Porous sound absorbers work best when positioned at a quarter wavelength from a wall

Evidence

Another form of energy loss occurs when the sound wave hits or travels through some porous material. In this case there is a very large surface area of interaction in the material due to all the fibers and holes. There are frictional losses at the surface of any material due to the interaction of the velocity component of the sound wave with the surface. A larger surface area will have a higher loss, which is why porous materials such as cloth or rock wool absorb sound waves strongly. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 344 (#expands, via linked claim)

An alternative means of achieving wideband absorption is to use a large depth of porous absorber, for example one meter, and this can ­provide ­effective absorption with a flat frequency response, but at the cost of ­considerable depth. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2110 (#qualifies, via linked claim)

Categorization

• XREF: connects to: Resistive absorbers are impractical at low frequencies because quarter-wavelengths are huge — , Toole, Floyd E - Sound reproduction-…, 481; Deep porous absorbers give flat wideband absorption at the cost of space — David Howard, Jamie Angus, coustics and Psychoacoustics, 2110; Higher frequencies produce higher particle velocities, improving porous absorber efficiency (+2 more)

Broader Connections

• Higher frequencies produce higher particle velocities, improving porous absorber efficiency #expands (0.90)
• Deep porous absorbers give flat wideband absorption at the cost of space #qualifies (0.85) — David Howard, Jamie Angus, coustics and Psychoacoustics, 2110
• Porous materials absorb sound because their large surface area creates frictional losses. #expands (0.85) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 344
• Effective sound diffusion demands surface depths proportional to the target wavelength #expands (0.80)