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Old 16th December 2008, 07:35 AM   #1
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Default Causes for blowing a speaker...

Hi

Recently someone told me that its possible to blow a speaker simply based on the waveform fed into it, not necessarily due to excessive power. Their explanation had something to do with the fact that under some conditions you may be asking the speaker to move in a way that it cannot physically accomplish. I agree that speakers may not accurately reproduce all waveforms but i don't see why this would blow them.

I understand the whole deal with clipping and square like waves delivering more power than sinusoidal type waves but they claim this is not they are referring too.

I am just wondering if anyone could pass me some info on whether or not this is really possible. If there is already another thread explaining something like this please point me to it.

Thanks

scott.
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Old 16th December 2008, 08:14 AM   #2
MadMutt is offline MadMutt  Australia
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Simple.
Square wave will in effect feed the speaker a dc signal.
An alternating positive/negative signal but still a dc signal.
Clipping an amplifier is a very mild form of dc output.

Don't see how you can get a speaker to move in a way that will damage it.
Other than polling on DC.
After all a voice coil can only move in two directions..
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Old 16th December 2008, 10:36 AM   #3
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Default Re: Causes for blowing a speaker...

Quote:
Originally posted by skittletata
Hi

Recently someone told me that its possible to blow a speaker simply based on the waveform fed into it, not necessarily due to excessive power.
Depending on the enclosure, driver, and amplifier a sufficiently high amplitude (Under 100W can do it on drivers with 300W of Pmax) low frequency signal can run the voice coil into the back plate, deform the former, and cause rubbing which leads to failure.

Sealed enclosures are relatively immune because their excursion with respect to frequency at a given SPL is constant for a given input voltage once the frequency drops below their lowest frequency pole.

Dipoles are more at risk because the drivers' poles haven't; increased due to the added stiffness of the airspace in an enclosued box.

With ported speakers the port unloads below its tuning frequency and excursion gets closer to what it would be in free air.

Some drivers have travel limited by their soft components and progressively stiff spiders so they are unlikely to bottom; some don't.
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Old 16th December 2008, 11:08 AM   #4
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http://sound.westhost.com/tweeters.htm
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 16th December 2008, 02:47 PM   #5
Thraxor is offline Thraxor  Canada
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My (very rudimentary) understanding is that some speakers don't handle certain frequencies very well. As a bass player, this was the first thing people told me when shopping for amps. "Stay away from guitar amps 'cause you'll blow 'em in no time." I'm know they are tuned to pick up bass frequencies better, but I'm not sure about blowing guitar amp speakers. I've met people who swear they've done it (or know a guy who did), but that's it. I've never experimented with it either. Expensive experiment if you ask me.

Other than that, I assume if you put 400W of power to a 100W speaker/driver that you'll eventually "blow it" due to unusually high power demands. This would probably result more from sudden peaks during transitions from rests to playing, but that's just a guess as well.

If someone has a difinitive answer, I'd be interested as well. I like Drew's answer since I never thought about severely underpowering a speaker. Good stuff to know. As to the other stuff I mentioned. . . . ???
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Old 16th December 2008, 03:21 PM   #6
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Causes for blowing a speaker..

1. Son
2. Daughter
3. Friend
4. Never me....

Hee hee...

Seriously..I've never over-dissipated a tweeter to death, my son and a friend did, all to to what Rod says.

What Rod has missed is the mechanism that I've caused failures of tweets...Fatigue of the tweeter leadin wire.

Most manufacturers of tweeters bond the coil using epoxy or polyimide, and where it exits the coil pack, they do not support it. It dangles in the breeze, and more importantly, it dangles in the gap. So the drive signal causes the wire to flex. Being solid copper, that can only work for so long.

I had this failure with 4 selenium D205TI 2 inch dia VC compression drivers, so detailed the failure mechanism to the USA Selenium Application rep, as well as how to eliminate it from their design.

If I can find the pic, I'll post it.

Cheers, John
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Old 16th December 2008, 04:25 PM   #7
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Clipping squares off the top of waveforms, which effectively generates tons of high frequency harmonics. This shifts the energy distribution which is normally slanted towards the woofer and causes a lot more power to be sent to the tweeter. This sometimes causes mids and tweeters to blow.

Woofers aren't immune either, especially if they're biamped and the woofer amp clips. Woofers generally have cooling vents that are sized to work best at low frequency, where cone excursion is relatively high. As frequency goes up, excursion is reduced. This is even more true in basshorns and other cabinets that limit excursion. You shouldn't run high frequency signals at high power levels to a woofer, because the cooling vents become ineffective at high frequencies. So clipping can hurt them too, because clipping generates high frequency harmonics.

The most common cause of speaker failure is voice coil adhesion failure. The glue that holds the voice coil to the former becomes so hot it weakens, and the coil unwinds. It begins to rub inside the gap, and eventually wears through or breaks. So the issue is really one of thermal management.
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Old 16th December 2008, 05:48 PM   #8
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Hi

thanks for all the replies.

Quote:
http://sound.westhost.com/tweeters.htm
ya actually i already showed this to the person already that i was debating with. esp is pretty packed with information, and whats discussed within the articlewas more or less my point.

they are alluding to something else apparently.

maybe something like Drew is talking about.

just to clarify though because im not a speaker expert by any means:

the current goes into the coil, which generates EMF from the magnet, and then the force field moves the coil? changes in current change the direction/intensity of the field and therefore change the acceleration of the coil?
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Old 16th December 2008, 08:14 PM   #9
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Quote:
Originally posted by MadMutt
Simple.
Square wave will in effect feed the speaker a dc signal.
An alternating positive/negative signal but still a dc signal.
Clipping an amplifier is a very mild form of dc output.
Just because the voltage is constant for a short period of time does not mean it is DC. The lowest frequency content of a square wave is the frequency of the wave- there is no DC component.

This is relevant because thermally and mechanically a speaker does not behave as if it were fed DC when it is fed a square wave. If you like thinking in the time domain, you might imagine that the cone can never reach the excursion (or current the steady value) it would with DC because before it gets there the voltage has switched and the motor is pulling the cone back (or the current reversing) the other way.
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Old 16th December 2008, 08:31 PM   #10
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skittletata,

IMO the whole thing boils down to two types of failure:

Mechanical failure: overexcursion

Thermal failure: too much power

Except for in extreme conditions (say an SPL vehicle), every cause of failure boils down to one of these. This where you should start when trying to determine if something could cause failure.

To me it sounds like what you are asking about is mechanical failure, and if playing certain types of signals through the speaker can cause this. By using the term "waveform" I think you are trying to say that the cause is not the amplitude of the signal but rather the shape of it.

There is no magic going on here. This is simply a matter of the cone moving too far. This is a function of BOTH the signal type and amplitude. It happens when the amplitude of the signal at some frequency causes overexcursion. For example, you might see this, as noted above, in a ported box below the tuning frequency. Down there it doesn't take much power to make the cone move a long way.
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