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Old 25th August 2004, 08:25 AM   #1
polsol is offline polsol  South Africa
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Default Acoustically absorbant materials for cabinets

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)?

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Old 25th August 2004, 02:07 PM   #2
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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.

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Old 25th August 2004, 04:02 PM   #3
markp is offline markp  United States
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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.
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Old 26th August 2004, 06:38 AM   #4
polsol is offline polsol  South Africa
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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.

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Old 26th August 2004, 03:33 PM   #5
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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.
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Old 26th August 2004, 11:16 PM   #6
RHosch is offline RHosch  United States
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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.
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Old 27th August 2004, 12:37 AM   #7
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Bill's suggestion is spot on, but any additional crap you
care to coat it with won't hurt, either.
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Old 27th August 2004, 01:06 AM   #8
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Bill and Nelson are spot on. There is no mysterious way to dampen a poorly built cabinet.

Mark
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Old 27th August 2004, 01:03 PM   #9
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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
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Old 27th August 2004, 02:04 PM   #10
polsol is offline polsol  South Africa
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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....

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