Power and Loudness in The Real World
Perceived "loudness" varies logarithmically with output power (other inversely proportionate factors are; frequency, number and material of objects through which the sound waves must travel, as well as distance between source and receiver) a given change in output power produces a much smaller change in perceived loudness. Consequently it is useful and accurate to express perceived loudness in the logarithmic decibel (dB) scale.
An increase/decrease of 3 dB corresponds to a doubling/halving of power. The sensitivity of loudspeakers,rather than merely the often-quoted power-handling capacity, is important when maximum volume levels are desired. Many high quality domestic speakers have a sensitivity of 84 dB for 1 W at 1 meter, but professional speakers can have a figure of 90 dB for 1 W or even 100 dB (especially for some large-coned woofers). I.E., An '84 dB' source "speaker" would require a 400-watt amplifier to produce the same audio energy as a '90 dB' source being driven by a 100-watt amplifier, or a '100 dB' source being driven by a 9.92 watt amplifier. This does not mean a bigger speaker can produce more sound with less overall power. Just that a larger speaker can typically handle more initial power and so requires less amplification to achieve the same high level of output. This means using a speaker with a higher dB rating can be more advantageous as very high power amplifiers become impractical.
A better measure of the 'power' of a system is therefore a plot of maximum loudness before clipping of the amplifier and loudspeaker combined, in dB SPL, at the listening position intended, over the audible frequency spectrum. A good system should be capable of generating higher sound levels below 100 Hz before clipping, as the human ear is less sensitive to low frequencies, as indicated by Equal-loudness contours.
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