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Sounds Of Science - SOFSCI - Sound absorption panels behind speakers.

The laws of sound physics (acoustics) or How does it work?

The human ear can generally hear sounds with frequencies between 20 Hz and 20 kHz (the so called “audio range”). 20 Hz is a very low frequency while 20 kHz is a very high frequency. Examples of low frequencies in our surroundings are the bass in music or the low humming that a fan generates. Examples of high frequencies are the treble in music or the high notes of a small child. Both speech and music are a mixture of high to low frequencies.

When sound is made, may it come from human speech or a HiFi speaker, the room is filled with it. And if it is not absorbed then it bounces around until drained of energy. In a room with hard walls this bouncing can go on for quite some time. These echoes, or reverberations, greatly impacts our daily lives – may it be during conversations, HiFi listening or working in an open office environment. 
  


Imagine the following two examples:


Example 1: You are talking to a friend in a stairwell. The stairwell is made out of concrete and has a lot of parallel walls where the sound bounces back and forth, creating a lot of echoes. You will find that both of you need to raise your voices considerably in order to make yourselves understood. And, although loud but understandable, the experience is not very pleasant and nothing you would like to endure for a longer period.
 

Example 2: You are talking to a friend outside, on a lawn. No walls or anything around you that can create any echoes or reverberations. You will find that you can hear your friend perfectly even when he/she is talking quite softly and that what you hear sounds very pleasant.

Example “2” is naturally the one to strive for, is it not? What if every environment we are in could be like this? According to many scientists our lives would be a lot healthier and less stressful if they were. And the way to get there is to either constantly be on that lawn or to absorb the echoes at home or at the office in order to create a similar acoustic environment. This can be done through the use of sound absorbers.
All materials have some sound absorbing properties. Incident sound energy which is not absorbed must be reflected, transmitted or dissipated. A material’s sound absorbing properties can be described as a sound absorption coefficient in a particular frequency range. Materials like polyester fiber and glass and mineral wool are known to be good absorbers. A thick concrete wall is an example of a very poor sound absorber.

What one does not want to do is to remove only one range of frequencies. Very many solutions on the market today do just that: They are only effective on high frequencies. This means that they remove part of the echo (reverberation) in the room but leaves the rest bouncing back and forth: Creating a very uneven acoustic environment. In an open office environment, for example, this leads you to constantly hear a low murmuring in the air, since the low frequencies are still bouncing back and forth – creating a bad environment for you and your co-workers.  
 
High frequencies are quite easy to remove and they are removed with a curtain or a thin piece of felt on the wall. Thus many products are made this way, while claiming to be sound absorbing. Which, in a way they are, but they only absorb part of it – leaving the rest unabsorbed. In the best of scenarios they do not change much at all while in the worst of scenarios they actually make things worse since they create a very uneven acoustic environment. Compare it with removing all the salt from your food: You still have food but what is left is not very tasty anymore.
Another thing we need to know about acoustics is that the absorption of low frequency sound increases with the thickness of the absorber. The absorption will be more effective where the particle velocity is high. Close to the boundary of the room the particle velocity will be zero and so this is not an ideal location for sound absorption. The absorption furthest away from the backing surface will be the most effective and this is why thick layers absorb at lower frequencies. An alternative to having super thick sound absorbers is to place the sound absorbing material where it does the most good: Away from the wall. Tests have shown that having an air column close to the wall is almost as good as having sound absorbing material there, when it comes to absorbing low frequencies. The best thing here is of course to have both a lot of sound absorbing material and an air column close to the wall. Most products on the market have neither but the products from Sounds Of Science naturally has both. 
  
 
For an animation regarding sound and sound absorption, please click here..