6 Loudspeaker cabinets

6.8 Damage

From the two types of power ratings 6.7 Power handling for a loudspeaker, it can be deduced that there are two types of systematical damage that can occur: thermally and mechanically induced damage. Of course, any speaker that doesn't have its cone protected by some sort of grille can sustain damage, but this is not considered systematical damage.

There's a lot of misinformation floating around as to what causes speaker damage. Power amplifier clipping is said to cause damage. This is not entirely true. Clipping in itself does not cause speaker damage. It's not that simple. For more info on this subject, refer to chapter 5.7 5.7 Clipping.

Thermally induced damage

Playing bass guitar loudly demands a lot from the voice coil in a loudspeaker driver. Because loudspeakers are such inefficient devices 6.10 Efficiency and sensitivity, nearly all input power is converted to heat.

Overheating a voice coil can really have only one cause: simple overpowering, either with continuous power or peak power. In theory, driving a speaker in a ported enclosure very hard at the port frequency can also cause overheating. At the port frequency, the voice coil exhibits very little movement and cooling capacity is derated. In the real world with signals with complex harmonic content, this hardly ever happens.

Overheating a voice coil can have various consequences:

  • The coil wire could simply burn up and cause an open circuit.
  • Wire isolation could melt and short circuit coil windings, causing danger to the connected amplifier as well.
  • The coil may deform because of the heat, or the glue with which the coil is attached to the cone assembly may melt and the coil may shift or start scraping the pole piece.

Mechanically induced damage or overexcursion

Overexcursion can have two causes:

  • A badly designed cabinet: good cabinet design will let the driver operate within certain excursion levels over its frequency range at maximum input power. Not observing this may cause overexcursion, even when the driver is used within its specifications.
  • A driver in a non-closed cabinet is used below its usable frequency range, called "unloading". Even moderate power levels could be enough to cause overexcursion. If there's danger of unloading, the frequency range should be limited with a high-pass filter at signal level.

And it can have various consequences:

  • The spider, surround or cone could deform or even rupture
  • The voice coil could tear itself loose from the coil former
  • The coil assembly could tear itself loose from the cone assembly
  • The voice coil former could buckle and become stuck
  • In very extreme cases, the coil assembly could slam onto one of the pole pieces

Mechanical damage is by far the most common cause of speaker failure. It is, however, easy to prevent, as the speaker usually warns the user by producing vast amounts of distortion, sometimes called "bottoming out".

Aging and wear

An often overlooked part of the damage equation is aging and wear. It should come as no surprise that old speakers can't take the beating they used to get when they were fresh. But speakers can age quickly if used near their limits. Playing a low B (31 Hz) on a 5 string bass guitar over a cabinet equipped with 8" drivers, which unloads below, say, 50 Hz, may not be exactly what the cab designer had in mind. However, determining whether you've crossed the line is close to impossible. When a speaker starts bottoming out, you're far over the line already. Then again, common sense probably comes a long way.

© Joris van den Heuvel 2001-2009