A resistive absorber behind the listener solves reflective-wall problems where diffusers fail

Evidence

If resistive absorption is to be useful, it must be at a distance from the room boundary—the portion of it that is adjacent to the boundary is doing nothing, and the effectiveness increases with distance from the boundary, or the thickness of the material, up to a maximum at the 1/4-wavelength distance where the particle velocity is maximum (Figure 4.21d). Look at Figure 4.19 and divide those wavelengths by four to get an idea of why resistive absorbers are not attractive solutions at low frequencies—even at a mid-bass frequency of 100 Hz the quarter-wavelength is 2.8 ft (0.86 m), which seriously intrudes on expensive real estate. — , Toole, Floyd E - Sound reproduction-…, 481 (#contradicts, via linked claim)

Figure 7.25 shows one method of working out where absorption should be placed in a room to control early reflections. By imagining the relevant walls to be mirrors, it is possible to create “image rooms” that show the direction of the early reflections. By defining a reflection-free space around the listening position and by drawing “rays” from the image speaker sources, one can see which portions of the wall need to be made absorbent, as shown in Figure 7.26. This is very straightforward for rectangular rooms but a little more complicated for rooms with angled walls. Nevertheless, this technique can still be used. It is applicable for both stereo and surround systems, the only real difference being the number of sources. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2299 (#expands, via linked claim)

Categorization

• XREF: connects to: Engineered diffusers attenuate the specular reflection toward the listener by up to 20 dB; Treating room walls as mirrors locates where early reflections need absorbing. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2299; Resistive absorbers are impractical at low frequencies because quarter-wavelengths are huge — , Toole, Floyd E - Sound reproduction-…, 481 (+1 more)

Broader Connections

• Resistive absorbers are impractical at low frequencies because quarter-wavelengths are huge #contradicts (0.92) — , Toole, Floyd E - Sound reproduction-…, 481
• Engineered diffusers attenuate the specular reflection toward the listener by up to 20 dB #qualifies (0.82)
• Treating room walls as mirrors locates where early reflections need absorbing. #expands (0.78) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2299
• Hemi-cylinder diffusers lose low-frequency diffusion when placed contiguously rather than randomly spaced #complicates (0.72)