A by-product of using too small of an amplifier kills speakers [If the output is not enough the user turns up the gains {15% distortion is hard to hear, at subbass frequencies} which leads to more average power over time ].
Manville quote:
The only thing that thermally damages speakers is power... more specifically: average power over time.
I'll explain...
If you take a given amplifier, let's say 100 watts and operate it just below clipping with music material, the "Crest Factor" of the amplifier's output is equivalent to the "Crest Factor" of the program material.
"Crest Factor" is the difference between the average level of the signal and its peak level. For example, a pure sine wave has a "crest factor" of 3dB, meaning that it's peak level is 3dB higher than its average level. We all know that 3dB represents a power factor of 2, so another way to look at it is that the peak power of the signal is twice that of its average level. So, if we play a sine wave on our 100 watt amplifier, just below its clipping level, the average power (over time) the speaker is needing to dissipate is 50 watts.
A true square wave, by comparison, has a crest factor of 0db, so it has equal average and peak power. Our 100 watt amplifier, playing a square wave, unclipped, into our speaker requires that the speaker dissipates 100 watts of power (twice the heat as a sine wave).
Music has a significantly higher crest factor than sine waves or square waves. A highly dynamic recording (Sheffield Lab, Chesky, etc.) typically has a crest factor of 20dB or more, meaning that its average power is 100 times lower than its peak power. So, if we play our 100 watt amplifier just below clipping with the typical audiophile recording our speaker is only needing to dissipate 1 watt of average power over time.
Modern commercial recordings typically exhibit crest factors of around 10dB, meaning that the average power is 10 times lower than the peak power. So, our 100 watt amp just below clipping would deliver an average power over time of 10 watts that the speaker has to dissipate.
Okay, so what happens when we clip the amplifier (which we all do at times). When the amplifier enters into clipping, the peak power no longer increases, but here's the KEY... THE AVERAGE POWER CONTINUES TO INCREASE. We can often tolerate a fair amount of clipping... as much as 10 dB or more above clipping with a reasonably dynamic recording... a bit less with a compressed commercial recording.
So, if we turn the volume up 10dB higher than the clipping level with our Sheffield Lab recording, we have now reduced the crest factor of the signal reaching the speakers by 10dB... so instead of needing to dissipate 1 watt average, we are asking the speaker to dissipate 10 watts average, and we're probably ok.
If we turn up the volume 6dB past clipping on a compressed commercial recording (or bass music recording), we have taken the crest factor of the signal from a starting point of 10dB to only 4dB, asking the speaker to dissipate an average power of 40 watts instead of 10 watts... that's FOUR TIMES the average power, which generates four times the heat.
SO, in most cases, the reason clipping can damage a speaker really has nothing to do with anything other than an increase in average power over time. It's really not the shape of the wave or distortion... it's simply more power over time.
When someone plays Bass Mekanik clean (unclipped) on a 1000 watt amplifier the average power is 100 watts (10dB crest factor). You can also make 100 watts average with Bass Mekanik by heavily clipping a 200 watt amplifier.
If someone is blowing a woofer with 200 watts of power due to a lack of restraint with the volume control... they will blow it even faster with a 1000 watt amplifier because they will probably turn it up even more and now they have more power to play with... this is the recipe for aroma of voice coil.
When woofers are rated for power, an unclipped signal is assumed. We use test signal with a crest factor of 6dB for power testing and can run a speaker at its rated power for hours and hours on end without thermal or mechanical failure. For example, a W1v2 can dissipate 150 watts average power for eight hours or more with signal peaks of 600 watts. So, we rate the speaker for 150W continuous power. This way, when a customer needs to choose an amp for it, they will hopefully choose one that can make about 150 W clean power... Even if they clip the bejeezus out of that amplifier, it is unlikely that the speaker will fail thermally. This is a conservative method, but it needs to account for the high cabin temperatures in a car (think Arizona in the summer) which significantly impacts heat dissipation in the speaker. A top plate that starts at 150 degrees F is not as effective at removing heat as one that starts at 72 degrees F in the lab... and this affects the ramp up of heat in the coil.
DISCLAIMER: The frequency components of clipping can affect tweeters due to their low inductance and lack of low-pass filtering. Clipping essentially raises the average power of high frequencies to a point that can damage tweeters... Woofers and midranges couldn't care less about these high frequency components because their filtering and/or inherent inductance knocks that stuff out of the picture.
Best regards,
Manville Smith
ok... from a non-audio professional, (but a big enthusiast, and car audio geek) I see one word here being misused a lot, and that word it.. dum dah dah dummmmm
Watt
We are not talking about watts here, we're talking about volt-amperes, and that makes the whole issue come together really. When we talk about watts, we are talking about the actual output of the speaker, the amount of electricity running through the coil that is actually doing work. In an inductive motor (such as oh, I don't know, a speaker) you have a massive amount of power lost due to the inductance of the coil, as well as moving the speaker cone, there is capacitance as well, in the windings of the coil, but generally that is by far overshadowed by the inductance generated by the moving coil. Lets look at inductance first, the inductance of the coil puts the voltage delivered by the amplifier, and the current drawn by the coil at that voltage out of phase, which causes a large reduction in wattage, this power is lost as reactive power, or VAR (volt-amperes, reactive) I have seen (calculated acutally) speakers that are being fed 600 volt-amperes from an amplifier, but put out less than 60 watts of power, at some frequencies, the rest is lost to inductance, as well as frequency and box related issues.
All this said, when a small amount of DC is a componant in the amplifier output, the voltage and current are in phase for that amount of time, causing a HUGE surge in the wattage that the speaker actually sees, imagine going from a 60 watt output, to a 600 watt output in a split second. As Mr manville Smith said, speakers are rated with an assumption of clean power coming from an amplifier, any DC output changes not only the amount of output, but the carachteristics of that output dramatically. in industry we call it "Power Factor" but it gets overlooked in car audio completely.
anyways, that's my view, take it or leave it.
"So if you run it with less power and even though you are not getting enough bass out of it, no harm will befall your driver."