I have a very general understanding about speakers and how they work. I was wondering what different types of “blow outs” there are. I’m assuming one way to ruin speakers is to overpower the coils and heat them to the point that they melt together or short out and don’t creat the magnetic field required to push the cone. But when that happens, does that make any sound? I’ve destroyed one speaker before, and it made a horrible crackling static sound and then went silent. Another speaker I have still works, but if the bass is loud enough it makes LOUD popping sounds, like that of large packing bubbles. The first time, it only popped once, I couldn’t figure out what it was. But the next time, it did it several times and I promtply turned off my music. It seems to work fine otherwise. I’m just trying to figure out what is happening inside to make that happen.
There are a couple of ways speakers can be damaged.
One is a simple overload of the voice coil. Too much current through the speaker causes the coil to overheat and distort or actually melt. This is pretty drastic, and you’ve got to pour on the watts to get it to happen. Once the voice coil distorts, even if it isn’t burned to the point of being an open circuit, the speaker is pretty much toast, as the coil will no longer move freely in the gap around the magnet, or won’t move at all if is open.
This kind of massive overload will almost certainly cause the type of damage described in the next paragraph as well, but since the driver is already dead, it doesn’t much matter.
A more common type of damage is a lesser overload that doesn’t burn the voice coil, but overextends the suspension mechanism, or what’s usually called the spider. This is what is happening when you hear that sharp cracking sound. This is usually the sound of the voice coil bottoming out against the back of the magnet housing. This may or may not hurt the driver, depending on hard and how often you bottom it out. But it’s best to avoid this, obviously.
If the coil hits the back of housing hard enough, it can distort the voice coil, or distort the spider beyond its ability to recover. Once this happens you may hear the cracking sound more, and at lower volumes. Once the spider has been stretched out, it may move more easily to the fully extended position, causing the bottoming problem, or not support the coil correctly in the center of the gap around the magnet. And if the coil becomes distorted, it may not move freely, or rub on the magnet, giving hums, scratches, or pops. The driver may still be listenable at this point, but at best it won’t sound quite right, and at worst it will sound terrible, making popping & scratching sounds.
Also, it barely possible to overload a speaker enough to cause damage to the surround (the soft, usually foamy looking stuff that holds the cone to the outside edge of the driver’s frame), but you’d destroy the voice coil in the process, so again it doesn’t matter much from a practical standpoint.
Note that the sounds made by these kinds of overloads usually apply only to bass or wide range drivers (the low frequency stuff). Tweeters (which reproduce the high frequencies) usually just die quietly when they are overloaded, and never work again. You might hear a pop, but more than likely the voice coil just burns out, giving you an open circuit, and then no sound.
Ugly
Interestingly enough, often, the cause for speaker failure is, as aften as not, using an amplifier that has too little power. This would apply to the high frequency drivers (midrange and tweeters) in a speaker system.
What will happen is that the amplifier will be overdriven to the poing of clipping the signal, which results in extrame amounts of high frequency harmonics that we hear as distortion. This distortion (sometimes in levels beyond what the amplifier is rated to put out) then overload the drivers and we have fried speakers.
I have known of this phenomenon for years, but recently found good information about it here:
http://www.jblpro.com/pub/technote/lowpower.pdf
One guy brought some Vega subs into Best Buy a few weeks ago, complaining they didn’t work. We looked at them, and we saw the spider had been blown completely clear of the magnet, and was getting caught up on the outside. I estimate he put about 1-1.5k watts into each one to do this. They were rated for, IIRC, 200-300 watts.
–Tim