We recently had a subwoofer come back from a rental with a funny noise in one of the drivers. It turned out to be not that funny…
Since we test all speakers (and earbuds, and lav microphones) every time they return from a rental, we knew that the problem had to have appeared quickly.
Sending the driver in to the manufacturer for warranty evalution revealed that the driver had been overpowered, causing the voice coil winding to come loose and start rubbing in the magnet gap—not covered by warranty!
But how could this have happened? This was a top-end professional speaker from a major manufacturer with built-in amplification and speaker protection circuitry. How was it possible for a customer to overpower the driver with this protection in place?
While it would be nice to have a speaker system that is completely idiot-proof, real-world protection circuitry is meant more to protect against honest mistakes and occasional light abuse. Determined abuse, aka “it sounded really crunchy, but the DJ was finally happy” is beyond the design specs for most speaker protection circuitry.
Speaker protection circuitry works by limiting or compressing the incoming signal. Usually a “brick wall” limiter will kick in when the signal gets too strong.
The difficulty in implementation is that the type of signal has a big influence on whether the drivers are in danger. A loud kick drum thump, even when it causes that red light on the speaker to blink, is over quickly and doesn’t cause much heat to build up. A sustained bass note might not be loud enough to cause the limiter to kick in but instead may cause heat to build up in the driver’s voice coil, eventually weakening the glue that holds the coils together.
The most insidious problem is when a lot of sustained music is squashed by the limiter – the signal getting to the drivers is now flattened out by the limiting, and the average power (and therefore heat) to the drivers ends up being higher than normally expected from “music.”
The basic rule of thumb still applies—if it sounds bad, it is bad. When the protection circuitry is being pushed into effect, when that little red “protect” light is constantly lit up and when the system sounds distorted, the next stage in the protection system is for the operator to turn the volume down.
Modern self-amplified speaker systems are much better than the old systems we used to mix on–a sudden squeal of feedback or an unplugged cable doesn’t mean “time to replace all the horn diaphragms” any more. The “honest” mistakes are mostly a thing of the past. But operators of these systems are still responsible for the “dishonest” mistakes–cranking up the volume when things are obviously too loud for the speaker system and when those protection lights are competing with the light show.
There is no substitute for good design, active listening, and regular monitoring of indicators.
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