Friends recently returned from a couple of weeks away to find one of their teenagers had hosted a party during their absence. As well as discovering chipping in the kitchen's marble bench top and a missing iPod, they also found none of their hi-fi speakers were working.
Speakers have delicate electrical windings behind their cones called voice coils. These are intricate little things that are surrounded by a fixed and usually very powerful magnet. The amplifier feeds electricity into the voice coils in varying amounts and the resultant electrical field drives the coil, which is connected to the cone, back and forth along the inside of the magnet. This moves the cone in and out, generating sound waves. The voice coil is, therefore, the very heart of the speaker. The voice coils inside these particular speakers had been reduced to gloop. They had melted and the bad news for my friends was that replacing voice coils is so complex and expensive that it's often best to throw away the speakers and buy new ones. Replacing melted voice coils can get so complex that lots of repairers don't want the work. It's often not just a matter of putting in a new voice coil. Frequently, the heat generated has also damaged components around the coil and, in some cases, melted the adhesives used in the speaker's construction. So why do voice coils melt? I wasn't at the party (it sounded like a ripper), but I suspect two things happened. First, someone cranked up the volume and, second, these being young people, someone turned up the bass as high as it would go. Most of the energy applied to a voice coil winds up as heat, which isn't a problem at normal listening levels when it can be dissipated by the air around it. But at high power levels, heat builds up faster than it can be dissipated and, if these high power levels are maintained, something's got to give. Usually, it's the voice coil. It melts, taking the insulation separating the windings along with it. Turning up the bass makes this happen faster. Speakers are placed in cabinets because the air inside the cabinet forms a natural suspension system for the cone as it punches in and out. This is why the internal volume of a cabinet has to be spot on, and varies from cone to cone, depending on its size and strength. But at very low frequencies, the air inside the cabinet provides little or no buffering, meaning there's nothing to stop the cone moving in and out as far as it can possibly travel. By turning up the bass all the way, the speaker is being directed to favour low frequencies over all others, so the cone works even harder. And things just keep getting hotter. Had anyone at the party been paying attention, they would have heard the danger signals. The speakers would have been distorting horribly, producing ragged, fuzzy bass that sounded ill-defined. Listening to it would have been uncomfortable. But maybe no one noticed, or maybe if anyone did and turned the music down, they were overruled by others. The teenager involved is working weekends to reimburse her parents and has discovered that speakers, while not at all cheap, have nothing on marble bench tops. Speaker safeguards When you're buying speakers, whether it be for the first time or to replace those that have been beaten to death, it's worth having a talk to a specialist supplier about how you're going to use them. If you often play music loud and long, finding speakers with sophisticated heat dissipation technology or automatic cut-outs is well worth the effort involved. Some speakers have sophisticated cooling provisions built in, such as vented pole pieces and heat sinks. Others have circuitry that shuts them down when they get too hot or energy levels get too high. They simply stop working until things return to normal. Some amplifiers and car stereos have a "loud" switch that increases bass. These are designed to emphasise bass notes at very low listening levels when they're at their weakest, but activating the switch at high volume can lead to all the problems we've been talking about here. In many cases protective circuitry ensures the loud switch has no effect above certain pre-set volumes anyway. [Non-text portions of this message have been removed]
