Acoustically absorbant materials for cabinets

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Does anyone know of any information that pertains to the absorption properties of different materials? In other words the absorption characteristics of materials at different frequencies?
I am looking to build some loudspeakers out of materials that are not "conventional", more specifically out of a liquid "Corian" type which will be rigid (and light) with an inner coating of thermoset material that will act as a sound dampener.
If this type of information is not readily available is there any rule of thumb as to the absorption characteritics of materials (such as rubbery materials are more absorptive than rigid materials etc)?

Regards,
 
The inner layer you propose is pretty much a waste of time and money. Such compounds are very useful in car doors or flimsily built cabinets that cannot be properly braced, but in a very rigid cabinet such as you would get with Corian you probably need not bother. These materials are good for absorbing low frequency vibrations, but if you build the cabinet right in the first place they are neither required nor desired on the cabinet interior. Such materials can be quite useful when used as viscous damping layers sandwiched between rigid structural layers.

What you must do is make the interior completely non-reflective of frequencies short enough that they can reflect back to the cone at significant angles of phase, ie, the midrange. That is best accomplished with an inch of high density rigid fiberglas batting, but if you can't find any or if it's not adequately user friendly use an inch of high-density polyester furniture upholstery batting.

www.billfitzmaurice.com
 
I respectfully disagree with Bill. The corian has a high resonant frequency by itself and the soft goop type of material will damp out the resonance at those higher frequencies making a very dead cabinet. I've made such cabinets but have also added absorbant batting to stop the reflected waves as Bill mentioned.
 
Thanks Bill and Mark.

What worries me about using just a rigid cabinet material is the internal reflections and their impinging on the cone material.

My idea was (is?) to use a rigid outer skeleton and say a 1" thick inner sound absorbant layer. The sound absorbant layer would be composed of a highly flexible material containing particulate matter. The theory behind this is that any sound waves that enter the flexible material will be dispersed within it by reflection from the particles thus absorbing and disipating the energy.

I stress that this is just a theory - which is why I needed the data on the acoustical response of various materials!

I take it that more flexible materials are better absorbents?

BTW, I am a polymer chemist by profession and formulating the products and moulding this type of enclosure would be up my alley.

Regards
 
Since you are molding the cab, interior shape is what you would want to use to diffuse and or dissipate the reflections because different materials absorb over relatively narrow frequency ranges. That inner coating, like Mark said, is a great idea for damping the resonance of the corian.
 
You probably need not worry about damping the resonance of the Corian, so long as it is cast sufficiently stiff. In enclosures, we are concerned not only with the frequency of resonance, but also the amplitude of resonance given a specified energy input. Damping helps lower the amplitude of resonance, but so does adding more stiffness for a given panel mode (i.e., adding more stiffness and mass in combination). Thick Corian will have both mass and stiffness in spades, relative to something like MDF, and thus it is unlikely that damping would be of (relatively) much benefit.

Also, the particulate thermoset will likely be reflective at high frequencies, though I don't have any data on hand to prove that. Bill's suggestion to use a porous broadband absorber like rigid fiberglass is spot on.
 
I think that polypropylene has very good internal damping properties, so if Corian's anything like that it should an ok material to use.

One thing to remember is that with conventional magnet-based speakers - due to the laws of physics - the bulk of the mass vibrates with reversed polarity to the cone. This means that if the speaker is solidly connected to the baffle, transverse waves will ripple outward along the box material. Making the baffle out of a stiffer material will increase the velocity of those vibrations, and will thus increase the lowest frequency of box/panel resonances. However, it may or may not decrease their amplitude. That depends on how absorptive the cabinet material is.

A suitable material would have to somehow absorb a lot of energy every time that it bends, rather than store the energy. Therefore I'd look for a material that has a very low yield strength, whilst being stiff enough and hard enough to be usable. Alternatively, use a block of granite/concrete etc for a box of such high mass that those transverse waves won't ever cause any problems anyway.

As for stopping internal air vibrations from affecting the cone (and thus escaping from inside the box), there are several methods...
1) Increase the internal volume of the box relative to the surface area of the cone.
2) Increase the rate at which acoustic energy is absorbed in the box by decreasing the thermal resistance between the air and its surroundings (adding stuffing materials, etc).
3) Optimize the internal design of the box to maximize the amount of time before reflections return to the cone (labirynth, snail shell, etc).
4) Choose a speaker with a very high mass per cone area, so that it's relatively unaffected by sound inside the box. Or, back in the real world: use a very stiff cone so it maintains the same moving mass even at high frequencies. Aluminium cones are decades better than paper in this respect.
5) Choose a speaker with a very low Qes value, meaning that the amplifier will very effectively dampen unwanted vibrations, as long as output resistance is close to zero = high damping factor. In practice, it's commonly observed that there's not much difference due to different damping factors, unless it's extremely low (by today's standards) which causes subjective improvement/worsening of sound.
6) Use a specially tuned and EQ'd amplifier with negative output impedance to effectively make the speaker's Qes even lower.
7) Use some acoustic feedback or microphone calibration system to cancel out existing resonances, to stop them from emanating from the speaker's cone. This relies on stiff speaker cones and a sturdy box for good performance, otherwise the off-axis sound will be badly corrupted.

For the perfect loudspeaker, I'd use as many of the above methods as possible. Achieving truly great sound is not that easy or cheap as you can see.

CM
 
Thanks for the detailed info CeramicMan.

However, Corian is nothing really like PolyProp although both are polymers. Corian is subjectively a filled version of PMM (polymethyl methacrylate) - Perspex or Plexiglass as you might now them.
The filler significantly increases the hardness of the polymer so the end result is a rigid, inflexible material with a specific gravity (density) of around say 2 (cf 1 for water). Thus I would expect it to be highly reflective of sound waves and the down side there-of.

How would a "prime number" cabinet work out i.e. 3, 5 or 7 sided? The back of the cabinet facing the drive units should be an angle in each and thus not a 180 degree reflective surface.
Standing waves could be a problem I suspect....

Regards
 
Corian is almost as reflective as glass which is why it is important to put some knid of goop on the inside surfaces to absorb the reflected waves. Henry's roofing material is great and then put 1" fiberglass on it using the Henry's as a glue.
I'm toying with the idea of putting a length of 8" diameter sonotube cut in half length-wise behind the driver to scatter the backwave and break up the cabinet resonance modes.
 
polsol said:
How would a "prime number" cabinet work out i.e. 3, 5 or 7 sided? The back of the cabinet facing the drive units should be an angle in each and thus not a 180 degree reflective surface.
Standing waves could be a problem I suspect....

From what I've heard, a ratio of about 1 to 1.5 to 2.6 gives the best distribution of modes for a regular (square-ish) box.
 
Absorption and damping

Hi,

Just a general commentary:

Absoprtion and damping are not the same properties. If you put a sticky goop stuff on the inside walls of the cabinet, that's damping. It will reduce vibrations of the wall but not absorb sound.

To absorb sound in the cabinet, the best things are fibrous materials like felt or fiberglass or foam.

I guess the best is a bit of both. Auto adhesive felt sheets can dampen vibrations and absorb sound. Or put the goop stuff you talk about and place absobant material above that.

F
 
Re: Absorption and damping

gary f said:
Hi,

Just a general commentary:

Absoprtion and damping are not the same properties. If you put a sticky goop stuff on the inside walls of the cabinet, that's damping. It will reduce vibrations of the wall but not absorb sound.

To absorb sound in the cabinet, the best things are fibrous materials like felt or fiberglass or foam.

I guess the best is a bit of both. Auto adhesive felt sheets can dampen vibrations and absorb sound. Or put the goop stuff you talk about and place absobant material above that.

F
Gee, maybe thats why I wrote what I did. Also, I have found the goop to be less reflective than one might think, in addition to the damping properties.
 
MarkMck pointed out recently that egg shaped enclosures in fact have terrible issues with standing waves,

Standing waves only exist when interior cabinet dimensions are at least a quarter wavelength and there are flat walls to support reflections. In typical cabinet sizes used today the frequencies at which standing waves can occur are so high and the wavelengths so short that they are easily controlled by an inch of fiberglas or polyester lining of the cabinet walls. Wave concentrations caused by parabolic enclosure dimensions are a different story entirely and have nothing to do with standing waves. Purely radial surfaces are extremely effective in eliminating internal reflection concerns.


www.billfitzmaurice.com
 
BillFitzmaurice said:


Standing waves only exist when interior cabinet dimensions are at least a quarter wavelength and there are flat walls to support reflections. In typical cabinet sizes used today the frequencies at which standing waves can occur are so high and the wavelengths so short that they are easily controlled by an inch of fiberglas or polyester lining of the cabinet walls. Wave concentrations caused by parabolic enclosure dimensions are a different story entirely and have nothing to do with standing waves. Purely radial surfaces are extremely effective in eliminating internal reflection concerns.
www.billfitzmaurice.com

I have too say i understand and agree with most of what Bill is talking about but for me personally the inards of a cabnet is more important than the outter shell, bracing is not always the solution, i have use cork, lead wadding, tar and anything else you care to mention inside subwoofer boxes to give them a more sympathetic sound, boxiness is something that subwoofers can suffer from and damping or absorbtion always helps reduce the sound quality, not standing waves Bill so dont bash me here ok :)

Like your new website Bill, its about time too.
 
BillFitzmaurice said:


Standing waves only exist when interior cabinet dimensions are at least a quarter wavelength and there are flat walls to support reflections. In typical cabinet sizes used today the frequencies at which standing waves can occur are so high and the wavelengths so short that they are easily controlled by an inch of fiberglas or polyester lining of the cabinet walls. Wave concentrations caused by parabolic enclosure dimensions are a different story entirely and have nothing to do with standing waves. Purely radial surfaces are extremely effective in eliminating internal reflection concerns.
I have to disagree with some of that. It's almost like saying that a mirror won't reflect light if it's curved, which is simply not true. Don't forget that sound doesn't have the same "line-of-sight" properties as visible light, so it can readily curve around corners in varying degrees. Normally a wide range of standing waves such as 3000Hz down to 150Hz will press against the speaker's cone to some extent regardless of the "architectural style" inside the box.

To effectively absorb standing waves (by my definition they resonate between the air in the box and must eventually return to the speaker's diaphragm), the damping material has to have a depth of at least several wavelengths. Afterall, reflections are caused by a sudden change between the mechanical properties of air and any solid surface that's completely unlike air. There are various electrical analogies where reflections are created because impedances aren't properly matched, etc. To minimize sound reflections, "go-between" materials should be used that gradually change between the properties of air and a solid wall of the box. Another analogy is the special coating on glass surfaces to reduce visual reflections. That simply has a refractive index somewhere between that of glass and air.

CM
 
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