6 Loudspeaker cabinets
6.7 Power handling
Contrary to what many believe, loudspeakers are fairly fragile devices. It's easy to overstress them on a live stage with high sound pressure levels. Cones are usually made of paper simply because it is light and stiff, voice coils are very thin because weight would compomise performance, surrounds and spiders are made of cloth - again to reduce weight. Yet it's not uncommon to drive hundreds of Watts into these superlightweight precision devices.
There are two types of power handling for a loudspeaker driver. Thermal and mechanical:
Thermal power handling (Pth)
The only electrical part of a loudspeaker is the voice coil. It is usually made of thin copper wire on a heat resistant coil former. The thermal power handling (Pth) is the maximum average power the coil can dissipate (heat conduct), before the coil wire or the glue, with which the coil is attached, will melt. Because the rise in temperature due to high power is fairly slow, short, high power peaks (in the order of tenths of seconds) will be averaged by the thermal sloth. A rule of thumb: peak power (Ppeak) for normal duty speakers equals twice the continuous power (Pth) rating. Professional drivers are often able to withstand peaks of four times their rated continuous power. Some even 10 times. The ratio of continuous to peak power is called "crest factor", and is expressed in decibels: 3 dB for normal duty speakers, 6 dB for medium duty ones and 10 dB for heavy duty speakers.
Mechanical power handling (Xdamage)
A loudspeaker is an electro-mechanical device. The mechanical part can, just like the electrical part, be overloaded. The cone suspension system is limited in its excursion (displacement). Please note: this is not the figure of Xmax, which is the maximum linear excursion of the cone assembly 
. The cone should be able to move a considerable distance past Xmax. For bass guitar, the mechanical power handling is crucial. The signal from a bass guitar contains extremely high peaks, with relatively low average power. This is especially true for pick and slap playing techniques.
Power compression
One of the physical characteristics of metals is their positive temperature coefficient (PTC), meaning conduction decreases with increasing temperature. Voice coil wire is usually made of copper, so voice coils also have that PTC effect. In practise this means that efficiency drops with high input power. While not so much of an issue with bass guitar because of the high peak to average power ratio, it is there and could have an impact on the power handling of a cabinet.
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© Joris van den Heuvel 2001-2009