Page 94 - Master Handbook of Acoustics

hands—a distinct echo will be heard (177 msec). As you move closer and continue to clap your
hands, the echo will arrive sooner and will be louder. But as you enter the fusion zone, your ears will
spatially integrate the echo into the direct sound; you do not perceive the echo.

Perception of Reflected Sound

In the preceding section, “reflected” sound was considered in a rather limited way. In this section, a
more general approach is taken. The loudspeaker arrangement used by Haas was also used by other
researchers; in this familiar stereo setup the listener is located symmetrically between two separated
loudspeakers. The sound from one loudspeaker is designated as the direct sound, that from the other

loudspeaker, the delayed sound (the reflection). The delay injected between the two signals and their
relative levels is adjustable.
With the sound of the direct loudspeaker set at a comfortable level, and with a delay of, say 10
msec, the level of the reflected, or delayed, loudspeaker sound is slowly increased from a very low

level. The sound level of the reflection at which the observer first detects a difference in the sound is
the threshold of reflection detection. For levels less than this, the reflection is inaudible; for levels
greater than this, the reflection is clearly audible.
As the reflection level is gradually increased above the threshold value, a sense of spaciousness is
imparted to the combined sound. This sense of spaciousness prevails even though the experiment is
conducted in an anechoic space. As the level of the reflection is increased about 10 dB above the

threshold value, another change is noticed in the sound; a broadening of the sound image and possibly
a shifting of the image toward the direct loudspeaker added to the increasing sense of spaciousness.
As the reflection level is increased another 10 dB or so above the image broadening threshold,
another change is noted; discrete echoes are heard.
What practical value does this have? Consider the example of a listening room used for playback

of recorded music. Figure 4-19 shows the effect of lateral reflections added to the direct sound from
the loudspeakers. Speech is used as the signal. Reflections below the threshold of perception are
unusable; reflections perceived as discrete echoes are also unusable. The usable area is the unshaded
area between those two threshold curves, A and C. Calculations can give estimates of the level and
delay of any specific reflection, knowing the speed of sound, the distance traveled, and applying the
inverse square law. Figure 4-19 also shows the subjective reactions listeners will probably have to
the combination of reflected and direct sound.