Determining The Amount Of Amplifier Power For A Variety Of Systems

POWER CALCULATOR
Some websites provide a calculator that determines the amplifier power required to achieve the desired SPL at a certain distance.

The Crown calculator, for example, also accounts for the number of dB of amplifier headroom needed for audio peaks.

Text accompanying the calculator gives the equations used.

You input the desired SPL, the listener’s distance from the loudspeaker, the loudspeaker’s sensitivity rating (SPL/1W/1m), and the desired headroom.

“Headroom” in the Crown calculator is actually crest factor.

So in the Crown calculator, entering 12 dB of headroom allows for peaks 12 dB above average power without clipping.

The calculator tells you how many watts of continuous average power you need to get that average SPL, plus “headroom dB” of peaks, at that distance.

Example:
Listener distance from source – 15 meters (49 feet).
Desired level at listener distance – 85 dB SPL average (fairly loud).
Loudspeaker sensitivity rating – 98 dB SPL/1W/1m.
Amplifier headroom – 12 dB (this crest factor is adequate for speech).
Calculation results – 179 watts.

So you specify an amplifier that provides 179 watts continuous average power per channel (at the impedance of the loudspeaker). The amplifier will reach 179 watts only on short peaks.

Most of the time its signal will be 12 dB lower, or 11 watts average. It’s a good idea to have a few dB of headroom above those peaks as well so that occasional peaks don’t accidentally clip. If you want 3 dB of headroom above those 12 dB peaks, the amplifier should provide 358 watts continuous average power. That’s if the loudspeaker is used outdoors.

If you use it indoors, you usually get about 3 dB of room gain due to sound reflections off the room surfaces. So indoors a power amp of 179 watts would probably be sufficient.

Some of the calculator parameters need a detailed explanation…