Room At The Top
Real-world audio waveforms have much lower RMS values (less area under the curve) than sine waves do, even though their peak values may be the same (see Figure 2).
A lower RMS voltage means a higher crest factor, which in turn means less power delivered to the load.
One way to determine how much power a complex waveform will generate is to use the peak power output based on the DC rail voltage as a reference, and then subtract the crest factor of the complex waveform from it. This will yield the power generated by the complex waveform.
For example, an amplifier rated at 500 watts continuous average power with a sine wave will have a theoretical peak output power 3 dB higher (the crest factor of the sine wave) – 1,000 watts in this case.
Using this as a reference (that’s all it’s good for), we can subtract the crest factor of the real-world waveform. Refer to the following chart for some common decibel relationships:
- 0 dB – reference value
- -3 dB – one-half power
- -6 dB – one-fourth power
- -10 dB – one-tenth power
- -20 dB – one-one-hundredth power
These numbers will be useful for determining how much power a waveform generates based on its crest factor.
Our 1,000-watt “theoretical” amplifier would only generate 500 watts for a sine wave signal (-3 dB), 250 watts for a 6 dB crest factor signal (highly compressed music or speech), 100 watts for a 10 dB crest factor signal (slightly compressed music or speech), and only one watt for a 20 dB crest factor signal (“raw” music or speech).
So, in the real world of live performances, a 500-watt “sine wave rated” amplifier is likely to deliver only a fraction of this power to the loudspeaker.
The implications? First, if a loudspeaker can safely dissipate 100 watts continuous average power (based on destructive testing – sounds like fun, doesn’t it?), then the required amplifier size to deliver 100 watts will be considerably higher.
This is because the amplifier is rated using a sine wave, which yields a much higher power output than a real-world audio waveform.
Assuming a crest factor of 10 dB, the amplifier would have to have a peak rating of 1,000 watts (sine rating of 500 watts) to actually deliver 100 watts into a resistive load.
This “extra” room is called “headroom”. This is why it is common practice to oversize the amplifier relative to the loudspeaker’s power rating.
But if you connect a 500-watt amplifier to this loudspeaker, and someone feeds it a low crest factor signal (like a sine wave), it is likely that the loudspeaker would burn up since its power rating has been exceeded.