B
5.3.1 Maximum Gain
(Available Gain Before
Feedback)
As a rule of thumb, then, a sound
system should be operated about 6 dB
below the onset of feedback. This
practice will allow a reasonable safety
margin to control feedback, and assure
that the sound quality will be reasonably
natural (or at least that it won't be
hollow or ringy due to feedback effects).
We are now ready to introduce the
important concept ofmaximum acoustic
gain.
Referring to Figure 5-7, assume that
both the loudspeaker and the microphone
are omnidirectional. The microphone
is one foot from the lecturer. The
level of the lecturer's speaking voice,
measured at the microphone, is 70 dB
SPL. To simplify our calculations, we
will assume that we are outdoors, so
we need not factor reverberation into
the equations.
With the system turned off, the level
of speech that the listener hears can be
found using a simple inverse square
calculation from the ratio of Do to DB