THD above 10khz - why does it matter?

[preload]

This is probably a dumb question, but I notice that it's common to show THD levels measured at frequencies above 10khz - however, at frequencies above 10khz, the 2nd harmonic is above 20khz, the 3rd harmonic is above 30khz, etc. We are all lucky to even hear 18khz. So why does THD above 10khz matter?

 

[Inner Space]

We won't hear the harmonics themselves, but they might introduce IMD at audible frequencies; in a wide-bandwidth system they use amplifier power better employed elsewhere; and they might set off a super-HF resonance in the tweeter, which will screw up its audioband response.

 

[preload]

We won't hear the harmonics themselves, but they might introduce IMD at audible frequencies;

Thank you. I have a couple of clarifications questions, if that's okay.
Why would IMD induced by a single >20khz spike at a very low level, say -60dB, added to the other frequencies stimulated by music be important?
Or put differently, if you were listening to music at "100dB," and I added a 25khz sine wave at 40dB, would that conceivably introduce enough IMD to be even remotely audible? Even at 100% IMD, that 25khz is going to generate only 40dB IMD products.

in a wide-bandwidth system they use amplifier power better employed elsewhere

Right, but again, the level of 2nd harmonic distortion is going to be very low, so I can't imagine more than a miniscule amount of power being used to drive it.

; and they might set off a super-HF resonance in the tweeter, which will screw up its audioband response.

And once again, the level of the 2nd harmonic distortion signal is going to be very low, and even if it did cause a resonance at the precise resonance frequency of the tweeter, the resonance would still be inaudible. I'm not clear how that would "screw up" the audioband response.

 

[Inner Space]

I said "might" throughout, because you're right: if THD levels are low enough, not much harm will be done lower in the frequency band. But non-harmonic IMD products can be damaging even if quiet.

About the tweeter: it's not that you would hear the supersonic resonance way up there above the rest of the perfectly reproduced music, but that excitation of the resonance will likely break up the stiff dome and turn it into a floppy mess, suddenly trying to go every which way, which means it won't be reproducing any of its passband without gross negative effects.

 

[preload]

I said "might" throughout, because you're right: if THD levels are low enough, not much harm will be done lower in the frequency band. But non-harmonic IMD products can be damaging even if quiet.

Thanks, Inner Space. However, I'm still not sure what you're saying makes intuitive sense to me. Let's say, that the H2 THD product for a 10kHz fundamental occurs at 20khz, at a level of -60dB. If we're playing music at a level of 100dB (for the sake of argument), that would mean that H2 has a level of 40dB. What you appear to be saying is that the introduction of a 40dB H2 at 20kHz can result in "damaging" IMD. If so, what dB level are you suggesting the IMD products would occur at based on this 40dB stimulus?

About the tweeter: it's not that you would hear the supersonic resonance way up there above the rest of the perfectly reproduced music, but that excitation of the resonance will likely break up the stiff dome and turn it into a floppy mess, suddenly trying to go every which way, which means it won't be reproducing any of its passband without gross negative effects.

Interesting, can you give me an example where you've seen this phenomenon (supersonic resonance triggered by a THD product) described or reported? Or is this perhaps more of a personal theory?

 

[Inner Space]

Let's say, that the H2 THD product for a 10kHz fundamental occurs at 20khz, at a level of -60dB. If we're playing music at a level of 100dB (for the sake of argument), that would mean that H2 has a level of 40dB. What you appear to be saying is that the introduction of a 40dB H2 at 20kHz can result in "damaging" IMD. If so, what dB level are you suggesting the IMD products would occur at based on this 40dB stimulus?

... can you give me an example where you've seen this phenomenon (supersonic resonance triggered by a THD product) described or reported? Or is this perhaps more of a personal theory?

It won't be just 10k's H2. The octave 20k - 40k could be sprayed full of harmonic spuriae, the H2s thru H5s, say, of lower pitched fundamentals. If two (or more) of them are close in frequency, they will "beat" and spray their own "wolf tones" around, downward as well as up, and given that the intermodulation product will be non-harmonic and possibly dissonant, the result might be audible even at a nominally low level.

The speaker world is almost 100% full of cones and domes that break up when driven by frequencies beyond what they're designed for. It happens with my speakers - they're nicely controlled up to 30k, but at 35k there's a gigantic tweeter break-up resonance. Normally of no concern, but if, say, an H3 distortion spike from an 8.75k tone hit it hard enough, then it would lose its shape, become annular and floppy, and produce nothing but garbage from its crossover frequency upward.

Neither thing is a real-world worry with competent electronics, which is another reason for buying competent electronics.

 

[preload]

Neither thing is a real-world worry with competent electronics, which is another reason for buying competent electronics.

I guess that was my original question. So how would you defined "competent electronics?" More specifically, at what THD% would THD above 10khz start to matter?

 

[LTig]

intermodulation product will be non-harmonica nd possibly dissonant

This.

 

[preload]

This.

Helpful

 

[Inner Space]

I guess that was my original question. So how would you defined "competent electronics?" More specifically, at what THD% would THD above 10khz start to matter?

For competency, I like the markets where engineers sell to engineers. Numbers there are usually good to spectacular. How bad would numbers have to get for supersonic HD to matter? It would presumably be only one problem among many, but in and of itself I wouldn't want it to be higher in level than elsewhere.

 

[wynpalmer]

This is probably a dumb question, but I notice that it's common to show THD levels measured at frequencies above 10khz - however, at frequencies above 10khz, the 2nd harmonic is above 20khz, the 3rd harmonic is above 30khz, etc. We are all lucky to even hear 18khz. So why does THD above 10khz matter?

At least one reason is that 20kHz harmonic distortion measurements can be used as an easy proxy test for Transient InterModulation distortion- which is indeed audible and has been determined, as of the 1980s with Mati Otala's work, to be one of the biggest sources of loss of audio fidelity.
It requires a measurement bandwidth that includes at least the second harmonic, and preferably the third, substantially unattenuated.
Also, many amps have relatively low gain bandwidth product, and as a result feedback factors are reduced at high frequencies allowing the distortion to rise. There is an audible relationship between the level of feedback and dischordant harmonics generated in amps. Basically zero feedback factor even in a fairly non linear amp (such as a SET) results in a pleasing audible output, but increasing the feedback factor a bit degrades the sound appreciably (it peaks at about 5-15dB depending on harmonic) and it's only at very high feedback factors that the sound becomes pleasing again.
Measuring the rise in distortion versus frequency allows you to estimate the GBW and the behavior of the dischordant harmonics.

 

[Human Bass]

Frankly, i believe that it would be better to nullify any frequencies above 20k, so there is no risk to overload the tweeter and jeopardize the audible stuff its doing.

 

[preload]

At least one reason is that 20kHz harmonic distortion measurements can be used as an easy proxy test for Transient InterModulation distortion- which is indeed audible and has been determined, as of the 1980s with Mati Otala's work, to be one of the biggest sources of loss of audio fidelity.
It requires a measurement bandwidth that includes at least the second harmonic, and preferably the third, substantially unattenuated.
Also, many amps have relatively low gain bandwidth product, and as a result feedback factors are reduced at high frequencies allowing the distortion to rise. There is an audible relationship between the level of feedback and dischordant harmonics generated in amps. Basically zero feedback factor even in a fairly non linear amp (such as a SET) results in a pleasing audible output, but increasing the feedback factor a bit degrades the sound appreciably (it peaks at about 5-15dB depending on harmonic) and it's only at very high feedback factors that the sound becomes pleasing again.
Measuring the rise in distortion versus frequency allows you to estimate the GBW and the behavior of the dischordant harmonics.

Interesting. So you're saying that thd at 20khz correlates with other types of distortion, like TIM? There's probably an obvious answer to this, but why not directly measure the other types of distortion instead of guessing that they will be audible? Also does the relationship between supra sonic THD and "other types" of distortion hold true for all amps? Or is it just some amps with a specific design? What about class D? Thanks

 

[wynpalmer]

Interesting. So you're saying that thd at 20khz correlates with other types of distortion, like TIM? There's probably an obvious answer to this, but why not directly measure the other types of distortion instead of guessing that they will be audible? Also does the relationship between supra sonic THD and "other types" of distortion hold true for all amps? Or is it just some amps with a specific design? What about class D? Thanks

20kHz harmonic distortion correlates with TIM, specifically, as TIM appears when the required slew rate is about 10% or so (as a rule of thumb) of the amps slew rate- with lots of caveats based on specifics of the amps architecture.
There are a couple of tests that seem to correlate reasonably well with human perception- one is the magnitude of the high order odd harmonics- for example ninth- they don't seem to be well masked in the auditory process and can be perceptible even at relatively low levels. These harmonics are often generated in the application of negative feedback and the sensitivity grows at a rate higher than the harmonic ratio- for example a 9th harmonic might need to be > 40dB lower than a just perceptible 2nd harmonic in order to be imperceptible.
The feedback ratio generally (but not always) falls with increasing frequency in the audio band due to finite GBW products and finite gain for the amps, and the relative increase in harmonic distortion at 20kHz can provide a measure of this- or you could just look at the data sheet ;-).
The best test for correlation with audio quality seems to be a non-evenly spaced many tone input signal with a 1/f power spectrum to emulate music.
This input signal should extend to say, 40 kHz, as out of audio band signals can IM back into the audio band.
This type of input can degrade the SINAD of the output signal. introducing a sea of spurii that causes the effective noise floor to increase.
In the case of real music these randomized spurii appear as audible "fuzz", degrading resolution and imaging and introducing dischordance into the music. Well designed amps, even those with global negative feedback- if the feedback factor is large enough- or those with zero negative feedback, will pass this test and will be subjectively transparent, in my experience. Generally bipolar input stages are worse than FET ones due to the larger number of meaningful distortion components that are generated by the exponential gain characteristic versus a near square law gain characteristic.
However, the higher the feedback factor the smaller the input signal to the distorting input stage is, which reduces the distortion generated and hence the distortion generated by the distortion due to the application of the feedback loop- which reduces the generated higher order harmonics etc. (3rd harmonic distortion in the output, for example, generates 9th harmonic also when the feedback loop is closed).