Sound System Design 1241again. The difference between the two readings is the
acoustic gain of the system. (Replace the talker with a
pink-noise source, through a small loudspeaker, for a
more consistent and accurate reading.)34.2.2.4 Feedback and Potential Acoustic Gain (PAG)The acoustic gain of this simple sound reinforcement
system can be increased by turning up the volume con-
trol, but, at some point, this process will be interrupted
by feedback (howling). Feedback is an undesirable
oscillation of the entire sound reinforcement system that
occurs when the sound from the loudspeaker feeds back
to the microphone at a level high enough that the system
begins to reinforce itself as well as it reinforces the
talker.
Potential Acoustic Gain or PA G is the maximum
acoustic gain that can be obtained from the system
before feedback occurs. For this simplified system
(neglecting reverberation and echoes), PAG can be
stated mathematically as
(34-3)where,
Ds is the distance between the talker and the micro-
phone,
D 1 is the distance between the loudspeaker and the
microphone,
D 2 is the distance between the loudspeaker and the
farthest listener,
D 0 is the distance between the talker and the farthest
listener.
34.2.2.5 Number of Open Microphones (NOM)
This example system has only one microphone. Adding
additional open (in-use) microphones increases the pos-
sibility of feedback and reduces the potential acoustic
gain. The basic PAG equation Eq. 34-3 can be modified
to include a number of open microphones (NOM) term
as follows
. (34-4)
34.2.2.6 Feedback Stability Margin (FSM)
Eq. 34-4 is theoretically correct but experience shows
that a system operated at or very near its PAG will
exhibit ringing and probably have an undesirable
peaked frequency response. In addition, a sound system
operated near its PAG will increase the effective room
reverberation time in an indoor system. Thus, a 6 dB
feedback stability margin (FSM) is normally subtracted
from the calculated PAG. Systems operated 6 dB or
more below their PAG are usually free of the problems
of feedback or ringing. The final PAG equation for the
simplified system, then, should include a FSM modifier
as follows.(34-5)Example:
Let,
Ds = 2 ft,
D 0 = 128 ft,
D 1 = 45 ft,
D 2 = 90 ft,
NOM = 3.34.2.2.7 NoiseUnwanted noise (traffic, wind, audience noises, etc.)
can interfere with the listener’s ability to hear the talker.
Ideally, the sound from the loudspeaker should be at
least 25 dB above the noise level; that is, there should
be a 25 dB SNR.
In some high-noise situations, a 25 dB SNR may not
be achievable. Nevertheless, 25 dB is a common rule of
thumb that will almost always insure that a listener can
hear and understand the talker in an outdoor system.34.2.2.8 Head Room and Electrical Power Required
(EPR)If ambient noise is 45 dB LP (usually measured on the A
scale of a sound level meter), and a 25 dB SNR is
desired, then the desired LP at the listener’s ears is
70 dB. That 70 dB, however, is the average level at the
listener’s ears, and the peak LP must be considered as
well. The difference between peak and average level is
referred to as the system headroom, Fig. 34-3. For a
speech-only sound reinforcement system about 10 dB of
head room is considered appropriate. Thus, the peak
level in this example system would be 80 dB LP for an
average level of 70 dB LP.PAG 20D 0 D 1
DSD 2= log-------------PAG 20D 0 D 1
DSD 2= log-------------– 10 logNOMPAG 20D 0 D 1
DSD 2= log-------------–6 10 logNOM– dBPAG 20 128 45^
290 = log--------------------–6 dB 10 3log –=19.3 dB