Quote:
Originally Posted by jacko  In theory to absorb a wave we need a porous absorber that is 1/4 wave length deep. Could someone point me to some documents where it is explained? |
The 1/4 wavelength is valid for very thin porous absorbers. Unfortunately there is no web page that I am aware that explains the loss mechanism well. Fortunately it is like most of acoustics in that it is not intuitive.
Trying to put it in succinct terms, there are three main factors involved.
1. As sound strikes a porous absorber at greater angles than normal incident, the sound wave goes goes through a longer and longer path, increasing low end absorption.
2. Sound in a porous absorber travels at ~70% of the speed in open air, increasing low end absorption.
3. In a porous abosrber the insulation conducts heat away from the compressed zones, causing #2 above, and a phase incoherence at the material - open air boundary, again increasing low end absorption.
For the physicists looking for an explanation of the speed of sound change, normal sound waves are adiabatic, with localized zones of increased and decreased temperature. In porous absorbers the material conducts heat from the high to low temperature areas. Taking a variation on the gas law, adiabatic sound and isothermal sound speed are related by the square root of the ratio of constant presure specific heat to constant volume specific heat of air.
The net result, as nosebleedaudio wrote, is that porous absorbers are effective down to around 5% of the wavelength of the sound in random incidence absorption, and 10% in normal incidence.
Phillip Newell has a good explanation in
Recording Studio Design.
Andre