Diffusers manipulate only part of the spectrum and can act as absorbers when fabric-covered

Evidence

If, for example, the diffuser has a lower limit of 1 kHz, it means that the sound sent off in the direction of the main (specular) reflection arrow will have a lot of energy up to the diffusion limit, and then the high frequencies in the directional reflection will be attenuated. In other directions, listeners will hear the reverse: high frequencies with insufficient spectral energy in the lower frequencies. It all depends on the specific circumstances, to be sure, but one cannot ignore the fact that only a portion of the spectrum is being manipulated by these devices. As the experts, Cox and D’Antonio (2009), caution us there is more to the acoustical performance of these devices than meets the eye. They become quite effective absorbers when covered with fabric, as is often done for visual reasons. When sellers offer only dimensions measurable with a tape measure as “specifications”, one needs to be suspicious. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1034

Categorization

• SEED: Potential article: ‘the acoustical con’ — how visual appeal and tape-measure specs mask devices that degrade sound, relevant to home-theater DIY culture.
• XREF: Connects to the caveat-emptor theme about specsmanship that runs through audio and hi-fi literature generally.