Power Versus Application
This section will suggest how big a power amplifier you need to fill a venue with loud, clear sound. Basically, the louder the sound system and the bigger the room, the more power is required. Loudspeakers with high sensitivity need less power than loudspeakers with low sensitivity.

The list below recommends the amplifier power needed for several applications. Each application has a range of power based on the desired loudness and the typical loudspeaker sensitivity.

In compiling this list, I made the following assumptions:
• Typical loudspeaker sensitivity is 85 dB SPL/W/m for home stereos, 95 dB SPL/W/m for small PA loudspeakers, 100-105 dB for medium PA loudspeakers, and 110 dB for large PA loudspeakers.
• The recommended power allows for signal peaks of 10 to 15 dB for folk, jazz and pop music. Actually the peaks might be as high as 25 dB, but we’re allowing for some inaudible short-term clipping.
• The recommended power allows for signal peaks of 6 dB for rock music that is highly limited or compressed.
• Amplifier continuous power and amplifier peak power are nearly the same. Typically, peak power is only 1 dB higher than continuous power, and depends on peak duration.

Continuous amplifier power per channel required in various applications:
• Nearfield monitoring: 25 W for 85 dB SPL average (with 15 dB peaks), 250 W for 95 dB SPL average (with 15 dB peaks)
• Home stereo: 150 W for 85 dB SPL average (with 15 dB peaks), 1,500 W for 95 dB SPL average (with 15 dB peaks)
• Folk music in a coffee shop with 50 seats: 25 to 250 W
• Folk music in a medium-size auditorium, club or house of worship with 150 to 250 seats: 95 to 250 W
• Folk music at a small outdoor festival (50 feet from loudspeaker to audience): 250 W
• Pop or jazz music in a medium-size auditorium, club or house of worship with 150 to 250 seats: 250 to 750 W
• Pop or jazz music in a 2000-seat concert hall: 400 to 1,200 W
• Rock music in a medium-size auditorium, club or house of worship with 150 to 250 seats: At least 1,500 W
• Rock music at a small outdoor festival (50 feet from loudspeaker to audience): At least 1,000 to 3,000 W
• Rock or heavy metal music in a stadium, arena or ampitheater (100 to 300 feet from loudspeaker to audience): At least 4,000 to 15,000 W

Although a rock concert in an arena could be powered by 15,000 watts (allowing only 6 dB of headroom for peaks,) you’ll often see large touring sound companies using 80,000 to 400,000 watts total. That much power is needed to handle 20-to-24 dB peaks without any clipping, and to power extra loudspeakers for even coverage of a large area.

If one loudspeaker won’t handle the total power required, you need to divide the total power among multiple loudspeakers and multiple amplifier channels.

For example, suppose you need 1,000 watts to achieve the desired average loudness, but your loudspeakers power handling is 250 watts continuous. You could use a power amplifier of 500 watts per channel. Connect two loudspeakers in parallel on each channel. That way, each loudspeaker will receive 250 watts (not considering the change in amplifier power at different impedances, and not considering cable losses).

Note that if you parallel two loudspeakers, their total impedance is halved. For example, two 8-ohm loudspeakers in parallel have an impedance of 4 ohms. In that case, each loudspeaker would receive half of the amplifier’s 4-ohm power.

Power Calculator
Some websites provide a calculator that determines the amplifier power required to achieve the desired SPL at a certain distance. The Crown calculators on ProSoundWeb, for example, also account 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.

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