Wow, we had a few parties at our house growing up that my folks never knew about, but we sure as hell never let things get out of hand. Well it was more like my sisters had the party and I either stayed around to make sure things stayed cool or came back to find they exercised good sense and only had a few friends. My kid would be paying for a really long time and I bet that will be the last party that girl ever has and certainly she'll have a real respect for speakers. Funny you mention the cut off circuits and such. My bass amp has a similar circuit mainly for the horn, but of course it is built to handle the lower frequencies, but I still wonder how it remains cool in such a box.
On Apr 10, 2009, at 1:55 AM, Ray Boyce wrote: > > > 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] > > [Non-text portions of this message have been removed]
