A Monster Construction Methods Shootout Thread

[augerpro]

Which wood to use? Which fill? What does bracing do? Is CLD best? I'm starting this project to start quantifying some answers to these questions. A lot of work has been done before in fits in starts by others (BBC, Kef, etc.) but they were often narrow investigations and you couldn't compare results because measurement conditions were different among the different studies. Some have cited manufacturer's damping specs, but I have no idea how that translates to the real world. Ditto on accelerometer data. So here I will be doing SPL measurements, that is after all, what we actually hear. The cabinet is ordinary, large enough for the panels to have some radiating area and magnify differences between changes.



I will be using a Peerless 830970 firing into the cabinet using MLS signals. I have to test mic placement, but it will probably end up near-ish field, probably at a side panel. When I do the fill/lining I will mount the SB15 as a dummy woofer and measure what radiates through it. I intend to measure SPL, CSD, and THD. Any other recommendations appreciated.

For the material test I currently have: high and low quality plywood in 1/2" and 3/4", and MDF in 1/2" and 3/4".

For the fill and lining I have: SDS CLD tiles, AcoustX coating, denim insulation, polyfill, 1" thick dacron blanket (what the TL guys use) and Sonic Barrier.

For bracing I will do: simple oak crossbrace versus oak crossbrace using CLD construction with Sikaflex 292i adhesive. I also have some microspherules to combine with the adhesive to see how that changes things.

I would appreciate all suggestions on any other additions to the above. A priority will be that the materials must be widely available and reasonably priced, and the methods must be accessible to an ordinary DIYer.

[ergo]

Thats a great idea and not easy undertaking but would yield some very interesting data indeed.

Depending on your timing I would be willing to post you this unit to try out and loan for as long as this project will need it

https://audioxpress.com/article/Test-your-speakers-performance-with-this-do-it-yourself-measurement-system

https://audioxpress.com/assets/upload/files/George%20NtanavarasAccelerometer.pdf

Can be used with ARTA or REW and the sensor is small and light enough so attaching it with thin double sided tape or a drop of hot glue should work well. To release a hot glue attachment a spray can of "cold/freeze" can do the trick super easily.

I bought this unit from George in Greece some two years ago, but only tried it briefly. So kinda feel bad it's not getting the use it deserves and that use is exactly what you are attempting and I have not had time for. So let me know if this offer interests you.

[neuro]

Droid based construction for your consideration. Printed shell will be filled with a poured material; possibly carbonite, possibly concrete, plaster, etc.

[augerpro]

Great idea! Let's flesh that out a bit. What sort of internal structure? What fill material?

[neuro]

The baffle in that picture has gyroid infill. 2% fill for most of it and 10% for the center section so that the bolt holes have something to land upon as they are printed up layer by layer. A poured filling should flow into the gyroid structure so that I end up with macro scale composite of PLA and whatever I fill with. I would have rather had slightly denser infill in the whole thing but the pictured design was projected to use almost a full kg of filament and a normal roll is one kg.

The current plan is to fill with El Rey Fiber-47 Sanded Stucco Basecoat. It's cheap, easy to source (Lowes and Home Depot), fiber reinforced, and no large aggregate so if should flow to small features inside the print. I'll likely add plasticizer to increase flow of the mix. I'll also make the mix fairly wet and hope it still hardens okay. Plaster would be a safer option for a successful pour, maybe add some fiber to increase strength (but its more expensive and my local Hobby Lobby only had one bag). Epoxy granite should be the ultimate choice for fill material (many readers may already know but for those who don't: common concrete is sand and gravel aggregate with portland cement as binder, epoxy granite is several sizes of graded granite aggregate and epoxy as binder). Some high end machine tool manufacturers have gone to epoxy granite for the base of the tools (from cast iron) because it is superior at absorbing vibration.

I'm still debating if I'll print and fill the body panels or just the baffle. On one hand printed panels might be a bit of overkill on the 3D printing, might cost more, take longer, and perform worse than plywood+mdf panels would. On the other hand, there's no kill like overkill. If you want to go even farther down this rabbit-hole, a gyroid has two volumes, each of the "holes" on the top connect to either volume A or volume B but the two volumes are separate through the structure. So, you could pour one material in one volume and different propertied material in the other volume.

[gregrueff]

Hi Brandon,

I'm a long time lurker, but your post got me so excited that I decided to register so I could participate.
I've contemplated doing something very similar to what you appear to be undertaking. It's a serious effort!

I guess I'd be curious to hear your thoughts on what mechanisms you're addressing.
Is it acoustic excitation from the cone or the motor or both?
I'm assuming from your musings on stuffing/damping that it is primarily a study on backwave propagation and attenuation before it leaks out the panels?
Have you considered the various deformation modes of the cabinet, what excites them, and how to mitigate them?

I've also studied the BBC & KEF literature and it's not clear to me which is a greater concern.
I've also built my own cabinets with the KEF implementation of constrained layer damping by sandwiching a damping material between a panel wall and a brace. But without a baseline to compare it to, I have no idea to its efficacy.

The test I've contemplated is a sealed box completely constructed with the chosen material and construction method with a centrally located baffle to which the driver is mounted. That is to say the driver fires forward into half the chamber and backward into the other half. Then measuring the energy production of the front panel would indicate both the acoustic transmission from cone energy and the acoustic transmission from motor energy. This is very similar to your test! But I wonder if the SB15 will be the path of least resistance and not give an accurate representation of what kind of energy is being transmitted through the enclosure. Depending on the compliance of the SB15, your results could exaggerated one way or the other.

Please don't take this as criticism though. I'm super excited to see what you come up with and applaud you for taking the initiative. I'd be right there beside you if I had more time.

Cheers!
Greg

[augerpro]

neuro> Intuitively I think we tend to gravitate towards concrete and other heavy materials, but I've been part of some enlightening discussions that breaks down the different modes of transmission vs frequency band. If it were simply bass, heavy bracing to prevent flexing was the primary consideration, but higher in frequency that changes. Geddes at one time used urethane as the enclosure material, very high internal dampening. I wonder if something like that should be tried to compare to filling with something heavy and solid?

Greg> the SB15 will only be mounted when testing fills and soft wall lining. For material and method (CLD) testing, and bracing, there will be no SB15 mounted (and no hole for it), just the Peerless 830970. So sort of like your idea, where the driver front and rear radiation is captured by some enclosure.

[Steve Manning]

Here's a little inspiration for the ultimate in cabinet construction. https://www.magicoaudio.com/news/m9

[1Michael]

This is going to be some interesting R&D :T

[neuro]

Here's a little inspiration for the ultimate in cabinet construction. https://www.magicoaudio.com/news/m9

Wow, carbon-fiber and aluminum honeycomb "reduces overall weight" and the speakers still weigh 1000 lbs each.
I went to a Magico listening event a couple months before COVID shut-downs, great sounding speakers. Even their "lower" priced speakers (the A series) have a machined aluminum enclosure and bracing.

Honeycomb panels are generally available but every time I've gone looking they are way too expensive just to try out a new material. I can only imagine the complexity and cost of a curved structure like Magico has created.

[JonMarsh]

Which wood to use? Which fill? What does bracing do? Is CLD best? I'm starting this project to start quantifying some answers to these questions. A lot of work has been done before in fits in starts by others (BBC, Kef, etc.) but they were often narrow investigations and you couldn't compare results because measurement conditions were different among the different studies. Some have cited manufacturer's damping specs, but I have no idea how that translates to the real world. Ditto on accelerometer data. So here I will be doing SPL measurements, that is after all, what we actually hear. The cabinet is ordinary, large enough for the panels to have some radiating area and magnify differences between changes.



I will be using a Peerless 830970 firing into the cabinet using MLS signals. I have to test mic placement, but it will probably end up near-ish field, probably at a side panel. When I do the fill/lining I will mount the SB15 as a dummy woofer and measure what radiates through it. I intend to measure SPL, CSD, and THD. Any other recommendations appreciated.

For the material test I currently have: high and low quality plywood in 1/2" and 3/4", and MDF in 1/2" and 3/4".

For the fill and lining I have: SDS CLD tiles, AcoustX coating, denim insulation, polyfill, 1" thick dacron blanket (what the TL guys use) and Sonic Barrier.

For bracing I will do: simple oak crossbrace versus oak crossbrace using CLD construction with Sikaflex 292i adhesive. I also have some microspherules to combine with the adhesive to see how that changes things.

I would appreciate all suggestions on any other additions to the above. A priority will be that the materials must be widely available and reasonably priced, and the methods must be accessible to an ordinary DIYer.

Oops, you slipped in the killer disclaimer- "widely available and reasonably priced".

Clearly you weren't talking to me.... carry on and I'll await the results!


:T

[Bear]

Here's a little inspiration for the ultimate in cabinet construction. https://www.magicoaudio.com/news/m9

I believe that was written by someone playing the black-out version of audiophile bingo. Dang. It's got all the words! Is there back EMF? No, it "regenerates AC". We had to throw an extra cap in there, so it's now "elliptical symmetry". I am almost surprised that they didn't talk about the terroir of the individual components.

On a more serious note, with my current projects, I've been looking at "lip" braces vs shelf braces. I'd nominate testing those if feasible. In that respect, a stress skin torsion box (essentially B&W's original Matrix concept) could test the tradeoff between internal bracing and the required mechanical properties of the outer shell (e.g., can you get away with 0.25" plywood if the bracing was done correctly).

[CADman_ks]

I believe that was written by someone playing the black-out version of audiophile bingo. Dang. It's got all the words! Is there back EMF? No, it "regenerates AC". We had to throw an extra cap in there, so it's now "elliptical symmetry". I am almost surprised that they didn't talk about the terroir of the individual components.

....

WOW! The current M6 requires at least 2 guys to setup, a 20 page uncrating/installation manual, all special tools required to unbox, and comes with it's own torque wrench. There's probably $200 / speaker in just the casters and mounts to get the speaker off the crate. There's probably $200 / speaker in the crate as well. There's easily a $1000 in packaging and tools for a pair of these things. I guess that's required when you're spending $172,000 / pair...

WOW, just WOW...

[neuro]

neuro> Intuitively I think we tend to gravitate towards concrete and other heavy materials, but I've been part of some enlightening discussions that breaks down the different modes of transmission vs frequency band. If it were simply bass, heavy bracing to prevent flexing was the primary consideration, but higher in frequency that changes. Geddes at one time used urethane as the enclosure material, very high internal dampening. I wonder if something like that should be tried to compare to filling with something heavy and solid?

The idea of too rigid a construction being sub-optimal makes sense. For my baffle I'm hoping that the pla shell and infill will disrupt modes sufficiently while still being rigid enough to keep the driver frames from moving. Stucco tends subjectively to have a bit higher damping than regular concrete so hopefully that will help too. However, what would be best in the realm of DIY possible options? I'm still guessing epoxy granite would be very good. However, the dynamics we are dealing with differ a bit from machine tools so there is quite likely something even better for speaker enclosures. My guess is that pragmatically, the standard for Ardent construction will be hard to beat for cost/effort/results; plywood laminated to mdf. Dreaming of something even better, perhaps epoxy with rubber aggregate. Sand and PVA glue was another option I considered, still think it would work well.

A tradeoff we are often willing to make for DIY is size vs. performance. I.e. we'll accept something larger than a commercial solution. So, I wonder if two to four inches thick of cheap plywood (and/or chipboard and/or mdf) and xps foam board would be as good as something with more engineering/design elegance to it.

[neuro]

Here's a little inspiration for the ultimate in cabinet construction. https://www.magicoaudio.com/news/m9

In the context of construction options, I'm not sure what to make of Magico's repeated use of aluminum. Even this latest version with aluminum honeycomb; wouldn't aramid honeycomb have been a better acoustic option? For the solid aluminum versions, seems like they would have had to significantly coat it (adding weight) to keep ringing down. Then again, they don't seem afraid of weight so maybe there is Dynamat everywhere inside.

Back to reasonable construction options, it would be nice to compare some version of Steve's method (translaminate with damping layers) also.

[Steve Manning]

In the context of construction options, I'm not sure what to make of Magico's repeated use of aluminum. Even this latest version with aluminum honeycomb; wouldn't aramid honeycomb have been a better acoustic option? For the solid aluminum versions, seems like they would have had to significantly coat it (adding weight) to keep ringing down. Then again, they don't seem afraid of weight so maybe there is Dynamat everywhere inside.

Back to reasonable construction options, it would be nice to compare some version of Steve's method (translaminate with damping layers) also.

I want to say they made the comment at some point, that in getting the cabinet super rigid they were forcing any cabinet resonances to a frequency that was well above hearing range and as such not noticeable. Though don't hold me to that.

[augerpro]

neuro> sounds like an interesting project, let me know if your adhesive holds the PLA (PETG?) together. I am going to try combos of plywood and MDF/HDF in different thicknesses. At some point I'll also try the bamboo ply, it's not terribly expensive, and becoming reasonably available.

[augerpro]

Measurements have started, need some feedback. Below is a measurement of the side panel and rear panel. Any reason to do one over the other? I'd rather not double my work by doing both, if possible. Will probably stick with 50ms gate, 1/6 octave smoothed, unless someone would like more accurate but messier plots, I can always reprocess past measurements so this is not something I need to decide now. Also, since there was some confusion on my test setup, here is a pic when measuring different materials and build methods. For the lining/fill tests there will be a dummy woofer below the source driver, and the mic will be nearfield on that dummy driver.

[xandresen]

I notice the second and third harmonic peaks are strongly visible for the rear panel but not for the side. Construction difference? Change in microphone position?

Why not standardize on a 5ms gate to eliminate room reflections? We aren't seeing anything below 200Hz with this cabinet.

I wanted to try the EAR C-1002 blue visco-elastic damping sheet material, been around for decades, bought some.
It's very soft and stretchy - I never could figure a glue to bond MDF to it. West System epoxy might work.
Small amounts are available from http://www.percyaudio.com/

my experience
I made test panels with MDF+BB composite and a relatively stiff 1mm thick visco-elastic sheet material made for car doors - adding the damping sheet had no effect at all!
The same sheet did work well with MDF and thin bathroom tile as the constraining layer. So the relative stiffness of the materials matters.
My current speaker cabinets use the MDF + bathroom tile construction.

http://www.htguide.com/forum/showthread.php?42597-Constrained-layer-cabinet-walls-(bathroom-tile-as-constraining-layer)

[augerpro]

Not sure on why those peaks are so different, but yes they do appear harmonic. Regarding gate, I wanted to do near field and longer gate to capture that low frequency behavior, just like measuring any woofer, but yes there isn't a lot going on down there other than normal low frequency rumble from the environment. Still, here is measurement of the source driver outside the box, from 33" away, same 50ms gate and 1/24 smoothing. I determined empirically the SPL difference between the two plots, so the SPL levels should be reasonably correct. I'll probably do a near field measure of the source driver as the future reference.

[JonMarsh]

Interesting way to use your time not worrying about a day job for now!

:T

[augerpro]

Interesting way to use your time not worrying about a day job for now!

:T

It's been great! Unfortunately I sense a job offer on the horizon, so I need to get this box study moving and measure a few more Bliesma T25B waveguides I printed.

[augerpro]

BTW Jon, could use some feedback on the measurement conditions. Here is a comparison mic distance at 1/2", 6", and 30". First plot is raw response, second is *approximately* corrected for SPL loss with distance.

[gregrueff]

Hi Brandon,

Assuming that you’ve completely sealed the test enclosure(s) and you have no pressure leakage, then you are measuring pure deflection/deformation manifesting as sound pressure level. As such, the pressure level is resulting from the summation of all deformation modes, from all surfaces, and including the wholesale deformation of the whole cabinet in complex modes. Because you’re not measuring in an anechoic chamber, your microphone may only be able to pick up the direct radiating surface and nearest deformation mode(s) apart from ambient noise.

However, as long as the geometry and dimensions are consistent from test enclosure to test enclosure, then your absolute comparison between the two is valid.

I would base your decision on which results are more stark. Do you see a bigger and clearer difference in comparing the side or the back panel?

My two cents.
Cheers,

Greg

[augerpro]

Regarding box dimensions: I cut the panels so the outside dimensions would be the same. Internal dimensions and volume will be a bit different given the 1/2" vs 3/4" material. I'm not sure this was the correct decision, but even the internal differences would be very small, standing waves would probably shift a few Hz, same with panel resonances. Thoughts?

I'm going to do both side and rear panel measurements for now. The box is sealed, and I'm hoping the 3/4" MDF and SDS CLD tiles on the driver enclosure removes any radiation it might be giving off.

[augerpro]

Here are some first results. For now, I'm measuring both side and rear panels. Mic distance is 1/2", 50ms gated MLS signal, 1/24 octave smoothing. Low frequency rumble starts showing up below 250hz, so ignore anything below that.

1/2" junk ply
Got this HD, looks like 3 ply plus veneers.





1/2" high quality ply
Also from HD I think, 7 ply plus thick veneers, birch ply




1/2" MDF
Typical MDF from HD




3/4" high quality ply
This is Russian birch from a local wood supplier. Very spendy. 13 plys.





3/4" mediocre ply
From HD, called A/C Radiata. 7 plys





3/4" MDF
typical MDF from HD



Comparing 1/2" sheets:



Comparing 3/4" sheets:



comparing 1/2" plys:



comparing 3/4" plys:

[augerpro]

Some thoughts. MDF does well. I need to decide on how to set up the CSD to see if there is anything more to learn there. It's really easy to GIGO those CSD plots.

Thicker is better. Crappy ply is crappy ply, but if it is thicker can be usable. I need to select a standard material when moving to the lining and bracing tests. I'm inclined to use the mediocre 3/4" ply, and maybe the 1/2" MDF for those future tests.

If anyone has some bamboo ply and can cut up a flatpack to send me asap, let me know and I'll send the panel dimensions.

[augerpro]

Measured baseline for the fill and lining testing. Empty box, no lining, no bracing. Dummy woofer is an SB Acoustics SB15, terminals shorted. I had both the poly MFC and aluminum NAC on hand so measured both. I would have guessed the poly would pass less sound, at least at higher frequencies, but...

[augerpro]

Initial box lining results!

Monacor MDM3 wool batting 2 layers
Looks like this isn't available anymore. About 1/2" thick batting of wool and IIRC poly. Given the performance of 2 layers (1") I would like to find a new source of wool batting, since my big complaint with the denim insulation is the amount of fuzz and fiber it gives off. Makes me worry it will get into the voicecoil of a driver.





And 1 layer:





1" Sonic Barrier
From Parts Express





1.8" Ultratouch Denim insulation
Available at HD (and other places), reasonably priced, performs well. No wonder the popularity. Now if would just stop shedding so much stuff!





A comparo:

[5th element]

Thick is better

What I see is that the first lot of strong peaks are less pronounce with the thinner materials. It really does reinforce the idea that a thinner walled enclosure with damping might provide better results than thicker stuff. I guess it depends on what is considered audible.

The denim stuff gets a good rep and you can see why. Can't get it easily, like time I looked, in the UK, which is a shame.

[augerpro]

It appeared to me the main resonances were shifted lower in frequency with the thinner material, not so much lower in amplitude. Which makes sense if it is a function of stiffness. Same with MDF vs plywood. Plywood is stiffer (IIRC) so main resonance is shifted higher in frequency.

I think you are going to like the wool batting (for quilts) testing, should be easy to get in the UK. 3 layers of the above MDM3 would be 1.5", and I'm betting at least as good as the denim (1.8"). Wool batting for quilts is widely available, and I found a supplier who makes .8" batting, so two layers should perform well.

[augerpro]

More plots.

[augerpro]

From a response on another forum:

Great work, Augerpro.

The results at 400 Hz are a surprise to me. I would not have guessed that there could be a ~ 15 - 20 dB reduction at 400 Hz for all 3 of the stuffing materials. I would have guessed that a big 400 Hz peak for the empty box would have been a cabinet panel resonance, and stuffing would make little difference to a cabinet panel resonance.... but that is not what the data says. I don't believe this is a panel resonance, it is an acoustic resonance. This is why the stuffing materials have such a large effect.

Notice between 650 and 800 Hz the bare box has a resonance, and this looks like panel resonance. The 3 stuffing materials have much less effect here... The MDM3 2 layer and the Sonic barrier foam are about 8 dB lower than the bare box, and the denim is about 10 dB lower. While this is more of a reduction than I would have expected for a panel resonance, it is far less than the 20 dB reduction seen at 400 Hz or at the upper frequencies.

Another surprise is at 2 kHz. Here the surprise is the bare box... we have large resonance, it is the largest SPL resonance of the experiment. The magnitude of this resonance is as high or higher than the 650 - 800 Hz peak, which is shocking. I believe this is an acoustic resonance, because all 3 stuffing materials have a big impact. 2 kHz is in the region where most insulation materials have their strongest absorption. The foam and the MDM3 have a 20 dB effect, and the denim has a 25 dB effect.

The fact that the data revealed 3 unexpected results shows that value of empirical data.

Great analysis! I'm only right now going over the plots. Here is something to chew on, rear panel response (empty box) and dummy woofer response (empty box):



I also modeled these in boxnotes. I now realize I'm not quite sure what the assumptions are for this program. Is the standing waves generated by the driver at the specified location in a given box, and the response measured (if you could) at that same spot? So looking at the boxnote of the source driver, would you necessarily expect to measure those resonances at the dummy woofer location? And the dummy woofer boxnote wouldn't be relevant at all because it is not creating sound?

[5th element]

Am I reading it wrong then that the 1/2" MDFs main peaks are about 5dB lower than with the 3/4?

[augerpro]

huh, why yes they are at the primary resonance. I'll have to dig into this a bit more. Why would that be? I still see it in the plywood examples, but much smaller. I'm wondering if this might be a measurement artifact, specifically smoothing of the response to 1/24 octave, or even in the FFT due to the gate causing less resolution at lower frequency, so the peaks are smaller? I can process it with no smoothing and open up the gate.

[gregrueff]

Hi Brandon,

It looks to me like your boxnotes results help explain some of what is going on in post #33.
The ~410 Hz resonance that is reduced with damping material is predicted by boxnotes to be a standing wave resulting from the inside height of the cabinet. The 410 Hz wave is 33 inches long and the inside height of the cabinet is exactly half that.

I feel like your plot in post #18 also kind of bears this out. Although the frequency is shifted about 50 Hz forward, both the side and rear panel show a resonance here, but then differ elsewhere corresponding to the depth and width not being the same dimension.

I also can't resolve in my mind how the panel resonances and acoustic resonances can be separated. There are only two mechanisms that drive excitation in the panels at their resonances: the first is the reactive force from the driver motor; and the second is the source signal creating standing waves at the acoustic resonances. Therefore we should expect some of the panel resonances to be addressed with damping as their genesis is an acoustic resonance, right?

Awesome data so far!
I'm still trying to wrap my head around all the conclusions we can draw from this as well.
Cheers,
Greg

[augerpro]

A couple years ago there was a threaded started at avsforum that encouraged me to start this project. Here are some selected posts that lay out the argument well:

I was recently chatting with some people about the best ways to build speaker cabinets and why that is. When I built my speakers, the cabinets were designed with a great deal of effort to address two issues that had been shown to be audibly important, inexpensive to correct, but commonly not addressed properly in other speakers. I'll also note, I am not the one who designed my speakers not came up with these solutions and what follows in this post is nothing original. What I've noticed however is that these "concerns" which remain easy to fix, still are not readily addressed in a lot of DIY efforts.



The first is edge diffraction, and to be honest, that isn't why I wanted to post this. None the less, I'm often amazed more efforts aren't made to reduce both edge diffraction on speaker cabinets as well as avoiding diffraction surfaces near to the speakers (i.e. the screen frame, screen wall, etc.).



The other is cabinet panel re-radiation. Basically a speaker cabinet is not a sound proof box. The sound produced by the driver is re-radiated through the cabinet. In addition, as much as 50+ db's of the rear wave is reflected off the rear wall and back through the cone of the speaker. These have a measurable and audible negative effect on the sound of a speaker. Research into the causes of reradiated sound both through the panels and through the cone have found some things that I never understood to be true with regard to the cabinet reradiation problem. Had you asked me about this 5-10 years ago, I would have said that I think very little sound re-radiates through the cabinet wall, has little audible effect other than at resonant frequencies, and that acoustic stuffing in the cabinet absorbs more than enough of the rear wave. Turns out that is wrong. Understanding and addressing this is difficult, it isn't that well studied, their are conflicting opinions, and unfortunately, even some conflicting research papers. Some of the most intensive research really has been done by manufacturers, not independent scientists, and much of it has never been published.

http://www.dagogo.com/a-whitepaper-the-audibility-of-cabinet-panel-resonances-and-pat-pend-method-of-reduction-of-audible-coloration

This was a "white paper" by Albert Von Schweikert, and while some of his claims start to verge on voodoo, most of this is scientifically valid and was researched in a believable way.

http://www.usenclosure.com/Our New Technologies and Abilities/Technology_and_Capabilities.html

These guys have investigated and analyzed different cabinet wall construction and landed on a proprietary approach that reduces the problem. What is more interesting is their graphs showing the reduction in re-radiated sound from the panels.

http://www.audioholics.com/loudspeaker-design/lowering-mechanical-noise-floor-in-speakers

and

http://www.audioholics.com/loudspeaker-design/lowering-mechanical-noise-floor-in-speakers-pt-2

"Mark" wrote a nice two part article on reducing re-radiation and in part 2 discusses the benefit of damping.



As for the significance of the problem,

http://www.aes.org/e-lib/browse.cfm?elib=12234

I think this is a good article. it's important conclusion, speaker walls re-radiate a lot of sound, sometimes more sound than the driver itself, and....bracing doesn't necessarily make it better. What they show is that relying on wall cabinet measurements alone to show that a stiffer and better braced wall cabinet does not necessarily mean lower total radiated sound from the cabinet. That is really important.



Here is where we get to my point. In the second Audioholics article Mark makes mention of two modes of damping a speaker cabinet. Extensional and Shearing. Extensional is what most people do, if they do anything. That is applying a damping material to the inside surface of the cabinet. The other is Shearing, and this is what we know as Constrained Layer Damping. It is by far the most effective means of damping panels and dissipating energy. It is the approach we use in soundproofing theater walls, reducing noise and vibration infiltration in our rocket ships, reducing the distortions in microscope images, you name it.



Some researchers have also talked about another mode of sound transmission through a cabinet, through the walls themselves. Instead of the sound emanating from the speaker inside the cabinet and then transferring through the cabinet wall (and in some cases exciting a resonances such that it transfers out of the walls at a greater total amplitude than the initial amplitude level from the front of the speaker) it actually transfers through the frame of the driver to the front baffle, through the front baffle to all other surfaces, which then act as a re-radiating surface.



For a DIYer the fixes aren't all that hard and require the use of materials we have access to, but I just don't see it done very often. I don't understand why?



CLD baffles are the most important, if nothing else is CLD, the baffle should be. This can be accomplished by using two layers of material with a damping layer between. Green glue seems an obvious DIY choice, but for what its worth, my speakers were made with urethane rubber as the CLD material. It has a durometer of 40 and slight amount of additives to increase its damping slightly (talc I believe). For all practical purposes the cabinet is held together with sorbathane, in case people are wondering what this stuff is. Sorbathane is Urethane.



I think the take away from these articles is that bracing is really important and you need to rigidly brace a box so that you tie opposing panels together and break up their unbound surfaces. This raises the resonant frequency and often raises its Q as well. What I think is the other important take away is that this can't be done without damping and that CLD damping is probably best where possible. Bracing doesn't fix the re-radiation problem, it just makes it easier to fix. The fix is still dissipating that energy before it exits the box.



Another important take away is that since the driver is directly exciting the box vibrations and the sound is traveling through the entire cabinet, we somehow need to break up these direct connections. This is where decoupling becomes important. This means things like mounting the speaker onto a ring that is itself decoupled and damped. My own speakers have the speaker driver mounted onto a cast urethane mounting ring that is internally composed of two layers of MDF (one is 3/4" and the other is a piece of 3/4" that was recessed for the driver .5", leaving .25" of MDF. This means the layers of CLD for the driver mount are driver, urethane, .25" mdf, urethane, .75" MDF. The sidewalls and rear panel can also be decoupled to help break this up. The side panels can be mounted to the inner "shell" of the cabinet so that the driver and outer-shell of the baffle is decoupled through CLD from the sidewalls and thus also the rear panel. If the rear panel is CLD, same thing, the outer shell of the cabinet can be decoupled from the side walls, further breaking up the transfer of vibrations.



Without CLD the bracing would actually contribute to the re-radiation problem considerably. However, if you have CLD panels and tie the walls together so that the brace is touching only the inner shell, and it is separated from the outer-shell by damping, then re-radiation through the bracing is minimized.



In terms of acoustic damping inside the cabinet, it is important to absorb the rear wave, but its important to remember that just like fiberglass insulation doesn't soundproof a wall, batting or fiberglass inside a speaker cabinet doesn't soundproof the cabinet wall. Sound waves reflect when they hit an abrupt change in acoustic impedance and do so anti-phase to the original source wave. I like the idea of creating a change in the acoustic impedance as you approach the barrier surface. Something like loose batting in the middle with fiberglass or cotton dense insulation along the wall.



Finally I'll just mention none of this applies to subwoofer cabinets in the least. They need to be pretty rigid and dense, but their resonance frequency is well above their operating range, meaning we don't need to dampen it.



I'd really love to see people start incorporating these principles into their DIY designs. We often talk about building DIY speakers that best manufactured speakers. These principles are common in well engineered commercial offerings and they have been shown to make a measurable and audible impact, so why not incorporate them.

http://www.acoustic-glossary.co.uk/sound-insulation.htm it should have said coincidence frequency. Sorry for the typo.





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http://wiki.naturalfrequency.com/wik...d_Transmission see this and http://www.mne.psu.edu/lamancusa/me458/9_trans.pdf for discussion of these zones. Mass dominates between the resonance frequency and coincidence frequency. Not above coincidence or below resonance.





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Noah much of what you discuss deals with what would matter for a sub enclosure



https://books.google.com/books?id=jD...ge&q=stiffness below resonance transmission loss&f=false



Below the resonance frequency mass has little impact on transmission loss. Stiffness dominates. Doubling the stiffness leads to a 6 db increase in transmission loss. At resonance the damping matters. Above resonance mass matters. With a full range speaker that produces everything you can't just build stiff or massive because all of he zones, including coincidence would come into play.



I think maybe for different boxes different ideas might apply. For a main speaker it might make sense to get the resonance below its operating range by keeping mass high and stiffness relatively low, with high damping to reduce the amplitude decay of the resonance (which would be unlikely to be excited in this scenario). With a sub box it probably makes sense to get the resonance frequency high and get stiffness up as high as possible. In this case stiffness matters but these articles indicate mass does not matter so much. Damping would matter only to reduce the effect of the resonance.



Do you think I'm mis-applying any of these factors to a speaker box. I noted earlier some of these wasn't totally clear to me. Either I don't understand transmission loss as well as I think some of what we say about full range speaker boxes may not be right.





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[augerpro]

Once I move on to CLD construction, I need recommendations on the adhesive. I've read silicone is not very good at dampening vibration, but would like cheap, common caulk to use as a baseline. To step it up a notch, the newer MS polymer adhesives are apparently the way to go:

https://www.amazon.com/WEICON-13305310-MS-Polymer-1-Component-Adhesive/dp/B004Z0MRU2/ref=sr_1_3?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-3

https://www.amazon.com/Red-Devil-0987-Advanced-Purpose/dp/B0843K399R/ref=sr_1_4?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-4

https://www.amazon.com/Rayt-1881-52-Sprint-Polymer-Transparent/dp/B017IPHREQ/ref=sr_1_5?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-5

https://www.amazon.com/Deka-FRA451072-Dekasyl-MS-2-White/dp/B00SYHMKV8/ref=sr_1_22?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-22

https://www.amazon.com/WEICON-Speed-Flex-MS-Polymer-Adhesive-Sealant/dp/B0886VVQNX/ref=sr_1_38?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-38

[Steve Manning]

Once I move on to CLD construction, I need recommendations on the adhesive. I've read silicone is not very good at dampening vibration, but would like cheap, common caulk to use as a baseline. To step it up a notch, the newer MS polymer adhesives are apparently the way to go:

https://www.amazon.com/WEICON-13305310-MS-Polymer-1-Component-Adhesive/dp/B004Z0MRU2/ref=sr_1_3?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-3

https://www.amazon.com/Red-Devil-0987-Advanced-Purpose/dp/B0843K399R/ref=sr_1_4?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-4

https://www.amazon.com/Rayt-1881-52-Sprint-Polymer-Transparent/dp/B017IPHREQ/ref=sr_1_5?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-5

https://www.amazon.com/Deka-FRA451072-Dekasyl-MS-2-White/dp/B00SYHMKV8/ref=sr_1_22?dchild=1&keywords=ms+polymer+sealant&qid=1596904242&sr=8-22

https://www.amazon.com/WEICON-Speed-...904242&sr=8-38

For gluing panels together you have something like this. https://www.atsacoustics.com/quietgl...gle-tubes.html

[augerpro]

Meniscus bonded dacron batting 1 layer





2 layers



Not great, but the reason I wanted to test this dacron batting was that it is very popular with the transmission line designers, who use it to stuff the first half of the line behind the driver, and this batting provides a consistent density. Here I stuffed the box:







polyester pillow stuffing @ 1/3 of a pillow

[augerpro]

CSD are tricky to set up and tricky to read. Don't just look at what drops into the floor first, look at the difference in amplitude between the first slice and the last slice.

[xandresen]

Once I move on to CLD construction, I need recommendations on the adhesive. I've read silicone is not very good at dampening vibration, but would like cheap, common caulk to use as a baseline. To step it up a notch, the newer MS polymer adhesives are apparently the way to go:

There is a very long thread on CLD damping where user GEDLEE offers specific adhesive suggestions. He says it's the filler that you mix into the adhesive that works with the adhesive to provide the damping. Also mentions that some of the adhesives he recommends are very runny even with filler added.
Check out his postings here. thread rambles and he has to repeat himself, leaving out details sometimes.

https://www.diyaudio.com/forums/multi-way/153419-constrained-layer-damping-mdf-ply-30.html?

Following his thoughts, I found you can buy a two part polyurethane adhesive with Shore 40A rating, in small quantities, not runny, here.
Have not tried it, no idea what filler to use.

https://shop.sculpt.com/smooth-on-brush-on-40-trial-kit.html

[augerpro]

Yep, Earl promoted mixing glass microspherules into the adhesive to improve performance of CLD. I have some, just waiting until I get to that point in testing.

[Carl V]

Back in the day george short of north creek speakers suggested or advocated the use of “goop” to be applied to the interior walls. My memory has faded, but I recall it being based on dry wall joint compound. Do you plan to look into black hole 5 or the various heavy damping products. No Rez. Etc. Good luck. My hats off to your study.

[augerpro]

I've got some Rezonix CLD tiles coming. It was created by the guy over at diymobileaudio that did the big test identifying SDS CLD tiles as top of the heap. When SDS went out of business this guy created Rezonix to fill that gap. He hasn't provided test results yet to see how close his product is to SDS yet though.