QSC Audio CX Series user manual Determining Power Levels, dBref1watt, PowerWatts

DETERMINING POWER LEVELS

After you’ve determined where to place the loudspeakers, you need to calculate the power each one requires.
If the installation is in an existing facility already in use, use an SPL meter, set for slow response, to measure
the A weighted ambient noise at the listener’s ear position. Try
to take this measurement at the noisiest time—in a factory,
					20.0
when the machines are running; in a restaurant, when it’s full											dB(ref1watt)
			18.0
of patrons, etc. Background music will require an SPL at least
						16.0
10 dB above the ambient noise. For good paging intelligibility,
			14.0
you’ll need an SPL approximately 15 dB higher than the
			12.0
ambient noise; 25 dB above ambient will yield excellent
						10.0
intelligibility.
			8.0
Next, use the distance from the loudspeaker to the listener’s									6.0
ears, along with the loudspeaker’s sensitivity rating (typically									4.0
									2.0
expressed as “n dB @ 1 watt, 1 meter,” which means n dB of																Power(Watts)
SPL with 1 watt input, measured at a distance of 1 meter) to									0.0
	0.1	0.2	0.5							1		2	5		10		20	50	100
determine how much power the loudspeaker needs to get from								-2.0
the distributed line. Use the inverse square law guide below			-4.0
to convert distance to dB; you’ll need to add this figure to the			-6.0							Decibels
desired SPL and then subtract the sensitivity rating to deter-							-8.0							referenced to
														1 watt
mine how much more or less than 1 watt the loudspeaker				-10.0

requires.

Distance from speaker

Power increase in dB; referenced to loudspeaker

sensitivity rating (1 watt @ 1 meter)

22