Handbook for Sound Engineers

System Gain Structure 1231

33.8 Power Ratings

It is important to note that the amplifier’s wattage rating
must be appropriate for the loudspeaker. The loud-
speaker must not be driven beyond its ability to dissipate
heat buildup or beyond its limits of mechanical excur-
sion. In most systems, heat will be a bigger problem than
excursion, so the rms value of the amplifier’s output volt-
age waveform must be managed. Since the continuous
power (based on the rms voltage) delivered to the loud-
speaker is largely a function of the type (crest factor) of
the program material, selection can be complicated.

33.8.1 Amplifier Power Ratings

The power ratings of power amplifiers and loudspeakers
have little in common. The amplifier is usually rated in
accordance with the maximum continuous power that it
can deliver reliably with sinusoidal input for a specified
span of time into a specified load impedance. This
yields a large number for the amplifier power rating
(due to the high rms voltage of the sine wave), and most
likely a wattage that the amplifier will never be called
on to deliver, since the signals that we present to audi-
ences usually bear little resemblance to sine waves.
Even so, this rating can be useful for amplifier compari-
sons and selection. Just remember, you won’t get that
much rms voltage across the loudspeaker with
real-world program material.

33.8.2 Loudspeaker Power Ratings

The loudspeaker’s continuous power rating describes
the loudspeaker’s ability to dissipate heat on a continu-
ous basis. A meaningful rating must state at a minimum:

1. The type and crest factor of the signal used.


2. The bandwidth of the signal.


3. The time duration of the test.


4. The rms voltage of the signal.


5. The impedance of the loudspeaker under test.

If the signal used has a crest factor of 6 dB, and the
loudspeaker is rated at 50 W continuous, the amplifier
size required to run the test would be

Power specifications are of little use to system tech-
nicians. They must be converted to an equivalent rms
voltage so that the system tech can measure the signal
with a voltmeter. A simple conversion is to multiply the

power rating by eight and take the square root to get the

voltage. Bear in mind that if the power rating has been

exaggerated, the voltage resulting from this conversion

will be too.

Due to the high crest factors of audio program mate-

rial, power amplifiers normally deliver far below their

theoretical maximum sine wave power. This makes it

possible (and necessary) to use an amplifier whose

continuous power rating exceeds the continuous power

rating of the loudspeaker if the intent is to produce the

maximum LP possible. Care is required on the part of

the user to insure that the crest factor of the program

material is not reduced excessively by dynamic range

control devices (compressors and limiters) and then

used to drive the amplifier to the point of clipping. Figs.

33-11 and 33-12 show the same waveform. The peak

output voltage of each waveform is the same. A peak

limiter was used to reduce the dynamic range of the

second waveform, resulting in a 6 dB increase in

applied rms voltage (and continuous power) to the loud-

speaker (four times). This example shows how that

amplifier power (and loudspeaker power dissipation)

are highly dependent on the nature of the waveform, not

just the amplifier rating. The amplifier selection and

setting should ideally depend on the target sound level

in the audience. There are no ramifications to operating

a loudspeaker below its power rating, and in fact it is

good design practice.

One is reluctant to formalize a formula for deter-

mining the required amplifier size for several reasons,

including:

1. The continuous output power of an amplifier is a
   function of the crest factor of the program material
   and can vary by 20 dB or more (a 100:1 ratio!).

17dBW+6 dB=23 dBW

23 dBW=200 watts continuous

Figure 33-11. Output voltage of a complex waveform with

large peaks.

Time–s

Amplitude