A question about sources of harmonic distortion

Ed_Perkins

I saw this statement on another DIY site:  "3rd harmonic peak at 2700 Hz is a product of cone breakup at 8100Hz".  By my understanding/definition of a 3rd harmonic the source of a 3rd harmonic problem at 2700 Hz would at 2700/3 Hz, not at 8100 Hz.  But putting semantics aside - does cone breakup at a higher frequency cause harmonic distortion at lower (sub-harmonic by my definition) frequencies?  I feel I can ask on this forum without the discussion degrading into the typical web forum mess.  Thanks.

Wolf

Usually, it's the lower inducing the higher, from what I've seen. A lot of drivers have a dip at the cone diameter wavelength, and this manifests higher up with a peak or 2 in freq. response. These are also spots where HD will be higher.

dcibel

The statement is just not clearly written. The 3rd harmonic of 2700Hz excites the breakup at 8100Hz, which is why you have a higher distortion at this frequency, a very common trait of metal cone drivers. Not all breakup modes are represented as distortion at the lower frequencies, basically meaning not all drivers need the breakup notched out, but most metal drivers do.

dcibel

Ed_Perkins said:

does cone breakup at a higher frequency cause harmonic distortion at lower (sub-harmonic by my definition) frequencies?

To answer this all you need to do is look as a distortion plot of a metal cone speaker. Here's a chart pulled from Zaphaudio. You can see the 3rd and 5th harmonics are amplified simply because the speaker has a much higher output at those harmonic frequencies versus the fundamental. This is a Seas W18EX.

Ed_Perkins

Thanks - I do see the resulting f3 peak at about 1.5 KHz and the resulting f5 peak at about 1 KHz.   I also found the following on John's website.  Would be nice to see an explanation why notching out the breakup node doesn't reduce/remove the peak that occurs below the breakup frequency.

" A breakup node of 4.2kHz directly correlates to a peak in 3rd harmonic distortion [at] 1.6kHz. Sharp peaks at a particular frequency will always cause a peak in the 3rd harmonic distortion plot roughly 2 octaves below. Notching out the breakup node is not enough to get rid of its effects. The only way to avoid it completely is to cross over below the frequency where the peak is not excited as a harmonic. With the W18 that means crossing over at 1.5kHz or lower."

dcibel

Ed_Perkins said:

Thanks - I do see the resulting f3 peak at about 1.5 KHz and the resulting f5 peak at about 1 KHz.   I also found the following on John's website.  Would be nice to see an explanation why notching out the breakup node doesn't reduce/remove the peak that occurs below the breakup frequency.

It is simply because the generation of that distortion is occuring post-crossover, by the speaker itself. If you look at the plot I posted above, notching out the cone resonance only gets rid of the resonance itself and all the distortion generated by it at ~5kHz. Its important to know that not all speaker cone breakup is accompanied by high distortion in the breakup frequencies, but this peak is common of metal cone drivers which is why the cone resonance of metal cone drivers is generally notched out in the crossover. It is nasty with a long decay, literally rings like a bell.

Lower in frequency in the pass band of the driver and crossover, the driver generates the 3rd and 5th harmonics you see there, this is beyond the control of the crossover as it is a signal generated by the speaker iteself, not by the incoming signal from the amp.

I hope that is clear.