This is probably the second most useless of all specifications. Exactly the same loudspeaker can be legally described as 100, 200, 300, or 400 watts depending on whether it’s RMS, continuous sine wave, continuous music or peak program, and heaven knows how many AES watts.
At the end of the day, the number of watts is totally meaningless unless you take into account the amount of power compression that all loudspeakers have to some degree, and which eats up those watts.
Several reputable manufacturers, for example, tend to quote power handing at zero compression, so 400 watts will give twice the output than 200 watts.
One rather large Italian manufacturer quoted the same power handling for one of its 15-inch drivers as 600 watts but with a staggering 6 dB of compression due to its design. This means the driver gives maximum output at 150 watts and is not 1 dB louder at 300 or 600 watts.
Power handling is basically down to the ability of the voice coil not to self-destruct at high temperatures. This is largely down to the glue and voice coil former (which is the same for most manufacturers). Although the best loudspeakers, after all of this technology, are still paper cones and wooden boxes, it is the voice coil that has vastly improved over the years. With modern technology, the engineer’s rule of thumb in bass drivers is 100 watts per inch of voice coil, so a 100 mm voice coil is sensibly rated at 400 watts.
I recall suggesting to one manufacturer, who was getting back a lot of blown drivers, to drill a series of holes in the loudspeaker’s back plate to help dissipate the voice coil’s heat. This he did with good results. This innovation would have resulted in extended life for that driver, but those wonderful people in the marketing department saw the improvement and instantly upgraded the loudspeaker to 1,000 watts! As a result, the customers just turned up the wick and the manufacturer got even more returns.
Usually measured as how many dB you get at 1 meter distance, with 1 watt input. This is a useful spec on the face of it, because it shows how efficient the system is, and the more output per watt means more headroom, less distortion, cheaper amplification, and so on.
Nice in theory, but how is it measured? Logically you put a voltmeter across the loudspeaker terminal, and an amp meter in line with the terminals. When you have 1 volt and 1 amp, you have 1 watt—simple! To make things easier, some manufacturers, rather than measure it, use the stated impedance (i.e., 8 ohms) then apply the correct voltage calculated to produce 1 watt.
Instead of the 8-ohm impedance, others use the DC resistance (approximately 6 ohms) so they get a different sensitivity figure. Some manufacturers don’t even bother to do that; they simply take the sensitivity published by the driver manufacturers, add up the number of drivers (which shouldn’t make any difference), then add a spurious figure because their driver is horn-loaded, which produces more signal on axis than the same loudspeaker on a flat baffle.
The result with many system manufacturers is a totally false figure, even to the point where two manufacturers show different sensitivities even though they are using exactly the same drive unit.