Page 101 - The Master Handbook Of Acoustics

76 CHAPTER THREE

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 is now added to the increasing spacious-
ness. As the reflection level is increased another 10 dB or so above the
image broadening threshold, another change is noted; discrete echoes are
heard.
This is all very interesting, but what practical value does it have?
Consider a specific example: a listening room in which recorded music
will be played. Figure 3-20 contains answers to the effect of sound
reflected from floor, ceiling, and walls being added to the direct sound
from the loudspeakers. 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. Simple calculations can give estimates of the level and delay of
any specific reflection, knowing the speed of sound, the distance trav-
eled and applying the inverse square law. Figure 3-20 gives the subjec-
tive reactions the listener will probably have to the combination of any
reflection and the direct sound.
To assist in the “simple” calculations mentioned previously, the
following equations can be applied:
(reflected path, ft) – (direct path, ft)
Reflection delay
1,130 ft/sec
This assumes 100% reflection at the reflecting surface.
direct distance, ft
Reflection level at listening position 20 log
reflection distance, ft
This assumes the inverse square propagation.

Occupational and Recreational Deafness

The hearing of workers in industry is now protected by law. The
higher the environmental noise, the less exposure allowed (Table 3-2).