In visual perception, if one sees a larger object far and a smaller object near, their relation can be recognized from context. Aural perception includes analogous effects. The magnitude of a sound is source intensity, having a physical, objective value. But the perceived source level is loudness, a mental perception. If small source intensity is close to a listener, it is recognized as such through reverberation, which serves as the context as in visual perception. Since reverberation is the environmental context, it must be constant across distance, as suggested by Figure 3 depending only on source intensity [KMW90].
The following are equations for source and reverberation
as a function of distance and loudness in the PSFC,
using loudness and distance look-up tables,
created through psychoacoustic calibrations:
The is input to the PSFC to control final source level, VOLUME. is the input to PSFC to control the final reverb level, LIVENESS. (Both and are maxima in dB.) The and are look-up tables, shown in Figure 4. The depends on the distance, so the adjustment must include both distance and loudness. Note that does not depend on the distance.