Page 100 - The Master Handbook Of Acoustics

75
THE EAR AND THE PERCEPTION OF SOUND

15

Echo level direct - dB 10

5

0
0 10 20 30 40 50
Echo delay - milliseconds
FIGURE 3-19
The precedence effect (Haas effect) in the human auditory system. In the 5 to 35 msec
region, the echo level has to be about 10 dB higher than the direct sound to be dis-
cernible as an echo. In this region, reflected components arriving from many directions
are gathered by the ear. The resulting sound seems louder, because of the reflections
and appears to come from the direct source. For delays 50 to 100 msec and longer
21
reflections are perceived as discrete echoes. (After Haas. )

Perception of Reflected Sound

In the preceding section, “reflected” sound was considered in a rather
limited way. A more general approach is taken in this section. It is inter-
esting that the loudspeaker arrangement Haas used was also used by
dozens of other researchers and that this is basically the familiar stereo
setup; two separated loudspeakers with the observer (listener) located
symmetrically between the two loudspeakers. The sound from one
loudspeaker is designated as the direct sound, that from the other, the
delayed sound (the reflection). The delay injected between the two sig-
nals and their relative levels is adjustable. Speech is used as the signal. 22
With the sound of the direct loudspeaker set at a comfortable level,
and with a delay of, say 10 ms, the level of the reflected, or delayed,
loudspeaker sound is slowly increased from a very low value. The
sound level of the reflection at which the observer first detects a dif-
ference in the sound is the threshold of reflection detection. For levels