Page 146 - Master Handbook of Acoustics

FIGURE 6-11 Results of investigations into the effect of a simulated lateral reflection in an anechoic
environment and with speech as the test signal. Curve A is the absolute threshold of detection of the
reflection. Curve B is the image-shift threshold. Curve C indicates the points at which the reflection
is heard as a discrete echo. (Composite of results: Curves A and B from Olive and Toole, Curve C
from Meyer and Lochner.)

In Fig. 6-11, curve A is the absolute threshold of audibility of the echo. This means that at any

particular delay, the reflection is not heard for reflection levels below this contour. Note that for the
first 20 msec, this threshold is essentially constant. At greater delays, progressively lower reflection
levels are required for a just-audible reflection. For small room, delays in the 0- to 20-msec range
are of greatest significance. In this range the reflection audibility threshold varies little with delay.