Nelson-Reed 8-04/B loudspeaker How to Calculate Maximum SPL

Sidebar 3: How to Calculate Maximum SPL

If you have a scientific calculator (or a slide rule, or a set of log tables), it is easy to calculate, from a loudspeaker's sensitivity rating, how much Sound Pressure Level it will put out with a given amount of available amplifier power.

Since a sensitivity rating is a statement of SPL of acoustic output with 1W of signal input (at 400 or 1000Hz), it will be increased according to the difference in deciBels between the amplifier's rated maximum output and that 1W reference level. To calculate this, take the amplifier's rated output, find the base-10 logarithm of that, multiply that by 10, then add that to the loudspeaker's sensitivity figure. The result is the maximum SPL the amplifier/loudspeaker will produce before the amplifier starts clipping.

For example, the Nelson-Reed 8-04/B has a rated sensitivity of 92dB at 1W of input. I am using them with amplifiers rated at 160W output. The base-10 log of 160 is 2.2, multiplied by 10 is 22, added to 92. That combination of amp and speakers will, in theory, put out an SPL of 114dB before the amplifier overloads. Most listeners would overload first.—J. Gordon Holt