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The crest factor of an audio signal is the dB difference between
the peaks and the RMS value of the signal. The RMS (Root Mean Square)
is defined as the “heating value” of the signal - the
voltage that would generate the same heat as a DC (Direct Current)
signal, over the same time. The RMS value of a complex signal must
be read with an RMS voltmeter. Alternatively, the signal can be
digitally sampled and the samples summed to yield the RMS value.
As such, the RMS value of a complex signal can be thought of as
the “area under the curve” of a signal as viewed in a
wave editor software application. This can be done with many wave
editor programs for personal computers, such as Sound Forge™
and Cool Edit™.
Some typical crest factors are shown on the next page. It is of
importance because the crest factor of the signal is one parameter
that determines how much actual power is delivered from amplifier
to loudspeaker. Since the main enemy of loudspeaker longevity is
heat, one must be careful to stay below the long-term average power
rating of the loudspeaker.
In light of this, it can be seen why a power amplifier’s actual
power output is dependent on the RMS value of the voltage waveform
that it is amplifying. A low crest factor signal will deliver more
power (and more heat) to a loudspeaker than a high crest factor
signal, even though both are played over the same amplifier.
Since a voltage waveform has a crest factor (in dB), this value
is useful in determining the appropriate amplifier size for a given
loudspeaker and desired level. Here are the steps:
1. Determine the continuous power rating of your power amplifier.
Manufacturer’s do extensive testing and are usually glad to
provide this information.
2. Convert this rating to a level in dBW.
dBW = l0log W
where W is the continuous power rating
3. Since this is most often measured with a sine wave (3 dB crest
factor), you can double the continuous power rating to estimate
the “peak” power rating of the amplifier. In dB, you simply
add 3 dB to the number that you just calculated. IMPORTANT - this
is a number that we will use for computation purposes only. Your
amplifier cannot sustain its output at this rating but should be
able to provide this level for short-term transients.
4. Estimate the crest factor of your signal. For music systems,
this usually ranges from 6 dB (very compressed music) to over 20
dB.
5. Subtract the crest factor from the peak rating of the amplifier.
This will be the continuous power in dBW that you are delivering
to your loudspeaker. If you feel more comfortable working with watts,
you can convert dBW back to watts:
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