Speaker Versus Amplifier Power

Aug. 4, 2008

Recently a friend asked me if he should use speakers that are rated for a lower power or a higher power than the amplifier used to drive them. Most, including myself, feel that the speakers should have a much higher power rating than the driving amplifier, especially for high fidelity applications, such as PAs and bass guitar. This reduces the chance of damaging the speakers with too much amplifier power.

But there are some that argue that it is high power distortion that actually damages speakers. When you are sending square waves, such as what you get when an amplifier is distorting severely, to speakers, the average power is equal to the peak amplifier power, which is much greater than the RMS power due to a sine wave. The RMS power of most amps is determined by the cleanest sine wave you can get out of an amp. A bigger amp is less likely to distort at a given volume level.

The reason I feel the former is better is that most PA and bass amps these days, and many hifi amps, have built in compressors that kick in during high power operation and minimize the distortion. Consequently, if you are getting square waves out of an amp, it is either broken or severely, obnoxiously, intolerably distorting, where any reasonable person would turn the amp down.

For guitar, many recommend using speakers that just barely can handle the amplifier power. The rationale is that the speakers will distort and naturally compress the guitar sound in a desirable way. I agree with this. This can be hard on speakers, though, if you play your amp at max power a lot.

I feel most speakers that fail are damaged by too much cone excursion (too extreme cone movement), which occurs when you send too much bass through them. The lower the frequency, the more a speaker cone has to move to reproduce that frequency for a given volume level. That is why many bass and PA cabinets are closed back with tuned ports, as proper tuning will significantly reduce cone movement. This is also why many say playing a bass through a guitar amp can damage it. I don't know of any guitar amp electronics that could be damaged by bass, though many guitar speakers can be damaged by bass.

Most speakers are specified by the maximum power their voice coils can handle before burning up. But cabinets also have a power rating, which is the maximum power the speaker in them can handle before its cone movement is excessive enough to damage the speaker. This cabinet power rating is dependent upon the lowest frequency that will be sent to the speaker. Often this cabinet power rating is much lower than the rating of the speaker that is in it. Many manufacturers overlook this cabinet power rating and it is not specified or they just use the speaker voice coil rating for maximum cabinet power.

This is another reason for having speakers that can handle much more power than your amplifier, as cone excursion drops in direct proportion to the area of the cones being driven, assuming the same type of cabinets and the cabinets are appropriately tuned for those speakers. In other words, if you are driving two 15" speakers with 200W of power, they will each typically move half as far as one 15" speaker being driven by the same 200W of power. It doesn't matter if the one 15" speaker has a voice coil that can handle 1000W of power and the two 15" speakers can handle 150W each (300W total). The one high powered speaker is more likely to be damaged by bass frequencies than the two lower powered speakers.

Incidentally, four 10" speakers have much more cone area than one 15" speaker, so, assuming their voice coils can handle the same power and they have similar suspensions, the 10" speakers are less likely to be damaged by bass. That is one reason 4-10" speaker cabinets have become popular for bass guitar.

So I recommend using speakers that are much higher power rated than the amplifier you are using.