Early reflections must be suppressed by about 15 dB before they disturb stereo imaging.

Evidence

In order to achieve a reflection-free zone, it is necessary to suppress early reflections, but by how much? Figure 7.24 shows a graph of the average level that an early reflection has to be at in order to disturb the direction of a stereo image. From this, we can see that the level of the reflections must be less than about 15 dB to be subjectively inaudible. Allowing for some reduction due to the inverse square law, this implies that there must be about 10 dB, or α = 0.9, of absorption on the surfaces contributing to the first reflections. … However, it is important to realize that the overall acoustic must still be good and comfortable, that it is not anechoic and that, due to the wavelength range of audible sound, this technique is only applicable at mid to high frequencies, where small patches of treatment are significant with respect to the wavelength. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2294

Categorization

• DEFINE: Introduces the reflection-free zone as a quantitative acoustic target tied to stereo image stability.
• QUESTION: Is 15 dB really the threshold, or is it material- and listener-dependent? Worth chasing Toole 1990.