Handbook for Sound Engineers

1232 Chapter 33

2. Amplifier power ratings are based on signals that
   can bear little resemblance to audio program
   material.


3. Monitoring actual power delivered to the loud-
   speaker requires sophisticated equipment and a
   knowledgeable operator.


4. Standard loudspeaker power handling tests require
   that the loudspeaker be driven to the point where
   no permanent damage occurs. This is a bit ambig-
   uous. The author utilizes a power handling test that
   drives the loudspeaker with increasing rms voltage
   until its response changes by 3 dB from the small
   signal (typically 3 Vrms) response. This rms
   voltage is used to determine the continuous rating
   of the loudspeaker, either in volts rms or power into
   a rated impedance.

Even so, a conservative approach is as follows:

1. Determine the loudspeaker’s continuous power
   rating in watts (from the specification sheet).
   Determine the maximum rms voltage by taking the
   square root of eight times the power rating. Note
   that the voltage is necessary for level setting and
   verification.


2. Quadruple this rating for the required amplifier
   size. This will allow program peaks to exceed the
   continuous rating by 6 dB.


3. Be careful to not clip the amplifier, Fig. 33-13.

If the crest factor of the program material exceeds
6 dB, and the amplifier is operated without clipping, the

loudspeaker will simply be operating further below its

continuous rating, increasing its reliability and

longevity. A careful operator could use a significantly

larger amplifier, provided that a high crest factor is

maintained and clipping is avoided, Fig. 33-14.

In essence, buy a big amplifier but use it carefully.

Don’t overdrive the loudspeaker or the audience! A

sound level meter should always be used to check the LP

produced by the system, and this value should be within

OSHA exposure guidelines.

33.9 Conclusion

A properly calibrated sound system allows the operator

to mix at or near meter zero on the mixer without dan-

ger of clipping any system component. Meter zero

should also correlate with the maximum desired LP in

the audience. In effect, all components in the system are

now functioning as one component, the only difference

being that they are housed in separate chassis and inter-

connected with cables.

Reference

Milton Kaufman and Arthor Seidman, Handbook for Electronics Engineering Technicians, 1976 McGraw-Hill, Inc.
Don Davis and Eugene Patronis, Sound System Engineering, 2006 Focal Press, Burlington. MA.

Figure 33-12. Output voltage of the waveform in Fig. 33-11
utilizing a 6 dB limiter and normalized to full scale.

Time–s

Amplitude

Figure 33-13. Waveform clipped due to insufficient head

room.

Figure 33-14. Increasing the amplifier size produces addi-

tional head room.

Clipping

Time–s

Amplitude

Time–s

Amplitude