Porous absorbers only work where sound wave velocity is nonzero

Evidence

Because porous absorbers interact with the velocity component of the sound wave, they are affected by the space between them and the wall and their thickness. This is due to the fact that at the exposed surface of a hard surface, such as a wall, the velocity component is zero, whereas at a quarter of a wavelength away from the wall, the velocity component will be at a maximum, as shown in Chapter 1, and so a porous material will absorb more strongly at frequencies whose quarter wavelength is less than either the spacing of the material from the wall or the thickness of the material if it is bonded directly to the surface. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2047

Categorization

• DEFINE: Explains why absorber placement relative to walls matters, which is counterintuitive to the layperson.
• QUESTION: Could verify the practical rule of thumb for optimum absorber-to-wall spacing distances in real rooms.