76 Chapter 2
2.26 Playback Systems in Studios ...................................................................................
Assume that a monitor loudspeaker can develop LP 110 dB at the mixer’s ears and that
in an exceptionally quiet studio we reach LP 18 dB at 2000 Hz (NC-20). We then have
LLLPPPDiff Total Noise (2.51)which is equal to 92 dB. Adding 10 dB to avoid the inadvertent addition of levels gives
102 dB. The distortion now becomes
100 10 0 00078(^10220)
.%.
In this case, extraordinary as it is, the previously esoteric fi gure becomes a useful
parameter.
2.26.1 Choosing an Amplifi er
As pointed out earlier, the loudspeaker will establish equilibrium around 1% with its
acoustic distortion. To the builder of systems, this means that extremely low distortion
fi gures cannot be used within the system as a whole. Therefore systems-oriented amplifi er
designers have not attempted to extend the bandpass to extreme limits. They know that
they must balance bandpass, distortion, noise, and hum against stability with all types of
loads, extensions of mean time-before-failure characteristics. Most high-quality sound
reinforcement amplifi ers incorporate an output transformer, giving us 70 and 25 V and 4,
8, and 16 Ω outputs. In fact, connecting across the 4 and 8 Ω taps yields a 0.69- Ω output.
Example
Let the rms speech value be LP 65 dB at 2 ft in the 1000- to 2000-Hz octave band
( Figure 2.21 ). Let the ambient noise level be LP 32 dB with the air conditioning on and
16 dB with the air conditioning off in the same octave band ( Figure 2.22 ). With the air
conditioning on the signal to noise ratio ( SNR ) is
SNR
65 32
33
dB dB
dB(2.52)
and with the air conditioning off
SNR
65 16
49
dB dB
dB.