Wattage Rating

Tour2ma

The Rane link above discusses clipping's generation of HF harmonics. Look for the table around mid-page.
http://www.rane.com/note128.html

While the article dismisses the harmonics as a cause of tweeter failure due to their low power, it chooses in the table to examine the clipping of a 100 Hz tone at 100 W... not a real world example with respect to either the single tone or the power level.

It follows the table with a more realistic example of a composite a HF sine wave atop the LF, but it does not repeat the harmonics analysis for the HF signal. Rather it erroneously states, "Notice that only the low frequency burst portion of the waveform clips...". Sorry, but I don't notice that at all.... in fact it is the HF "rider" signal that intially exhausts the amp's voltage capability and clips before the 100 Hz carrier does in the subsequent extention of the example.

The actual HF frequency is not stated in the composite signal (It looks to be around 3 kHz, but no need to state it if you've already decided to ignore it), but if we assume one of 15 kHz (approaching, but not at the upper limit of music) at a 5 W power level (probably greater than real world, but bear with me) and redo the table to show the generation of ultrasonics, i.e., HF exceeding 20 kHz...

Harmonic - Amplitude - Level - Power - Freq (kHz)
.....1................1/1..........-0.........5.00........15
.....3................1/3..........-9.5......0.55........45
.....5................1/5..........-14.......0.20........75
.....7................1/7..........-17.......0.10.......105
.....9................1/9..........-19.......0.06.......135
....11..............1/11.........-21.......0.04.......165
....13..............1/13.........-22.......0.03.......195

Almost 1 W total of ultrasonics that the tweeter will try, and fail, to reproduce. I say fail because we know from the specs that 24k is about as high as tweeters go. And this failure means that little or none of the electrical energy supplied above this frequency is converted into acoustic energy.

No driver is 100% efficient in converting electrical energy into acoustic. The energy that does not go to acoustic purposes goes to heat generation. I want to say that 60% is about as good as it gets, but I may be off here. However, with the ultrasonics it approaches 0% efficiency. Bottom line is that the ultrasonics' signals go to no good purpose, but only add to the tweeters' heat dissipation load.

So if in the above example a 60% efficient tweeter with a 5 W rating is used, it can safely dissiapate 2 W of heat energy and is fine with the base 15 kHz signal. But add the 1 W of ultrasonic energy and heat begins to build up, and failure will occur if left in this situation long enough.

I stated above that a 5 W - 15 kHz signal might not be real world (the power distribution in music does decrease with increasing frequency), but a music signal is so complex no one wants to deal with its analysis in detail. But any frequency over 3500 Hz is going to add an appreciably powered harmonic thaty exceeds 24 kHz, so with multiple frequencies > 3500 kHz in music, during clipping all generate harmonics that are additive in their impact on tweeters.

I have to think a bit more on the ultrasonics' impact on protection devices, but initial thought is that their added electrical current is just that to a polyswitch or fuse, no different than the music signal's current. Issue is I'd assume that the protection is designed with the tweeter's acoustic efficiency in mind, not the 0% that accompanies the utrasonics.

No claimed expertise here, just a simplified case based on what I read many, many years ago applied to more contemporary theory that does not ring true with me. And there are even more of them out there...

PolkThug

Originally posted by McLoki
I but I cannot figure out how a peak sound that is clipped generates more heat than a peak sound that is not (assuming the same power).

I think a clipped waveform takes on a square shape which creates more stress on the tweeter to try to reproduce it.

John K.

Possibly the best analysis available online of the damage that may sometimes be done to a tweeter when an amplifier clips on high-level lower frequency signals is found on the St. Andrews site . Both the "harmonics" and "compression" hypotheses are analysed and other possible contributing causes are discussed briefly.

Tour2ma

Been through that paper before. Very even handed all the way through to his inconclusive Conclusion...

The main conclusion is that there is no obvious reason indicated by the analysis why we should either rule out any of the above factors entirely, or to assume that only one of them is always (or usually) a sole or primary cause. Which is in practice a more common cause, or whether they tend usually to act together, would be hard to say without more information on typical systems and musical waveforms in order to do a relevant statistical analysis, etc.

Author must be a consultant...

dorokusai

Flip-flopper.

It is a good read, fairly straightforward, nothing groundbreaking.