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LOUDSPEAKERS - AN IN DEPTH LOOK:

Why Speakers "Blow Out"

 

Most people have blown a speaker at one time or another. A typical scenario is a large drunken party where the stereo volume control is cranked as far as it will go (despite the offensive sound quality), and the next morning the house owner realizes that one or more speakers are no longer functioning properly. What happened?

 

There are two major ways speakers can fail: mechanical failure (most often limited to woofers, resulting from too much cone excursion) and thermal failure (the voice coil overheated and melted or burned).

 

Mechanical failure of a speaker (most often a woofer) occurs because the speaker's cone is being asked to move farther than it was designed to safely do. When this happens, parts of the speaker become stressed due to stretching and excessive vibration, and in severe cases the cone and/or parts of the voice coil structure can collide with the frame of the speaker. If operation of the speaker in this fashion is not ceased, sooner or later something will rip, break or come loose. If the woofer's spider (a component made of a mesh like material that holds the voice coil in alignment) comes loose, the voice coil will basically be allowed to flail wildly inside the voice coil gap, and the insulation on the coil will be damaged as it rubs against the metal parts of the magnetic structure. When this occurs, the sound quality will take on a very gritty tone, it should be very obvious (to a sober person anyway!); if left to continue in this fashion, the coil will eventually break (resulting in no sound at all). Mechanical failure of midrange and tweeter components can and does occur, but it is much less common than woofer failures. Almost all of the time, failure of a midrange or tweeter speaker is due to thermal failure.

 

Thermal failure of a speaker occurs when the speaker is fed more power than it is designed to safely handle. This basically causes the voice coil to get too hot, and two main things can happen. First, the adhesives used to hold the voice coil together can soften, resulting in the coil coming apart (this is a partial mechanical failure). If the coil gets too hot, it will simply melt or burn the wires in the coil, most often resulting in an open circuit (in this case there will be no sound at all).

 

In the majority of cases, a speaker blowing out is usually due to an accident or carelessness. An example of an accident is someone making cable connections to components (a CD player) at the back of the amp while unknowingly having the volume at full blast while the amp is on (the very loud hum that is generated could blow up a speaker). More often, carelessness is the cause (the drunken party example above). It should also be mentioned that it can sometimes be easier to blow a speaker with an amp that puts out less power than the speaker is designed to handle! The reason for this is that an amp that is being overdriven can generate a lot of distortion and the RMS level of the amplifier output will be a lot higher than what the operator thinks it might be, and the speaker can blow. You may sometimes hear the statement "speakers can take a lot of power if it is "clean" power". This is true to a point. However, if you try to drive a speaker rated at 100 W with an amp that puts out 1000W, chances are you could wreck the speaker even though the power was totally clean (no clipping or distortion)!