SR6500 Tweeter protection

MacLeod

I cant remember if Ive posted this before or not but it is the best example of clipping Ive ever head. I know we're not necessarily talking about clipping here but its still relative and very educational. Its a post from another forum by Manville Smith; super master engineering genius for JL Audio.

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
JL Audio, Inc.

Vestax

Toxis wrote:

distortion is not ONLY a square wave. Distortion is also lack of control over a speaker which can add heat to the VC causing it to breakdown. Distortion comes in many forms and fashions. BTW, you cannot give a speaker 45w of pure distortion and have them not blow if you have the volume at a decent volume. What you're calling distortion in that sense is a signal of noise. Not the same thing.

Can you elaborate on this, I'm curious? So if I were to send 45 watts of "pure" distortion to my subwoofer that has a thermal handling of 200 wrms, in the perfect alignment enclosure, it'll have a chance to blow? Is this at all frequencies?

neomagus00

what is 'pure distortion'? we have to distinguish between electrical distortion (which is relative to the signal coming into the amp) and distortion from the speaker (which is what you hear compared to what goes in the speaker)

the second has nothing to do with blowing a speaker (unless you drive beyond xmax so hard you break it), it's all the first kind that matters. if you send 45 watts of average power (rms), no matter what shape the wave is, it will not blow a speaker that can handle 45 watts or more, ever.

disclaimer: again, this ignores the movement of the speaker causing cooling.

MacLeod

Vestax wrote:

So if I were to send 45 watts of "pure" distortion to my subwoofer that has a thermal handling of 200 wrms, in the perfect alignment enclosure, it'll have a chance to blow?

It would have absolutely no chance in hell of damaging that sub. Remember, the only thing that will kill a speaker (aside from sticking a screwdriver thru the cone) is too much power so as long as that pure distorted signal is below the subs thermal limts, itll never be harmed.

jimmyjam_

Anywhoo just an update when I had my SR tweeters in the kickpanels I had the xover set to +1.5db and it still didn't keep up with the low end. now I have them in the bottom corners of the A-pillars and on -3 db they sound fantastic; really loud without seeing the protection lamp flicker at all. the lesson for the day is that great speakers are nothing without a good installation.

1996blackmax

That goes for car audio in general...