6 Loudspeaker cabinets
6.9 Protection
All the talk about power handling 
and damage raises the question as to how to protect loudspeakers from overheating and overexcursion. The best advice that can be given is: know what your equipment can do and know what it can't do. But that would be too easy. As a performing musician you don't want to concentrate on your cones, but on your craft.
Overexcursion
The No 1 bass guitar speaker killer is overexcursion. The No 1 reason for overexcursion is unloading. The No 1 way to prevent this is to use a low cut filter before the power amplifier stage, sometimes referred to as "rumble filter" or "subsonic filter". Common sense can also go a long way, ie. not using 8" drivers to play a 31 Hz low B.
Overexcursion sounds much like an overdriven tube amplifier, and in extreme cases a distinct roaring or even rattling may be heard. The amplifier may very well be putting out only a fraction of its maximum power. But at this point, you're simply pushing it way too hard. This is often referred to as "farting". If an edgy sound is what you're after, be extra careful, or invest in a tube (pre)amp that may give you the desired effect without stressing things.
As stated in chapter 5.7 , overloading an amplifier may cause it to become unstable and behave erratically. There is no proper way to prevent damage to loudspeakers with an out-of-control amplifier. Even a low cut filter won't help you here, because this filter can only prevent very low frequencies from entering the power stage of the amplifier. The only way is to prevent the amplifier from behaving erratically by the use of a limiter . In other words: by simply preventing it from going into overload.
Overheating
Overheating coils is far less of a problem than overexcursion. Because of the high crest factor in a bass guitar signal, the average power going into a bass guitar cabinet is usually low. Exceptions to the rule are synthesized sounds and extreme compression , which can drive up average power. An overloaded amplifier also increases average power.
Tweeters and horns
Tweeters and horns seldomly have the same high power ratings as low frequency drivers. Because of this lower power, it's easy to protect them using circuitry inside the cabinet. A fuse would be the most obvious choice, but once it fails, it has to be replaced or reset. Another effective and simple measure is a filament light bulb of sufficient power that heats up and dumps the excessive power. But a light bulb may also fail. A socalled "polyfuse" device can also be used. It has the same working principle as a light bulb, but won't fail.
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© Joris van den Heuvel 2001-2009