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post #1 of 179 Old 04-13-2017, 11:58 AM - Thread Starter
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The Science of Speaker Cabinet Design

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-t...ble-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%20New...abilities.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/loudspeak...or-in-speakers
and
http://www.audioholics.com/loudspeak...-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.
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post #2 of 179 Old 04-13-2017, 01:01 PM
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re'
cast urethane mounting ring

did you cast it yourself? if so, please tell us how

I've cut up sheets or strips of sorbothane to make mounting rings but its expensive with all the waste...
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post #3 of 179 Old 04-13-2017, 01:37 PM - Thread Starter
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Quote:
Originally Posted by JackNC View Post
re'
cast urethane mounting ring

did you cast it yourself? if so, please tell us how

I've cut up sheets or strips of sorbothane to make mounting rings but its expensive with all the waste...
I did not! I said it was a good idea, I didn't say I knew how. Thats what all the great DIY minds are here for.

I don't think you must use sorbathane or urethane to make this happen. Again, you could use two layers of MDF in a CLD, recess the driver only part way through the outer layer (which ensures the driver is isolated from the inner shell) and then use a foam rubber, straight rubber, recycled rubber, cork rubber, cork gasket for the surface between driver and cabinet. All of these have been used by companies for this purpose and should work
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post #4 of 179 Old 04-13-2017, 02:40 PM
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OP I think your CLD construction is pretty spot on! There have been good discussions on this (though few measurements) over at diyaudio.com over the years. My own experiment would be similar to yours but I would only use 1/4" panels (other than the baffle). Basically box-within-a-box where the inside box was well-braced, then some 1/8" sorbothane (possibly some 3M VHB tape instead), then the outer box. Completely for convenience the baffle would be solid and at least the inner box would be glued to it. I know, not completely following the design logic, but I figure this is as much a surface area problem as anything, and if 80% percent of the panels are decoupled from the sound waves it would be acceptable. Still without measurements I don't know.

I do have a current project that does a lot of what you are thinking about other than the CLD construction. Here is an early CAD which doesn't have holes int he braces yet:


1" chamfers on the baffle for diffraction control, (after experimenting I really like chamfers, a lot of bang for buck). The upper half of the speaker behind the drivers is filled with dacron batting from meniscus audio, the same stuff people use for transmission lines as the the entire volume can be filled with an even .75 lb/cf density. Also Acoust-X was applied to the interior to absorb panel energy.
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post #5 of 179 Old 04-13-2017, 02:48 PM
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BTW anyone that knows me when I was more active on the forums 10 years ago knows this sort of experimenting with actual measurements is right up my alley. So I have an offer. I'm in an apartment right now, so construction is out, but if anyone in the Denver area is willing to build to my specs, we can start measuring various designs.

My basic idea would be to use a wideband 2" driver, like those from Peerless, attached to the outside of the box (with it's own small enclosure of course) so it would fire into the box. This way it would mimic a driver, but only the interior sound's contribution to panel radiation, and re-radiation through a dummy driver's cone would be measured. We would also build control boxes of MDF and Birch ply. Start working through variations and find the most efficient and economical method.

CLD with sorbothane or 3M VHD tape interests me.

Acoust-X interests me.

MDF vs high quality Birch ply interests me.

Cast urethane REALLY interests me.
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post #6 of 179 Old 04-13-2017, 02:51 PM - Thread Starter
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Measuring reradiation is really hard, but you can measure panel response with something like a contact mic. I have one that I can connect to a mic preamp and measure like an accellerometer.

Because CLD is a damping approach it doesn't have to be the entire surface. Just that more is better. You can apply a viscoelastic compound to the panels and a block of MDF and get benefit.

Try applying a denser liner than Dacron such as felt, fiberglass, or bonded cotton to the cabinet walls. This helps reduce reflections better than Dacron alone (at least in theory).


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post #7 of 179 Old 04-13-2017, 04:03 PM - Thread Starter
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I've had the same issue AugerPro. I used to live in a house, had acquired loads of tools and test equipment. I had oscilloscopes, B&K measurement mic, accellerometer, 3000 watts of giant test load resistors, table saw, routers, air tools, you name it. Then I moved to Chicago and sold, gave away, or threw away most of this stuff. Then I had a baby and we moved to the suburbs, but I can't really justify debuying anymore.

At one point I had a new speaker or amp project almost monthly and it was just filling up every room I had.


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post #8 of 179 Old 04-13-2017, 04:26 PM
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Thankfully all my tools are in storage, just waiting for when I get back in a house. I think they mate because two Festool tools have also appeared...

Regarding your comment on the lining, that's exactly why we need real measurements, since there are different takes on that. As I understand it sound energy is best absorbed while in the velocity mode (middle of an airspace) versus pressure mode (at a boundary). Accordingly, using a lower absorption product that fills the entire air space *should* be better than a more absorptive material at the walls. My own interest in this stemmed from the often glowing reviews of the midrange of a TL speaker (versus simply the bass), for example Salk Songtowers.
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post #9 of 179 Old 04-13-2017, 06:35 PM
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I dont agree DIYers dont take measures to combat diffraction. I just think sometimes cost or asthetics overule. Like augerpro said, a chamfer or roundover is easiest. It does ok for a dome or ribbon tweeter. Horns are good for this as they limit radiation along the baffle.

As for the rest, your wording makes it seem to be somewhat mixing internal reflections with cabinet resonance. Internal absorption is very important, so it doesnt reflect through the cone, as you said. As for bracing vs CSL damping, I understand that rigidity and damping work against each other. This is often missed and counter intuitive. If CSL works, it may just be the result of an overbuilt cabinet, rather than the impressive CSL technique. I could be wrong and Id love to see someone explore the issue. I cant be bothered at the moment.

With low XO horns, the most common approach is to brace and stiffen like crazy and raise the resonances as high as possible, eventually into the stop band. An infinitely stiff or infinitely damped speaker baffle cant resonant. If the baffle is sufficiently stiff to not resonant within the woofer pass band, then you dont need to damp.

Ive never known the sound to pass through the cabinet walls to be a problem, but I may experiment with this. Or did I misunderstand you?

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post #10 of 179 Old 04-13-2017, 06:44 PM
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Quote:
Originally Posted by tuxedocivic View Post
As for bracing vs CSL damping, I understand that rigidity and damping work against each other. This is often missed and counter intuitive.
This is the crux of the current debate that most interests me. And most in need of measured verification.
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post #11 of 179 Old 04-13-2017, 07:40 PM
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Ive never known the sound to pass through the cabinet walls...
Sound doesn't pass through solid materials, it vibrates them. If they're vibrated with sufficient force they'll in turn become radiating surfaces that cause the air on the other side to vibrate sufficiently to create audible sound. If they have a combination of stiffness and mass that prevents them from vibrating they won't become radiating surfaces, making them 'soundproof'.

Constrained layer construction dissipates energy in the damping layer(s) between the solid layers, which reduces the energy available to vibrate the outermost layer. I've found it to be pound for pound and inch for inch less effective than the combination of mass and stiffness. It's a method that makes more sense in room construction, where inches and pounds are less of a consideration, and panel to panel bracing is impossible. You can't very well have a matrix of braces spaced every six inches connecting all the room walls, floor and ceiling.

One method of reducing or even eliminating outer panel vibration not mentioned is a vacuum 'bottle'. Using dual wall construction with a vacuum in the space between the walls sound waves that vibrate the inner layer would have no means of transmission to the outer layer to cause it to vibrate.

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post #12 of 179 Old 04-13-2017, 07:56 PM
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Sound doesn't pass through solid materials,
Yes it does. Liquid too.

Thanks for trying to teach me something in that long post though.
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Dipoles get rid of one problem. Well, sort of. Turn it into a benefit I suppose is more accurate.
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I

Quote:
Originally Posted by Bill Fitzmaurice View Post
Sound doesn't pass through solid materials, it vibrates them. If they're vibrated with sufficient force they'll in turn become radiating surfaces that cause the air on the other side to vibrate sufficiently to create audible sound. If they have a combination of stiffness and mass that prevents them from vibrating they won't become radiating surfaces, making them 'soundproof'.

Constrained layer construction dissipates energy in the damping layer(s) between the solid layers, which reduces the energy available to vibrate the outermost layer. I've found it to be pound for pound and inch for inch less effective than the combination of mass and stiffness. It's a method that makes more sense in room construction, where inches and pounds are less of a consideration, and panel to panel bracing is impossible. You can't very well have a matrix of braces spaced every six inches connecting all the room walls, floor and ceiling.

One method of reducing or even eliminating outer panel vibration not mentioned is a vacuum 'bottle'. Using dual wall construction with a vacuum in the space between the walls sound waves that vibrate the inner layer would have no means of transmission to the outer layer to cause it to vibrate.

If you have put your ear to a rail road track, you can 'hear' a train coming from a long way off, yet the vibration is basically nil. Same with underwater sounds, the sounds of propellers can be heard at great distances, yet the local 'displacement' is nil.

Put your 'thermos' bottle in a high velocity air stream and tell me that you have no means of transmission from the inner to outer layers. They still are attached at the neck.

Using a short broomstick or long screwdriver pressed to your temple, you can hear various noises inside an internal combustion engine, for instance the whirring of a timing chain set, or the valvetrain, or placed on an alternator or power steering pump, can be used to diagnose noises.

http://www.engineeringtoolbox.com/so...ids-d_713.html

http://www.engineeringtoolbox.com/so...ter-d_598.html

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Quote:
Originally Posted by augerpro View Post

As I understand it sound energy is best absorbed while in the velocity mode (middle of an airspace) versus pressure mode (at a boundary). Accordingly, using a lower absorption product that fills the entire air space *should* be better than a more absorptive material at the walls.
This is correct, and any acoustic panel also perform better when spaced away from the solid surface behind it.

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post #16 of 179 Old 04-13-2017, 10:07 PM
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Originally Posted by Mpoes12 View Post
I'd really love to see people start incorporating these principles into their DIY designs.
I do and have done for years and have posted about them at various times. The CLD VS mentions is very similar to what I use for example.

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post #17 of 179 Old 04-13-2017, 11:12 PM - Thread Starter
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Quote:
Originally Posted by tuxedocivic View Post
I dont agree DIYers dont take measures to combat diffraction. I just think sometimes cost or asthetics overule. Like augerpro said, a chamfer or roundover is easiest. It does ok for a dome or ribbon tweeter. Horns are good for this as they limit radiation along the baffle.

As for the rest, your wording makes it seem to be somewhat mixing internal reflections with cabinet resonance. Internal absorption is very important, so it doesnt reflect through the cone, as you said. As for bracing vs CSL damping, I understand that rigidity and damping work against each other. This is often missed and counter intuitive. If CSL works, it may just be the result of an overbuilt cabinet, rather than the impressive CSL technique. I could be wrong and Id love to see someone explore the issue. I cant be bothered at the moment.

With low XO horns, the most common approach is to brace and stiffen like crazy and raise the resonances as high as possible, eventually into the stop band. An infinitely stiff or infinitely damped speaker baffle cant resonant. If the baffle is sufficiently stiff to not resonant within the woofer pass band, then you dont need to damp.

Ive never known the sound to pass through the cabinet walls to be a problem, but I may experiment with this. Or did I misunderstand you?


As the AES article I linked states, it reradiates the sound. You can explore If you want but they already have the results.

I'm not mixing up issues they are inter-related. The studies looking at the sources of panel resonances found they come from the driver directly transferring vibrations through the cabinet as well as sound exiting the back of the driver and into the cabinet space then exciting the molecules in the cabinet wall to again reradiate out of the cabinet. They will also reflect off the panels and through the driver cone. They are all from the same problems and would have the same I'll effect.

CLD has been tested by numerous manufacturers and researchers and found to be highly effective with speaker cabinets. A lot of this never seems to make it in the public in the right way, which is sad. Companies would rather keep it proprietary rather than put it out their for peer scrutiny and to advance the field.

The AES article found specifically that a very stiff box still reradiate sound.

I really don't know a reasonable way to test the deleterious effects of cabinet wall radiation. I don't have a laser inferterometer or a means to isolate the effect. I do have a contact mic/accelerometer for measuring the wall vibrations but I don't believe that's enough.


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post #18 of 179 Old 04-13-2017, 11:20 PM - Thread Starter
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I do and have done for years and have posted about them at various times. The CLD VS mentions is very similar to what I use for example.


I didn't mean to suggest no DIYer does this. I was suggesting it isn't common and I think that's true. There are common cabinet construction techniques and they typically go for massive and rigid. I think there are a good number of studies looking at vibrations in speaker cabinets as well as structures generally to suggest that stiff and massive is not enough, it needs damping to dissipate the energy.

I've seen a handful of diy projects testing this. It just seems uncommon. A lot of the diysg kits were coming with double front baffles which most people rigidly glued together. Why not apply a viscoelastic compound instead?


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post #19 of 179 Old 04-13-2017, 11:25 PM
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I've looked at accelerometers recently, but they appear to have the same problem they did before, which was very unstable results. Just resticking it to the same place would measure different.

In the end though that method seems pointless, since what we really want to know is is it audible? How far down from the fundamental is the re-radiation? That is why I would use a captured driver so any sound measured would only be from the cabinet or dummy driver cone. With this method you could quantify the difference between the fundamental and re-radiation. Which would inform the qualitative result: if I can barely hear the sound when the driver is driven >90 dB then there is no problem. Or if a mod helps or hurts, you can judge how much.
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post #20 of 179 Old 04-14-2017, 12:49 AM - Thread Starter
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If you look at the AES article I linked they already measures this in a much more sophisticated way and describe the levels. If you google.this you will also find various measurements just like you describe. Typically at the resonant frequency of the panel it is louder than the measurement of the direct radiation of the driver. At in incident frequency of the material it is equal. At other frequencies it is 10-20dbs below the direct radiation level, but often dropping at very high frequencies to the noise floor of the measurements.

I'll just say I've not experiences what you did with accellerometers. I've owned two and both have reliable results. I'm happy to share repeated measurents if you like.


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post #21 of 179 Old 04-14-2017, 01:26 AM
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Driver mounting screws...

Would I be correct in assuming that one would only screw into the inner baffle of a dual front baffle? ie: Drill larger holes in the outer baffle so that the screws/bolts don't contact it?

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post #22 of 179 Old 04-14-2017, 01:59 AM
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I'm not an AES member so if you can share any info from the paper that would be great. Ditto on the other studies you are referring too.

How did these other studies differentiate between the sound contribution of the driver versus the cabinet? Or the sound radiated back through the cone? Did any of them use a sound source inside the box?

I guess I was hoping to see/discuss actual measurements, not just personal experience.
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post #23 of 179 Old 04-14-2017, 02:23 AM
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My other issue with accelerometers is interpreting the results. What's audible, or at least an SPL level high enough that it *should* be audible (or not)? Seems to me the only way to do this is for the driver to only radiate into the box and measure the SPL with a microphone.

Last edited by augerpro; 04-14-2017 at 02:31 AM.
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post #24 of 179 Old 04-14-2017, 06:07 AM
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Originally Posted by Mpoes12 View Post
As the AES article I linked states, it reradiates the sound. You can explore If you want but they already have the results.

I'm not mixing up issues they are inter-related. The studies looking at the sources of panel resonances found they come from the driver directly transferring vibrations through the cabinet as well as sound exiting the back of the driver and into the cabinet space then exciting the molecules in the cabinet wall to again reradiate out of the cabinet. They will also reflect off the panels and through the driver cone. They are all from the same problems and would have the same I'll effect.

CLD has been tested by numerous manufacturers and researchers and found to be highly effective with speaker cabinets. A lot of this never seems to make it in the public in the right way, which is sad. Companies would rather keep it proprietary rather than put it out their for peer scrutiny and to advance the field.

The AES article found specifically that a very stiff box still reradiate sound.

I really don't know a reasonable way to test the deleterious effects of cabinet wall radiation. I don't have a laser inferterometer or a means to isolate the effect. I do have a contact mic/accelerometer for measuring the wall vibrations but I don't believe that's enough.


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I didnt mean you were mixing them up. I said the wording does. I mean, when I read your post I had a hard time separating the issues and understanding your point. Thats all. More my problem than yiu.

Ryan
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post #25 of 179 Old 04-14-2017, 07:58 AM - Thread Starter
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Originally Posted by augerpro View Post
I'm not an AES member so if you can share any info from the paper that would be great. Ditto on the other studies you are referring too.

How did these other studies differentiate between the sound contribution of the driver versus the cabinet? Or the sound radiated back through the cone? Did any of them use a sound source inside the box?

I guess I was hoping to see/discuss actual measurements, not just personal experience.
Let me see if I can get images from the various studies that are public. Via years of stalking industry folks and getting to become friends with various people who do this kind of research I've been able to hear about or even see a lot of the studies I'm eluding to. I even have a copy of one such paper that I know I cannot share because I was told it isn't going to be made public. The problem is that a lot of this research lead to proprietary techniques and the companies only file patents, not publish papers. In fast, Sony, of all companies, did one of the most exhaustive studies on CLD speaker enclosures, but the only public evidence I can find right now is their patent. The Patent makes lots of big claims, no data. Others with big research programs are B&W and KEF, but I can't find any public research papers. I have an email exchange with one of Kef's engineers from that project and some data he shared, but nothing useful, it was basically like marketing material.

Earl Geddes also extensively investigated this and put out papers on the topic, I'm really not sure which were for contracts and thus private and which were his own work and public. I'll ask him if I can share anything and provide that as well.

In the AES paper I linked to, they used a technique for measuring box radiation that doesn't rely on a microphone to pick up the sound. Instead they made use of a Laser Doppler Vibrometer and a Computational Boundary Element model to analyze the degree of sound radiating from the box. This approach relies on light and thus doesn't have to detangle the sound sources, it can see them. It is the state of the art right now for this kind of measurement and is currently considered the only true "right" way to do it. That approach allowed them to measure the amount of sound radiating out of the speaker enclosures under test. They were studying NHT 2.9 speakers with and without bracing. What they found was that the braced enclosure actually radiated more sound than the unbraced enclosure at certain frequencies, even though the bracing did reduce the wall vibrations.

https://www.comsol.com/paper/downloa...nchi_paper.pdf
Here is a similar publicaly available paper published by researchers from B&W. They make the point that the time domain behavior would be especially interesting because the high Q nature of some of these resonances (this was of their matrix enclosure with damping) have such long decays that even if down in level, would still add coloration to the sound. I think that is conjecture, they didn't test that, but at the same time, how would you. You would have to remove it and have someone blind AB to do that. Nearly impossible to do.

http://orbit.dtu.dk/files/51952164/Sound_Radiation.pdf
This is a paper on the BEM approach. Gives measurements as well. This author makes almost the exact same claim, but with the same problem, its based on theory rather than actual tests (again, because that isn't really a practical test).
"It should be remarked that even for cases in which the disturbance from the cabinet to the steady-state response may be too small to be readily discerned, it is still potentially audible" The citation for this claim is Stanley Lip****z article investigating the contribution of cabinet radiation to loudspeaker performance from 1991. I don't have this article and can't seem to gain access, so can speak to what it found. The article I've linked however has a lot of appendices that I think are what you are looking for in some ways. They give the output of the panels, they show that it is non-linear, and they provide graphs suggesting their audibility.

As for really testing the audibility of the detrimental effects, panel radiation is a kind of distortion and its non-linear in behavior, it actually changes as the speaker gets louder. In Appndix A.6 they specifically compare the speakers direct radiated sound via a laser to that of the cabinet via an accellerometer and compared the harmonic components of the distortion. They found that the harmonic components were greater in amplitude from the enclosure and showed that they were contributing to the total harmonic distortion of the speakers radiation and that its relative contribution would increase with greater level. In the same way that a driver distorts more as it gets louder and overloads, the cabinet will do something similar, but this test shows that the cabinets behavior IS NOT caused by the speaker, it is its own non-linear behavior, and thus adds to the non-linear behavior above and beyond that of the speaker by quite a bit.

I think what you really would like is a test that AB's reliably the contribution of this distortion. That really is a hard thing to do and so I think often what is done instead is that you first objectively investigate (without regard to its audibility) the actual problem. Once you have isolated it, you can than develop an experimental condition by which you add and remove only this isolated component to the signal. The time domain (impulse response) measurements showed a lot of ringing from the cabinets, so I think what you want to add and remove from the signal is this kind of distortion. Auralization would be my method of choice for this and while there have been auralization studies looking at these kinds of distortion, none that I've found specific to speaker enclosure induced distortion (which doesn't mean they don't exist, I just haven't found them yet). I would be shocked if Harman has not conducted these studies. I know they looked at auralization for cabinet diffraction, so why not cabinet re-radiation.

You asked if anyone has measured a captive speaker to see what is audible. People have done that, I've found even forum posts like that. One person posted their measurements with a tweeter mounted in reverse, since this would not radiate any sound out the back of the tweeter. It was pretty interesting, but the tweeter doesn't radiate low enough to be fully useful here, especially since a lot of the most non-linear behavior is far lower in frequency. I think building a speaker box that radiates into a larger soundproof speaker box would be a good way to investigate this ourselves. It won't "equal" the BEM method, but it was the standard old way as I understand it. I wasn't to sound critical of wanting to measure these effects, I just recognize how difficult it would be to seperate the contribution of cabinet re-radiation from the speakers direct radiation with our testing capabilities and then discern reliably the audible effects and to do so better than the existing published research. I think it is ok to look at this research and decide that the phenomena is real, the solutions are obvious, and it is just cheaper and easier to fix the potential problem rather than prove the problem. We can measure the fixes, we just can't measure if the fixes were audibly superior.
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post #26 of 179 Old 04-14-2017, 08:23 AM - Thread Starter
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Originally Posted by tuxedocivic View Post
I didnt mean you were mixing them up. I said the wording does. I mean, when I read your post I had a hard time separating the issues and understanding your point. Thats all. More my problem than yiu.
I probably didn't write it clearly. I'm famous for that. Many of my scientific articles have be well edited to ensure clarity. Sometimes they are nice and say something like "You are just really smart and understand things differently so its hard for people to understand what you are saying." That is code for, "what you wrote very well may be right but nobody can understand your incoherent manner of stringing words together."
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post #27 of 179 Old 04-14-2017, 08:26 AM - Thread Starter
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Originally Posted by Jekyll & Hyde View Post
Would I be correct in assuming that one would only screw into the inner baffle of a dual front baffle? ie: Drill larger holes in the outer baffle so that the screws/bolts don't contact it?

J&H
CLD approaches don't actually short out in this way, so no you don't need to do that. It wouldn't be the case that screwing through the entire baffle suddenly kills the benefit of the damping. However, for decoupling purposes alone, yeah it would potentially have some benefit to find a way to isolate the screw mounts from the baffle. Reducing speaker vibrations in this way is already dealing with the last 10% of sound quality. Isolating the screw mounts is now into the .5%. I wouldn't lose sleep over it, the CLD process in the first place is a big step in the right direction (or any heavy application of damping to a rigid massive box), this is just icing.

My earlier point about the way my cabinet was made was that I see value in mounting the speaker on to a CLD sandwich because that will actually help dissipate some of the energy that it transfers into the front baffle.
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post #28 of 179 Old 04-14-2017, 08:32 AM - Thread Starter
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Originally Posted by michael hurd View Post
This is correct, and any acoustic panel also perform better when spaced away from the solid surface behind it.
That is true, but not what I was talking about.

When I suggested lining the panels with something denser like fiberglass, cotton, or felt it was in addition to batt insulation like polyfill in the center. The reason is discussed in the VS white paper. When sound waves hit a barrier they are effectively hitting an abrupt change in acoustical impedance. This abrupt change causes some or all of the wave to reflect off the barrier with the higher acoustical impedance. If instead the wave progressively travels through a higher acoustical impedance that allows for a gradual change, then less of the wave will reflect back. We want none of the wave to reflect back, all of it to transfer all of its energy either into the damping material itself or into the damped enclosure. The cabinet filling is very important, but it can't absorb all of the sound inside the cabinet.

I would guess that gluing fiberglass, felt, cotton, etc. to the inner wall of the cabinet would provide additional damping as well and so has a good side benefit.
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post #29 of 179 Old 04-14-2017, 09:42 AM
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If cld is the biggest help why not just build a cabinet with two layers of 1/2" mdf and green glue? The main issue I see is that you need real glue at the edges so you might have to use a router to thin one layer in the middle leaving an edge that you can titebond. Green glue is not an adhesive and would likely be detrimental to finishing the cab. I do wonder though if you could lay up two layers of 1/2" as a whole sheet and then cut and join as normal. Veneer might be sufficient to prevent green glue coming out at the edges. Haven't played with green glue yet but have read of potential issues with it due to it staying squishy forever.

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post #30 of 179 Old 04-14-2017, 10:37 AM
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Interesting subject. I went through a lot of internet links and papers before I built some cabinets about a year ago. It was not easy to find something really substantial resulting in a holy grail step by step method for DIY.

I ended up with the decisions mentioned above. Double baffle with CLD (19+0,8+19 mm), internal bracing at non equal intervals, partial CLD for the side walls. It seemed to me, isolating and damping vibrations from the driver basket and the magnet to the cabinet was important. For fastening the driver basket to the baffle I used oversized holes (Ø 6,5 mm) in the baffle and M6 machine screws together with viton o-rings as sealing and damping elements. A larger viton o-ring to seal around the rim of the basket. (For common elastomers, viton has the best inner damping and a very low rebound at room temperature and above and also for the frequencies of interest, 100 Hz and upwards.) Cross section were Ø3,53 mm with about 15% compression, as suggested in a NASA report I came across.

In the link below they cut away the driver cones, so there is no internal pressure in the cabinet. Noice from the cabinet is solely due to the magnets reactionary forces exiting resonances in the cabinet walls. A neat way to get rid of one variable, expensive though for a DIY investigations. Movements were measured with laser. https://www.comsol.com/paper/downloa...nchi_paper.pdf

Last edited by Adhoc1; 04-14-2017 at 10:41 AM.
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