the original horn as a baseline, and in the previous plots above
« Last Edit: April 18, 2020, 08:27:54 am by J. Wilhelm » Logged
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« Reply #30 on: April 18, 2020, 08:23:31 am »

Your right angle issue is sounding remarkably like one we studied regarding radar waveguides!
Consider musical brass horns, and how they are also a waveguide - smooth interiors are a must.

Have you considered making the angle part out of moldable fireable material like clay, plaster or concrete?

clay has the advantage of lending itself to slab fabrication .. that is, making slabs of clay rather like boards, then bending and molding to shape and gluing the edges with clay slip. Then firing the piece.
Low fire clay can be baked in a homemade wood oven, just a few bricks holding up some sort of metal or concrete lid with a front and back “door” piece .

Concrete or plaster  saturated fabric might also work-well,  and not require firing... and lends itself to
Hand smoothing and prototyping.

Prf Mvl
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J. Wilhelm
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« Reply #31 on: April 18, 2020, 08:46:53 am »

Your right angle issue is sounding remarkably like one we studied regarding radar waveguides!
Consider musical brass horns, and how they are also a waveguide - smooth interiors are a must.

Have you considered making the angle part out of moldable fireable material like clay, plaster or concrete?

clay has the advantage of lending itself to slab fabrication .. that is, making slabs of clay rather like boards, then bending and molding to shape and gluing the edges with clay slip. Then firing the piece.
Low fire clay can be baked in a homemade wood oven, just a few bricks holding up some sort of metal or concrete lid with a front and back “door” piece .

Concrete or plaster  saturated fabric might also work-well,  and not require firing... and lends itself to
Hand smoothing and prototyping.

Prf Mvl

Indeed, all options are on the table.. But weight is an issue and I'll have to experiment a bit. There is also wood fiber filler putty, which is lightweight. I think that a large size pvc elbow with suitable filler inside a box would be enough with some putty to smooth the transition . Otherwise I'll see what I can do with wood. The main problem, I think, is that the "elbow" became a separate acoustic device by way of the smaller box that served as connection to the horn.

Basically this is a Helmholtz resonator hooked up to the wrong end of a horn. The acoustics equations can be solved for complex impedance in series, first a spherical cavity where the Sony driver is,, then a short tube, then an inverted horn, and last a radiative loss at the exit of the horn.

I want to match the mass of the air in the bass reflex enclosure and the resonance frequency. I think that the 132 Hz resonance in the Sony speaker is the first overtone of the 59 Hz resonance of the bass reflex enclosure.

I also noticed that the 132 Hz signal is quite prominent on the Sony enclosure, regardless of the source material I'm playing. Me thinks that the 132 Hz overtone is very much intentional sound coloration provided by Sony to make things rattle in your home. It should not be there for *every* song that I play on my phone.
« Last Edit: April 18, 2020, 08:49:56 am by J. Wilhelm » Logged
J. Wilhelm
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« Reply #32 on: April 18, 2020, 08:59:59 am »

These are the parameters I'm looking at. Black text is input, orange text is output

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Deimos
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« Reply #33 on: April 18, 2020, 09:11:11 am »

Smiley
Hahaha!  Grin No not a professional tabletop instrument! Though that'd be awesome if I did! You need to check out the kind of apps you now have available for your Linux PC and Android / Apple smartphone nowadays!  Grin

No.....I gave up all that when I retired.  Wink
And besides I'm spoiled.... This was the Spec An I was using the last year I worked: R & S FSU43
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J. Wilhelm
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« Reply #34 on: April 18, 2020, 09:41:00 am »

Smiley
Hahaha!  Grin No not a professional tabletop instrument! Though that'd be awesome if I did! You need to check out the kind of apps you now have available for your Linux PC and Android / Apple smartphone nowadays!  Grin

No.....I gave up all that when I retired.  Wink
And besides I'm spoiled.... This was the Spec An I was using the last year I worked: R & S FSU43

Fancy... I remember the good old days when I was taking the measurements and instrumentation class at the end of my first degree in college.

In this case I'm just looking for clues as to what I'm doing. It does help in some ways and others not. I can hear differences between the two speakers that the spectrum analysis is not telling me, and vice-versa, the analysis can tell me that I'm not that far off in performance from the commercial product, and in some ways better, because my homegrown speaker enclosure is very flat in response (which makes me happy). I still don't have what I want, but I'll get there.

The same app revealed to me last year how ridiculously bad the frequency response of the subwoofer of the Mk. II Boombox was. That was instrumental in me giving up on the project. At some point you're putting out too many fires, placing too many patches, and it didn't make sense to fix something that was so broken since inception (I think I was dealing with a kareoke machine, really). This time, I'm reasonably sure I can make the box sound "normal"  Grin
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« Reply #35 on: April 21, 2020, 06:01:39 am »

So the first step to testing the hypothesis that volume matching coupled with lengthening  the horn would trigger resonance was to build a simple box to extend the length. Made exactly like the outer cover of the waveguide. Then I just placed the speaker on the end with some window insulation tubing as a gasket.


The first setup was done without any batting, to take a look at the general behavior and letting my ears do the judging. Cranking up the stereo to the same familiar tune by Kenji Kawai, I noticed right away that the excursion of the driver's cone was off the charts. The sound from the output slot was not as loud as the bass port of the Sony enclosure, but itwas  much better this time. I did notice that at some point in the music program the lowest frequencies (drums below 100 Hz) were definitely louder out from the slot than the sounds coming from the cone. That is significant, because it's the first indication of resonance. The sound was horrible though. Very garbled.

My first impression was that the speaker was clipping the sound, because the motion of the cone was really strong, like you'd expect a subwoofer to move, but remember, this is no woofer. I hadn't seen it move like this before. This is a rather stiff full range speaker, and if you lift that box from the horn the excursion of the cone really is reduced to a small fraction of the amplitude (just using my eyes) , and you can hear the bass go to zero. The horn is definitely doing something now... Not just serving as an infinite baffle, which is what it was doing before.

I couldn't infer anything too obvious from the white noise test, because in the setup above the full range is pointing upward, the bass port is facing sideways and the tweeter in a tangential direction far away from the horn's port (which I need to align so I can hear high frequencies that should not be in the horn)... It's really hard to take measurements this way. The box needs to be modified to have the speaker and tweeter facing the same direction and the bass slot pointing at least in a similar direction. The geometry of the horn is way too long this way, so really the speaker needs to be mounted sideways - which will surely waste energy and decrease the resonance effect, but at the moment the setup is impractical and impossible to measure. A smooth bend on the horn is what is needed, literally a wave guide.

The other thing that I noticed is that the box I made for the driver was really vibrating this time. Putting my ear next to the box, I could hear sound emanating from the box. The opposite is true for the SoundBeam™ because of all the compartments to make the horn which make the walks very stiff. This is not a surprise, because I made a simple box with 1cm walls, and no bracing of any kind. I think that the box was making its own sound, and obviously that's not good Roll Eyes. The box will need to be braced, thickened, and a few reflectors added to smooth the interior. I'm thinking of using a large diameter PVC pipe to slice a quarter circumference of it to create an interior curved ramp inside, to guide the sound waves toward the horn. And I think that's critical, actually, given my experience with the "elbow" enclosure. Smooth rigid dense walls lead to cleaner sound. The rubber gasket has got to go. You can see the whole driver frame bobbing up and down, so that's wasting a lot of energy, and also distorting and mudding up the sound, dampening the sharp drum sounds.

Will it work this time?


The other thing I noticed is that I started to see a very strong peak at 127 Hz during the drum routine of Kenji Kawai's Ghost in the Shell theme. I did use the spectrum analyzer to take a snapshot of that peak. I also saw a strong peak at 25 Hz, which I think it's some spurious distortion (it's present as a minor peak in all my measurements of the Sony Bass reflex and the horn. So I don't know if this is a natural sound distortion for the Sony driver. Prior to the test I thought this peak was an error from the spectrum analyzer. But this time that 25 Hz peak is loud - inaudible to most humans, but it's there to be measured by the frequency analyzer. Maybe it's a resonance as well.

Drum bass in Kenji Kawai's Ghost in the Shell theme
The 125 Hz signal probably is the 1st overtone of the 59 Hz theoretical resonance
The 25 Hz peak was thought to be a measurement error, but now has increased too.



That peak looks a lot like the 132 Hz peak in the Sony bass port reflex speaker. I very much think that is the overtone of the fundamental, so my horn and the bass reflex speaker are doing the same thing now. Maybe that's good news  I hate to think I'm introducing an artificial "boom." But before I can tell you for sure, I need to have the full range, tweeter and bass slot aligned to reasonable orientations. Otherwise I can't do my measurements.
« Last Edit: April 21, 2020, 07:46:48 am by J. Wilhelm » Logged
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« Reply #36 on: April 21, 2020, 08:05:04 am »

SNIP
Consider musical brass horns, and how they are also a waveguide - smooth interiors are a must.

Have you considered making the angle part out of moldable fireable material like clay, plaster or concrete?

Concrete or plaster  saturated fabric might also work-well,  and not require firing... and lends itself to
Hand smoothing and prototyping.

Prf Mvl
Could I make a curved wall out of sheet rock? I've never tried before, but I know that funky things happen to sheet rock when it gets wet  Grin
« Last Edit: April 21, 2020, 08:07:01 am by J. Wilhelm » Logged
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« Reply #37 on: April 21, 2020, 09:03:22 pm »

SNIP
Consider musical brass horns, and how they are also a waveguide - smooth interiors are a must.

Have you considered making the angle part out of moldable fireable material like clay, plaster or concrete?

Concrete or plaster  saturated fabric might also work-well,  and not require firing... and lends itself to
Hand smoothing and prototyping.

Prf Mvl
Could I make a curved wall out of sheet rock? I've never tried before, but I know that funky things happen to sheet rock when it gets wet  Grin

It may be worth trying. Typically sheetrock cuts best by scoring and snapping.
It can be bent quite far by spraying both sides of the paper with water until visibly saturated, then bending slowly around a form, clamp until dry.

I have seen folks that can make a simple form using rigid foam!

I can envision snap cutting triangular pieces of sheetrock (cutting with a saw makes a huge mess, but it is doable) spray with water, wait a bit then slowly bend over the form. If it breaks, one can patch it up with plaster or drywall mud.

Oh look, these guys score one side with a table saw and bend it dry!
https://m.youtube.com/watch?v=WzYyF2e8-jk

If that works, but is too heavy you could use the sheetrock prototype as a model or form for either lightweight thin plywood or to mold heated sheet plastic.

I have been experimenting with plastic from bottles, both the clear two liter ones and the heavy colored bottles for things like laundry detergent. The stuff is quite amazing. One can cut a large panel from the main body, then warm it up with a shop type heat gun and watch it flatten out and “shrink and thicken” . The stuff seems to have a sort of memory of it’s prior flat shape and thickness prior to blowforming into a bottle. To get a good flattish sheet I found I need to squish it betwixt a pair of very smooth heavy panels like formica counter cutouts (from installing sinks) . Then one can cut and form it much like kydex... not sure if you could get the desired density, tho.

Yhs
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« Last Edit: April 21, 2020, 09:06:54 pm by Prof Marvel » Logged
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« Reply #38 on: April 25, 2020, 07:44:00 am »

Today I tried to implement the idea of curved waveguide. I did make a major mistake in choosing my materials, but I'll let it go for now and continue with the tests. So I went to the hardware shop and I bought a wire mesh meant to be used as a rain gutter cover. It's fairly rigid, weatherproof, and it's ideal for spackling or plastering.







When looking for cementitious materials, I picked a bucket of stucco. I thought, stucco is a form of cement, textured with sand, and it should be both hard and dense. The problem is that I didn't realize that what I picked turned out to be a space age like material which is an imitation stucco. It has sand alright, and it's very heavy, but the matrix is not cement but rather a type of silica infused latex or acrylic foam with coarse sand for texture, and the matrix is somewhat flexible to the touch when dry. I didn't realize my mistake until after I had used said material. Unfortunately it's too late to fix the problem. This type of foam does stick fairly well to the wood.

If there is any positive side to this situation is that the faux stucco material will not break or crack, and perhaps it may double as a kind of batting, to absorb unwanted resonance. So I'm keeping the box for now and since the box is so simple to build I'll just make another box of I have to. Initial tests do seem to confirm that it's very good at sound absorption. It's all that sand. It makes the box rather heavy, even if it's suspended in a matrix of what could be described as "egg whites beaten to ultra-mega stiff peaks." I actually remember buying a similar version of the same thing without sand last year (Boombox Mk II project, for something I can't remember), and that versión was super light weight, and I decided it was good for nothing. This version satisfies the mass requirement, but I just don't know how good it is at sound reflection. I'm prepared to be disappointed on that end.

The wooden box I showed you before was fit with the steel mesh, making a smooth 90 degree turn inside the box. The grate or mesh was fixed first with latex glue by the edges, and spread the faux stucco over and throughout the mesh with a spatula. I cut a hole to fit the driver, making sure it was a tight fit. The nice thing about the rain gutter mesh is that it could also be used as a grating for the finished speakers. It's very inexpensive ($3 for two 3ft long pieces) and easy to cut with metal sheet snips.







It's a fairly difficult thing to achieve a smooth surface on the mesh, however, because the stucco material is fairly liquid and it takes a long time to cure. So with patience you have to smear the stucco from behind the mesh, and once you have enough material smooth it out on the other side, until all the holes of te mesh are blocked. Repeat over and over, until you have covered the whole area, making sure that the concave surface is smooth, and following the shape of the mesh. The back can be rough, with dimples (from pushing the material through with the spatula, and the thickness in the back will be to taste, to satisfy the mass density requirement.

Sound tests will follow. I'm undecided whether to make a hole to fit the tweeter. That tweeter will need a horn to match the Sony enclosure. The space age stucco will cure overnight as its still soft. There is also the matter of a vibration of the mesh sitting next to the wood when you tap the box on the side. That is a serious problem, which I discovered while the faux stucco was still fresh. I can't do anything about it now, and I'll wait to see if the vibration goes away as the matrix cures. If it doesn't, I may introduce a screw with a wide washer to pin the mesh down against the wood... Ugh!

Oh, well. Tomorrow will be another day.
« Last Edit: April 25, 2020, 05:59:24 pm by J. Wilhelm » Logged
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« Reply #39 on: April 28, 2020, 10:22:23 am »

So I have good news and bad news. And in the process I have learned new things and cemented a theory as to what Sony's "Secret Sauce" is. I think I've reverse engineered their bass reflex speaker.

I started by adding the Tweeter to the wall where the blue Sony driver was set. I also estimated that the ramp I built was going to take some volume and effective length away from the horn, so I built a wooden "ring" to extend the length of the horn by about two inches (it's entirely arbitrary, I eyeballed the length by looking at the ramp).





The bad news first. The new ramp plus the speaker mounted in one of the walls perpendicular to the wave-guide has made the horn lose resonance again.

The good news. The performance of the horn improved from the last stable configuration, and it has achieved performance parity with the Bass Reflex below 83 Hz, reaching a minimum audible frequency of about 42 Hz (!) and it definitely has achieved a marginal superiority over the Bass Reflex in the rest of the spectrum (much cleaner sound) all the way from 83 Hz to 20 kHz (remember the Sony box and the Horn were similar but only above 83 Hz - that is no longer true). The sound is now "crystal clear" for any frequency I hear when playing music. And  yes! The speaker has good performance all the way up to 20 kHz (even though I can't hear anything above 10 kHz because I'm old and have hearing loss - but the  spectrum analyzer can detect it quite clearly), so I can detect actual performance from 30 Hz (audible to be above 43 Hz) up to 20 kHz (audible to me below 10 kHz).


What I have learned in the process: The Sony bass reflex system is definitely a two pipe Helmholtz resonator, and it was tuned somewhere in the vicinity of 60Hz. However, by doing further tapping tests on the Sony driver, I realized that the fundamental frequency of the blue Sony driver is not 83 Hz as I originally measured. You see, when connected to the Sony tweeter, the electrical current induced by the coupled solenoids of both speakers is enough to change the fundamental resonance of the blue Sony driver, from 83 Hz to 132 Hz! In other words, 132 Hz is the resonance of the full range Sony drive + tweeter pair! The 132 Hz signal is in turn the *first overtone* of the Helmholtz resonator. You are getting a down-conversion of acoustic energy from 132 Hz down to 60-something Hz by way of the Helmholtz resonator. The price you pay for that performance kick is a permanent coloration of sound at 132 Hz. Sony improved the response of the full-range+ tweeter pair by accepting an artificial tone. There is no way that the blue Sony driver can reach all the way down to 57 Hz by itself, especially when coupled to the Tweeter without a decent crossover (The crossover in this case is just a capacitor coupled to the inductance of the speaker coils themselves).

I measured the performance of the horn speaker setup with and without the extension ring and made a comparison to the Sony Bass Reflex speaker.  I tested both in pure sine wave tones at 57 Hz and 67 Hz and using white noise from 20 Hz to 11 kHz (remember 59 Hz is the "design" frequency of the horn - supposedly matching Sony's Helmholtz resonator). There is no point in comparing much above 10K, really, because the tweeter takes over and that is an encapsulated speaker that will sound exactly the same for the Sony Bass Reflex and my horn speakers.

At least for white noise, the horn speaker is visibly flatter. Note the 132 Hz peak, the overtone for the Sony Bass Reflex when playing white noise. That is the sound coloration I was talking about. There is also a bit of a "dip" on Sony's profile around 200-300 Hz(?). You always see those features no matter what you're playing.

First overtone and dip in 200-300 Hz are clearly visible for Sony Bass Reflex


What is more interesting to me is the frequency response all the way down to the design frequency of the horn. Finally I achieved parity with the Bass reflex, even if I can't detect an obvious resonance. The performance is a perfect match between the Sony Bass Reflex, and the horn speaker - with and without the extension ring ring. Except for one thing: the sound is visibly cleaner again. The 132 Hz overtone is absent (or at least very muted) in the horn's response to a 57 Hz and a 67 Hz sine wave, but you can see the 132 Hz overtone quite present for the Sony Bass Reflex.


Horn speaker reaches parity with Sony Bass Reflex below 83 Hz, down to design frequency of 59 Hz and below
There is a very reduced - or no overtone around 130 Hz for the horn





To me that overtone is an indication of resonance. The reason being that now I feel that the blue Sony driver plus tweeter simply can't reach all the way down to 57 Hz without some kind of resonance involving the first overtone. And we know for sure that resonance can be achieved in the horn, because I managed to overdrive the horn on the vertical setup (blue driver in line with the horn, pointing upward - see photos above) to the point that the frequency analyzer saw a 125 Hz peak, similar to s Sony's - and the blue driver was also "falling apart on me." It was clipping like crazy and sounded horrible - which tells me that in the correct setup for resonance the forces are too high for the blue Sony drive.

It may be necessary to re-design the whole horn (make it wider) or get a different stiffer driver to actually make the horn work to its full potential. In most practical Transmission line speaker designs I've seen online, the speaker is never placed in line directly facing the horn, It's always mounted sideways at 90 degrees, which is why I though I could do the same.  So who knows? You never see the slot of the horn blowing air the same in the way that the Bass Reflex speaker does (it can probably pit out a candle with a good drumbeat). It's quite obvious to me what resonance is, but what is not obvious is how "parity" in low frequency performance is reached without the overtone for the horn.  I'm thinking that I may in fact have resonance already, and it's just that standing waves are impeded by the curved ramp, and I could actually get stronger resonance by promoting standing waves with a wall. Again, all transmission line speaker enclosures I've seen lack a curved ramp. It's just a "chopped" at 90 degrees, so to speak, so maybe that's needed to promote standing waves. Curiously after the speaker's location, the horn can have as many bends as you want  Huh So that supports the idea that you need that 90 degree wall to promote standing wave formation. I'm just trying to extend the bass as far low as possible in the spectrum, but with what I know now about the Sony Bass Reflex design, my horn may already be doing the same that their Helmoholtz resonator was. That was a sneaky trick Sony used to enhance the performance of a full range speaker.


One thing I haven't tried as a compromise is a 45 degree corner. Have the driver's horn pointing at a 45 degree angle to the waveguide to avoid directing too much energy against the opposing wall which could be dissipated into the air - but that assumed the speaker is a good surface for standing wave formation. It kind of reminds me of the American sewage line requirements whereby all lines must have bends not sharper than 45 degrees to avoid the muck from getting stuck at the bends Tongue (sorry for the visuals). In my case what I aim to do is prevent energy loss. What I do know is that as you approach resonance, a 125 Hz overtone will appear in the plots. I'm not sure I want to see that. And if you go further, sound distortion will happen with this Sony driver.

I made a plot of the frequency response of the horn speaker. Unfortunately a single set of measurements is not enough for this un-calibrated equipment (my phone) and I get a lot of hysteresis (lag in measurement response), which means I have to make multiple measurements at every tone to avoid a very "choppy" plot. At first the frequency below looks like a roller coaster, but note that in the continuous sine wave sweep it looks much better than that (I haven't figured out how to make a plot during the sweep - I just made individual measurements for a discrete set of tones, and the decibel scale indicated a non-steady state response over time, hence the "choppy" plot. The important part of the plot, however, is the frequency cutoff (I guess that would be like the -3dB critical frequency of a low pass filter), which happens between 80-90 Hz...


Qualitatively, the spectrum analyzer can't tell me how something sounds in detail. I can see that the horn is "cleaner" sounding, but I don't get much confirmation from the plots. I did a sound test with Kenji Kawai's GiTS theme again, and I was really happy to hear parity of both speakers, but the best part was the absolutely perfectly clarity of the sound. That makes me feel good about what I already have.

I don't think I'll be using the enclosure with the ramp as you see in the picture. I'll build a new one because there's a number of issues that cropped up in the sound tests. The wire mesh I used, hits the wood and vibrates, creating this buzzing sound when you knock on the box or get an explosive sound. Also, the wood enclosure is too thin without bracing. Plus the the extension ring is needed to recover the volume lost by the ramp. To my ears, the sound is nicer and cleaner with the extension ring, while the spectrum analyzer says it's the same. I figure the new driver enclosure will need 1.3 mm thick wood and it'll need to be about 29 cm long to recoup the volume from the curved ramp and the thicker walls of the enclosure.

Oh well. I have to go back to work tomorrow, I'll return later this week with more to tell.

Cheers,
J. Wilhelm
« Last Edit: April 28, 2020, 10:59:03 am by J. Wilhelm » Logged
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« Reply #40 on: April 29, 2020, 09:26:14 am »

I wonder if the difference in tone bewteen ear and SA could be the tendency of the human ear and brain to  "mash" things together and pay attention to certain things versus others (much the same way as the eyes and brain tend to "see" something one is looking for, rather than recording the entire visual salad, as it were, like a camera does), versus the SA's tendency to present the results as exact number values on a graph.

Just wondering...
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« Reply #41 on: April 29, 2020, 02:54:36 pm »

Am I the only one not seeing any of the images in this thread?
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« Reply #42 on: April 29, 2020, 05:13:20 pm »

Am I the only one not seeing any of the images in this thread?

Someone else was having the same issue, and it turned out to be some security settings on the browser...
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« Reply #43 on: April 29, 2020, 05:38:24 pm »

I wonder if the difference in tone bewteen ear and SA could be the tendency of the human ear and brain to  "mash" things together and pay attention to certain things versus others (much the same way as the eyes and brain tend to "see" something one is looking for, rather than recording the entire visual salad, as it were, like a camera does), versus the SA's tendency to present the results as exact number values on a graph.

Just wondering...

On this particular subject I know a fair amount. The answer is yes, 100%, but not as much "mashing" as decomposing. There is a lot of post processing that happens to sound in the brain. To begin with, we have a sort of logarithmic hearing sensitivity that allows us to hear very faint sounds and yet listen to loud sounds without falling to the ground writhing in pain. People with hearing loss (like me) can compensate up to a point by increasing sensitivity to faint sounds until one day the brain can't do it anymore, and some sounds will just start "dropping" which is when you realize you needs to get hearing aids. Kind of what happens with your vision as you grow up to an adult, when suddenly you realize you can't focus any more and have to get glasses

The other thing that happens is that we decompose sound into its constituent sound frequencies in order to process sound. Literally it's an electromechanical (cochlea) Discrete Fourier Transform, so we can decompose the signal into various sine waves of different frequencies, each one of which goes to a separate nerve for processing. So the brain can amplify each one at will, like the sound equalizer settings in your mp3 player on your phone or computer.

We use out hearing system to locate sounds in a sphere around our head, so the brain measures 1) Loudness, 2) Phase* difference (for sufficiently low frequencies) 3) time of wavefront arrival (speed of sound from left ear to right ear for higher frequency) and 4) relative intensity* of the Fourier Transform frequencies as sound bounces around your head, into your ears. With those 4 parameters you can determine from which direction sound is coming. That gives you an idea of the amount of post processing that's happening in you head!

So it's entirely possible the brain is locking into one or two things I expect to hear, even based on the instruments I see playing on the screen! "I know what those drums are supposed to sound like"!!




* Mathematically, the frequency and phase cues can be bundled together into a Laplace Transform called "Head Related Transfer Function" you can use to generate 3D sound with a digital computer or even an analog computer.
« Last Edit: April 30, 2020, 09:23:46 am by J. Wilhelm » Logged
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« Reply #44 on: April 30, 2020, 01:31:50 am »

Ah My Dear J

"The Brain is Decomposing the music"

I love it! Right up there with "too many notes" ( from Amadeus)
https://youtu.be/AUFwk1Ibwkc

I have been cogitating on the matter and perhaps the wood you are using is too thin?
I do recall from back in the day, the "common knowledge" was to use the thickest densest particle board possible.
But that really defeats the portability doesn't it.....

yhs
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« Reply #45 on: April 30, 2020, 07:58:41 am »

Ah My Dear J

"The Brain is Decomposing the music"

I love it! Right up there with "too many notes" ( from Amadeus)
https://youtu.be/AUFwk1Ibwkc

I have been cogitating on the matter and perhaps the wood you are using is too thin?
I do recall from back in the day, the "common knowledge" was to use the thickest densest particle board possible.
But that really defeats the portability doesn't it.....

yhs
prog marvel

One of my favourite movies. "Just cut a few here and there, and it will be alright."

Well, if you want the mathematically correct term, then the "cochlea maps the acoustic signal from the time domain and time delay domain to the frequency and phase domain in discrete samples."  "Decomposing" is much easier to say  Grin

The wood is too thin, for sure, but most importantly the stucco ramp is too thin as well, which for all practical purposes is the wall behind most of the driver, but I'm not 100% convinced that wood thickness alone is the main obstacle to reaching resonance at the moment, because I did see evidence of resonance on the setup where I placed the speaker in line with the horn (see about 8 posts above)- using the very same wood box!  I got an overtone signal of 125 Hz, and an increase in bass from the outlet slot (not phenomenal, but it was there), but the driver's excursion was very large (more like a woofer), and I believe the driver was clipping the sound as well - it sounded horrible. That gave me a hint of the limit on the Sony driver and also an idea of what actual resonance looks like for this horn. Right now the only difference is that the speaker is at 90 degrees from the line, and there's a ramp. First impression is that it's enough to discourage standing waves.

For the moment I can put more stucco behind the ramp, until I make a solid plug, but I have an issue with vibration from the mesh *inside the stucco *  which I can only cure by making the mesh shorter for the next box and a new box needs to be made with 1/2 thick wood. Before I rebuild mesh in a new box, I could also try to carve a. solid "plug" for the new box with a built-in ramp out from a 6x6 post. That way I can test a simple plug versus a plugewith the ramp and see if my theory that standing waves are discouraged by the ramp is true.

Either way, there may be a limit to how hard you can push this driver - so full resonance may not even be possible without harming this driver. Normally transmission line horns have a much greater cross sectional area and makers warn that one must use a specific kind of driver with certain physical parameters (large stiffness to mass ratio). There may not be enough air compliance for this particular driver.
« Last Edit: April 30, 2020, 09:25:12 am by J. Wilhelm » Logged
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« Reply #46 on: May 01, 2020, 06:18:00 am »

brain drizzle ( as opposed to brain storm) takeaways:

thin wood can be good... think guitars and violins/violas ... but then the wood must be bent, supported, carved, and sculpted just like those instruments,
and the masters can do it by ear, whilst us wood butchers follow meticulous instructions and end up with a "dull thud buster that twangs" no better than a cigar box banjo.

I "feel" something wrong with the mesh and artificial stucco... your description of vibrations only amplifies that feeling.
I think but cannot proof that the artificail stuff is not rigid enough?

Perhaps 1/2 inch dryway misted with water and slowly bent over a large mayo jar or coffee can? Then reinforced with ordinary concrete mix (leave out the rocks).
Use plastic or fiber mesh (as opposed to metal) if needed for structural strength... something along the lines of burlap-crete. - burlap soaked in concrete mix,

Or experiment with smoothing ordinary plaster of paris over the front and back of the existing artificial stuff, building up the back
until the desired rigiditity is obtained?

I have this strange and perturbed feeling that the Sony Corp actually DESIGNED the plastic speaker housing with various thicknesses and internal support beams and
flying butresses and blind alleys ON PURPOSE! to achieve the insanely good speaker effects...

I am still surprised and amazed the the relatively inexpensive modern boom boxes outperform my ancient really good stereo & 3 foot tall 3 way tuned speakers...
and sorta dissapointed in a way....

yhs
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« Reply #47 on: May 01, 2020, 07:32:08 am »

brain drizzle ( as opposed to brain storm) takeaways:

thin wood can be good... think guitars and violins/violas ... but then the wood must be bent, supported, carved, and sculpted just like those instruments,
and the masters can do it by ear, whilst us wood butchers follow meticulous instructions and end up with a "dull thud buster that twangs" no better than a cigar box banjo.

I "feel" something wrong with the mesh and artificial stucco... your description of vibrations only amplifies that feeling.
I think but cannot proof that the artificail stuff is not rigid enough?

Perhaps 1/2 inch dryway misted with water and slowly bent over a large mayo jar or coffee can? Then reinforced with ordinary concrete mix (leave out the rocks).
Use plastic or fiber mesh (as opposed to metal) if needed for structural strength... something along the lines of burlap-crete. - burlap soaked in concrete mix,

Or experiment with smoothing ordinary plaster of paris over the front and back of the existing artificial stuff, building up the back
until the desired rigiditity is obtained?

I have this strange and perturbed feeling that the Sony Corp actually DESIGNED the plastic speaker housing with various thicknesses and internal support beams and
flying butresses and blind alleys ON PURPOSE! to achieve the insanely good speaker effects...

I am still surprised and amazed the the relatively inexpensive modern boom boxes outperform my ancient really good stereo & 3 foot tall 3 way tuned speakers...
and sorta dissapointed in a way....

yhs
prof marvel

I would not want the enclosure to vibrate though. That is for the musical instruments. I will aim to make that wooden plug and thicker box tomorrow. I was too worried about my financial situation today after work to go spend money on wood.

Spoiler (click to show/hide)

Well I haven't compared to my 3-way white-cone 1980s Garrard stereo system (in storage). Or the even larger 1970s Garrard speakers my grandmother had (picture below, I sold them years ago). That used to be my standard. But don't call it "out-perform." because that 132 Hz signal on the Sony shelf speakers is very artificial, though... In that sense I am a purist.



Well. you're right on the Sony plastic case. The back (3/4 of the box) is very thin particle board actually with no batting or anything at all. But the front face of the speaker is very strong and heavy molded plastic full of horrible Non Euclidean geometries that can make a man go insane (actually it's not that complicated it's mostly mechanical support for the drivers). The front molded face does have a semi-ellipsoidal cross section though, I had to guesstimate the average depth of the plastic face to arrive at the overall depth of the box.  There's these curious black foam inserts filling inexplicable thin rectangular cavities along the interface of the particle board and the plastic face. I know they have a purpose, I just don't know what that purpose is. Vibration control /dampening by way of mass inertia? Vents to equalize air pressure (you don't need them you have the ports already). They're too small to provide resonance of any kind.


You're also right on the stucco. It's more like a sand-infused rubber material - very dense, but flexible, which menas it's good for absorbing sound. It needs to be more rigid, not just heavy.
« Last Edit: May 01, 2020, 07:40:27 am by J. Wilhelm » Logged
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« Reply #48 on: May 02, 2020, 03:09:51 am »

SNIP

I "feel" something wrong with the mesh and artificial stucco... your description of vibrations only amplifies that feeling.
I think but cannot proof that the artificail stuff is not rigid enough?

Perhaps 1/2 inch dryway misted with water and slowly bent over a large mayo jar or coffee can? Then reinforced with ordinary concrete mix (leave out the rocks).
Use plastic or fiber mesh (as opposed to metal) if needed for structural strength... something along the lines of burlap-crete. - burlap soaked in concrete mix,

SNIP

OK. This is my response for now. Two "wood plugs" for the new box made from 3 layers of 2x6 standard wood studs. Exactly 4 ½ x 5½ inch, which would be the internal dimensions of the new box made of 1/2 inch craft plank . As close to solid wood as possible, the thinnest part would be 2cm, a little over ¾ inch thick. If I need to sand it'sOK, or fill a gap it can be done with latex glue. This should not vibrate and be rigid enough while still being relatively light.

« Last Edit: May 02, 2020, 03:11:59 am by J. Wilhelm » Logged
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« Reply #49 on: May 07, 2020, 05:46:37 am »

In this update, I'll talk about what I found by using solid plugs and a thicker-walled enclosure. Plus some surprising findings and maybe a directive for a new approach.

I ended up building a thicker speaker box made from 1/2 inch pine. I used one of the ramps above to build a cap that is removable. The idea being to make comparative studies between a 90 wall and a ramped wall at the start of the line. My theory was that the ramp could serve as a waveguide and save energy by keeping the sound waves from reverberating and the top of the line. I had also observed that the previous box (left side in the picture below) was radiating a significant amount of sound. It turns out the latter as very important, but probably not because of what you expect.


I made a cap for the box using a piece of wood and the wood ramp. For the results I present today I did not smooth over the ramp with stucco as shown in the picture, and I just left it in raw wood, but I could also flip the cap upside down and that would be my "no ramp" test subject.  The reason for covering the ramp with stucco is to smooth the curve (the wood ramp is not curved it's three segments with different slopes), and to rule out any possible effects due to sound reflection off a semi-flexible surface during testing. I will test with the ramp with the stucco on it next time.


The comparison would be more perfect if I build a block of wood with the same volume as the ramp to take out the volume of air that the ramp needs during the "no ramp" trials - maybe I'll do that for the next round of tests, but I didn't for this informal test, knowing that  a change of +/- 5 cm in the length of the waveguide doesn't seem to give me that much of a difference (unmeasurable, < 1dB according to previous tests). The length (hence volume) of the new thicker box is adjusted to include the volume of the ramp and the volume of the extra wood in the walls (the channel is 4mm narrower in one direction).

I did some preliminary studies, but I don't have plots for that yet, because it was just an informal exploration. Nothing was calibrated. I was just anxious to see what I would get. The comparisons were made against the Sony Bass Reflex speaker which is my baseline. The trials also compared between ramp and no-ramp caps and I measured sound levels for sine wave inputs (57, 67, 77. 87 Hz) and also measured sound levels for white noise. To compare between 90 degree wall ("no ramp") versus ramp wall performance all you have to do is flip the cap. I can do that even while listening to some material in real time to make a comparison with my ears. This turned out to be important as well.

Between the frequencies of 57 and 87 Hz white noise profiles were indistinguishable between the old speaker box and the new one, and between ramp and no ramp caps. So basically no new information could be gathered from the white noise plots; all profiles are flat.

However I did find significant differences between the no-ramp and the ramp caps. I found that using a ramp yields output levels 3 dB higher than a 90 degree ("no ramp") cap, quite consistently (+- 1dB time -dependent drift/error) throughout the 57-87 Hz bandwidth. So this settles the question; I had theorized that perhaps having a 90 degree wall against the speaker would be beneficial to standing wave formation, in direct contravention of the energy analysis that states that you need a smooth curve to direct the energy into the horn - it's basically an argument for energy savings versus accumulation of energy by resonance. I just wasn't sure which was the right answer and I had to test it. Normally wave-guide speakers don't use ramps like these, they just use 90 degree walls and deal with the consequences by adding batting, so I had to test it out. But it turns out that 3 dB is a significant difference in favor of using a ramp, especially since this is a logarithmic scale. Why curved ramps are not used by others is a question mark. It could still be that a 90 degree wall is needed for standing wave formation, but at a ramp length of 10 cm and a wavelength in the order of 4 meters, it's hard to see how you would impede standing wave formation.

Unfortunately for sine wave results in the 57-87 Hz range, I was much surprised to see that I *lost* 1 dB of performance across the band quite consistently, when compared to the Sony Bass Reflex baseline. This baffled me. I did everything right. The ramp is rigid. I made the box with plenty of mass. The box is more rigid and dense. By all accounts energy loss is mitigated by the new design, and yet I lost 1 dB, so what gives?

Let's analyze what I have learned so far:

1. The resonance frequency is 57 Hz. At this frequency a 5 cm difference in the length of the waveguide did not make much difference (wooden extension ring, I talked about above). At 2,5 cm of added length, this new box should not change that equation.

2. The shape of the ramp turned out to be significant. A no ramp configuration lost 3 dB versus a ramp design. Shape and smoothness in general is very important near the top of the line (speaker).

3. According to the latter study, the added mass of the box walls and ramp were not beneficial, but detrimental. That doesn't mean that energy saving is not important, but a loss of 1 dB in measurements can be attributed only to addition of the mass/thickness, because everything else is basically the same versus the mesh-stucco ramped box

4. For the mesh stucco ramp box , I had detected a significant amount of sound coming from the walls of the speaker box. That, in fact was my impetus for building a new wall and ramp, more solid that before

It's as plain as the nose in your face that that extra decibel I'm missing is the radiation coming from the thin-walled box, a combination of radiation from the mesh-stucco ramp (which is very thin) and the thin pine walls.  Huh  Roll Eyes

Traditional speaker theory states that the walls of a speaker should not radiate. The box should not resonate, like a violin or a guitar, and to that effect you make the box of the densest most rigid material possible, to both absorb any sound attempting to escape and reflect it back into the system.

The reason why you do that is that the sound waves coming from the back of the driver's cone are 180 degrees out of phase with the sound waves coming from the front of the cone. The box's function is to separate the rear waves from the front waves, and either make those waves disappear (infinite baffle) or harness them in a suspension speaker (sealed cavity), or in a bass reflex system (Helmholtz resonator).

But there is no reason you can't have a spherical radiator. In a spherical radiator, a speaker is entirely surrounded by a thin wall, very much shaped like a balloon. all the waves coming from the back of the speaker are trapped in that balloon, but when the driver of the cone pushes inward, the whole balloon expands, creating waves. If the driver is producing sound waves that are very limited in spread  (the cone "throws" sound into one narrow region of the room) the sound cancellation in that region is limited to a narrow angle about the speaker, and most of the sound coming from the passive radiator is useful sound. Also you can design the mass density of the balloon to have enough inertia to force the balloon surface to move out of phase with the sound inside the balloon, avoiding sound cancellation. In other words, the whole box becomes the driver and the driver becomes part of the motor.

It is possible to design such an enclosure, but more often than not you will sometimes see membranes used as "dummy speakers" or "Passive radiators." Basically they are made the same way as a regular driver but without a magnet, and they behave exactly the same way as a driver with a magnet, but are using the pressure energy from the enclosure and their mass dictates how far away in phase they are from the sound waves in the enclosure.

https://en.wikipedia.org/wiki/Passive_radiator_(speaker)

You can always design a piston located strategically on the box that is tuned mechanically (spring-mass-damper) to resonate at the desired frequency, thus helping you radiate more sound. But it's a difficult thing to get right, because that piston or "passive radiator" needs to be tuned properly, oriented properly, and be designed mechanically and acoustically as part of the whole system. In spite of the probability of sound cancellation, I think that my lost decibel was being produced by the the stucco/wood box itself acting as a spherical/ passive radiator of sorts. Maybe that stucco ramp? Where is Stradivarius when you need him?

 Tongue

So at this point I'm not sure what to do. I have a working theory of what is happening, and that gives me some ideas. But barring any further theory, the horn setup with the mesh-stucco ramp seems to be the best that I can achieve so far. And I need to treat it as a "sacred geometry" because I can't readily calculate the resonance and phase behavior of that particular stucco/pine box without using a good structural dynamics program - even a very old one such as the venerable NASTRAN  Grin  Grin

It makes no sense to add more weight to the system by adding solid ramps and thick walls trying to save more energy, unless I see an increase in performance in some other way, be it 1) placing the drivers at the endpoints which produced very strong - out of control- resonance, 2) making a new horn design (I suspect my horn's  exit slot is too small it should be at least 1/9 of the input area) or 3) by designing a passive piston/spherical radiator setup, which significantly increases the amount of physics and math I have to calculate, but it's definitely not going to look the same.

If I had another pair of large cones, a passive radiator design seems like a very interesting idea to me, and a way to put my stamp on the design, but I'm not sure I want to go that way. I don't feel like I have mastered the problem of transmission line speaker yet, because my horn design is no longer ideal, having been modified so many times.

As an aside, I must state that the particle board box of the Sony Bass Reflex was unusually thin for a speaker enclosure (the board is only 8mm thick).  Roll Eyes Secret sauce?

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