Absorbers used for sound isolation???

[chickenfingers]

Hi.
This one is a bit long, so thanks for your patience.

Say we have a certain absorbing surface with a 0.6 absorption coefficient for a frequency f.
After a sound wave hits this surface, I understand that 60% of its energy is dissipated by the absorber, which is quite a lot of energy being taken away.

Then, why are these objects not good for sound isolation if that much energy is taken away from the sound wave? What happens with the energy after the sound wave hits the absorber? does it turn into heat within the, say foam, then hits the wall and keeps on dissipating on its way back?

In the case of bass frequencies (which are everybody´s headache,)I could theoretically mount a bass trap covering the complete surface of very lightweight wall. This bass trap has very high absorption coefficients, lets say above 0.6 for all low frequencies. However this will do very little in stopping the airborne sound going across my lightweight partition.

Can anybody explain me what happens with the sound´s energy after it hits an absorbing surface, and why are they not good sound isolation objects if they absorb so much of the sound wave´s energy.

Many thanks!!

 

[chickenfingers]

Hi, Me again.
Change the 0.6 of the original post to say 0.9, as 0.6 doesn´t really take away much energy from the incident wave.

Cheers

 

[chickenfingers]

Absorbers for sound isolation

Hello

I will now reprashe all the above...

Imagine a room where sound can only leak out through one wall, let´s ignore rooms modes and the reverberant field or any room effects. The inner surface of this wall is fully covered with very high absorption material at certain frequencies, say 0.97 (my previous example of 0.6 was DUMB). If a sound at those frequencies is reproduced, I assume reflections off that surface will be perceived as weak (say standing 1m from the surface) and will be around XdB (I reckon at least 10dB) below the level of the incident wave. Meaning that the absorber has indeed, absorbed/dissipated this energy.
So, will the partition beyond the absorber see the attenuated sound wave, without the already absorbed energy?
My reasoning comes from: if the absorber has already dissipated energy amounting to XdB, then the incident wave on the partition should have the original energy minus the dissipated energy.

Thanks for your PATIENCE!!!
And sorry if you find the english a bit sloppy but it´s not my first language.

cheers
bye

Sergio

 

[PeterWhite]

Hello


So, will the partition beyond the absorber see the attenuated sound wave, without the already absorbed energy?


Sergio

Yes. Whatever percentage of the wave's energy is absorbed, that percentage doesn't pass through the absorber, but instead heats up the absorber (No, you can't fry an egg on it.) ;-) and is then dissipated as heat, rather than as sound waves. So you're converting sound waves into infra-red waves, I suppose. ;-)

I wonder how long you'd have to play Pink Floyd at realistic volume levels to see the absorber glow with an infra-red sensor? ;-)

 

[WmAx]

Sound absorption function, in regards to reducing/preventing reverb/reflections, is different from sound transmission blockage. The backing(behind the absorber) is playing a critical part of changing the velocity of the wave so that it can be maximally absorbed. If you, for example, put an absorber in the middle of a room, it won't absorb nearly as low as it can, near a boundary surface.

If you wanted to physically block sound from a path; the the absorber needs it's own solid back and the absorber needs to be physically decoupled from main wall/structure to prevent vibration transmission, and the absorber needs to be significantly larger than the wavelength being absorbed(so that the wave does not simply wrap around the absorber). However, even this is not so simple, as sound is usually radiated over a very wide area and in different directions - and the lower the frequency - the more difficult it will be to deal with. To actually produce substantial isolation, the entire thing needs to be air-tight/enclosed and de-coupled from the main environmental structure, less vibrations transmit throughout and act as a breach. Proper industrial sound isolation rooms have every possible air leak sealed, and the entire room is suspended from the main structure of the building on a big shock absorber. Even in home audio applications, where full isolation/suspension is not possible, all of the walls/ceiling/etc. are hung from special shock absorber brackets. The easiest way to do some decent isolation at home is to build a full surrounding constrained layer wall/ceiling assembly - using drywall, a layer of Green Glue and then another layer of drywall. If you do this in a room with a concrete floor, it will work superbly, otherwise, you also have to deal with the floor, which further complicates things. You still have to make sure no air leaks occur. Then you have to make special accommodation for heating/cooling. Special acoustic isolation baffle systems are typically used to connect to the room to prevent most sound from breaching the HVAC to the rest of the house.

-Chris

 

[Glenn Kuras]

I wonder how long you'd have to play Pink Floyd at realistic volume levels to see the absorber glow with an infra-red sensor? ;-)

Well if you smoke enough of the wacky stuff you would see it much faster!! Or should I say, THINK you would see it much faster.


Glenn