Sound absoption conficts with Sound transmission loss? 5

[dzhoucam]

What is the relationship between Absorption coefficient and Sound transmission loss? I read a paper which says that the absorption ability confict with the sound transmission loss for the same material. Is it right? Why?

 

[Hatch]

Absorption coeficient is a measure of the reduction in sound when it reflects off a material. Think light and fluffy (fiberglass insulation) for a high absorption coefficient.

Sound transmission loss is a measure of the ability of the material to block the sound travelling through it. Think heavy and dense (concrete block wall).

As you can see you can't have high absorption with high mass. For both properties a composite can be used such as a rigid fiberglass panel on a concrete block wall. Of course there are a million variations of composites. Many are used in the automotive industry.

C. Hugh

 

[GregLocock]

here is another way of looking at it.

In order for a system to absorb sound it has to transfer energy out of the air into the system, and then convert the energy to heat. In order for this to be effective then the impedance of the system must be similar to, ideally the same as, the air, in order to get maximum transfer of energy.

However, for a barrier to work it is best not to absorb any energy at all from the air, so it is a good idea to have an impedance that is very different to that of the air.

As hatch says, in reality you use a mixture of approaches for each path, partly because the impedance of systems varies a lot with frequency.

Cheers

Greg Locock

 

[dzhoucam]

Many thanks to Hatch and Greg!

Greg makes me clear about the energy transfer in the process of sound absorption. For a barrier, however, I wonder what is the energy transfer process. My idea about the process is that when the incident sound arrives at the barrier, part of the energy is reflected, part of the energy is restored in the barrier (right? if so, is it strain energy?) and the following is transmitted through the barrier.

I am studying the acoustics attenuation of a very thin sandwich plate( only 1.2mm thick including the face and core). The core is made of fibres. At first, I did a lot of theoretical work for sound transmission loss of the mamterials. But it seems that I should focus on the sound absorption according to both of your answers to my question since my material is too thin and fibrous. Do you think it will have a good sound transmission loss according to your experiences?

Thanks again.

Daowu

 

[MikeyP]

If the fibre material is the "filling in the sandwich" then it will have very little absorption capability as it is not directly in contact with the incident sound wave.

It is also unlikely to provide good (high) transmission loss. There are 2 mechanisms involved in sound transmission through a partition: Non-resonant and resonant transmission

In non-resonant transmission the incident sound wave impinges on the partition and forces it to move. As it does so it pushes the air on the other side of the partition causing the transmitted sound. Note the use of the word "force". The partition is NOT resonating and does not store potential energy, it is simply moving because it is being pushed. This is (usually) the dominant transmission mechanism at low frequencies. Because there is no potential energy, the only attenuation is provided by the inertia of the partition. This means that (to a reasonable approximation) non-resonant transmission loss is affected ONLY by the MASS of the partition (the so called "mass law&quot. More mass = more attenuation. A brick wall blocks more sound than a sheet of paper. The filling in your sandwich may add damping to the panel, but damping only works on the potential energy in the panel of which there is none.

At higher frequencies, the resonant transmission loss may start to dominate over the non-resonant. Resonant transmission occurs when in incoming sound wave strikes the partition in such a way that the pressure distribution matches the modeshape of one of the panel's modes of vibration at the same frequency as that mode of vibration. Some of the sound wave energy is now transferred to the panel in the form of a modal vibrational energy (a stored potential energy). On the other side of the panel, exactly the reverse happens. The resonating panel causes the air to move and transfers the stored modal enery to the transmitted sound wave. Of course this radiation will also occur on the incident side of the panel as well. Because the panel now contains potential energy, the damping of the fibrous filling can now have an effect. It reduces the modal energy in the resonating partition meaning that there is less energy left to be radiated. Hence an increase in transmission loss.

Would your sandwich panel give improved transmission loss compared to an equivalent solid panel of the same thickness made from the skin material? Probably not. At low frequencies it would probably have a worse performance. At high frequencies it may just improve, but as well as adding damping you are also altering the panel's stiffness. Depending on whether the filling material is bonded to the skin material or not and the properties of the filling, the stiffness could be higher or lower.

Have a look at the book "Noise and vibration control engineering" by L Beranek. It may give you some clues as to how to proceed with your problem.

M

 

[izax1]

Interesting discussion.

It ties up well with my thread "How is sound generated"

Greg, what do you mean by

"In order for this to be effective then the impedance of the system must be similar to, ideally the same as, the air"

Sorry for my ignorance, but what is meant by impedance here, and how do you find if there is a match??

Bernt

 

[GregLocock]

I suggest you go and find an electrical textbook, and look at power matching or impedance matching theory, for a decent explanation. This may help:

http://www.bluejeanscable.com/articles/impedance.htm

Here's the crude version for the acoustics world: impedance =p/v in the system for small waves, for most (all?) solids it is a frequency dependent complex number, dependent on the mode of transmission of the wave, ie longitudinal or bending. You can measure it in an impedance tube (Beranek describes this well, I expect Tom Irvine's website has something on it

http://www.vibration.com).

The impedance of air is 315 rayls, in SI units. At this point I will add that I have never bothered to work any of this out for air/surface systems, it is the idea that is important. I have worked it out for mass spring damper systems, where it is somewhat useful, but very confusing.

If the impedance of the two systems matches then they will be able to exchange energy optimally. If the impedance differs then the power flow into the receiving system will be reduced - the loss in dB is something like 10*log(modulus(Zr/(Zr+Zt)^2)) but I don't think that is right.

This subject is closely allied with sound intensity, it is probably worth reading up on that.




Cheers

Greg Locock

 

[Rob45]

dzh:
Two points here: As Greg suggested, when you've got a really good barrier, there will be *no* sound energy transfer to the barrier - it will all be reflected.

Second, your idea of a thin sandwich with fiber in between to metal sheets will be ineffective at absorbing sound because there is no opportunity for the sound to get to the fibers to be dissipated - the covering sheets prevent airflow, and without airflow you won't get any appreciable amount of sound absorption.
In simplest terms, cellular or fibrous materials dissipate sound, with greater thickness absorbing lower frequencies; while mass per unit area is the key element of sound barriers, a heavy limp material being the best barrier.