If the program material was recorded with 20 dB of headroom, then the long-term average signal level is 20 dB below the highest peak of the music—100 watts minus the first 10 dB of headroom is 10 watts, and 10 watts minus the second 10 dB of headroom is 1 watt.
If the signal source was a properly mastered commercial CD, there is a good chance that the headroom for the recording is 14 dB. If 1 watt results from 20 dB of headroom, then 6 dB less headroom would be 4 times the delivered power, or 4 watts.
This is why the UL listing on the back of amplifiers is never at the rated power capacity of the amp. It’s tested at 1/8th power, in this case, 12 watts, which is a program level with a significant level of clipping distortion where only the drunkest of partiers could stand to listen to it.
Think of the loudspeaker’s sensitivity rating, for example, 95 dB SPL (1 watt/1 meter). The level at 1 meter in our 100-watt amplifier test case would be between 95 and 101 dB SPL. It doesn’t seem that we get much out of that 100-watt amp does it?
Forget tube amps if you really want to listen to rock music at realistic levels. They just can’t supply enough wattage for the headroom of high-level music. What you end up with at higher levels is even-harmonic clipping, which is the tube amplifier’s warm tone that everyone talks about. It’s nice, but not accurate.
When I’m asked what the wattage rating for the amplifiers in a sound system design should be, my answer is always, “How big is your budget?” Amplifiers are rated in watts, and should be able to pass the peaks of the program material without clipping.
This means they ultimately should be sized to pass not only the long-term program-power capacity of the loudspeaker, say 500 watts, but also the peaks that the program headroom allows without clipping. At 20 dB, that would be a rather ridiculous 50,000 watts!
How We Got Here
The standard 20 dB of headroom was established way back in the early days of radio broadcasting when it was noted that uncompressed speech and music would never produce peaks more than 20 dB higher than the long-term average program levels. This is how we standardized on a +4 dBu nominal program level and a clipping level of +24 dBu for most audio electronics today.
But there is a problem in maintaining a 20 dB crest factor with the loud music played today. If the amplifier is below clipping, then the program level is often not loud enough for today’s “enthusiastic” listening levels.
Standard CD mastering has 14 dB of headroom as a general rule of thumb. This enables program levels to be turned up 6 dB higher, while using only 6 dB of peak limiting, which is not generally noticed by your typical listener. In sound reinforcement systems, lowering the headroom to a 10 dB peak-to-average ratio is common to attain maximum program level without clipping, though the 10 dB of limiting is easily detectable by the listener.