Audible distortion percentage frequency dependence

[SilverEars]

Anybody have any info on this?  Is there variation of audible distortion level depending on frequency?  Also, is harmonic distortion necessarily bad?  Isn't tube sound added harmonics which gives it a different timbre?  Wouldn't that be less noticeable compared to higher ordered harmonics?  You can possibly have high THD and will sound ok, and mostly likely have colored sound one may or not notice, correct?
 
I've read that more harmonic content there are, noticibly unpleasant the sound becomes.  Does higher order become unpleasant because it's in the high frequencies? 

 

[stv014]

  Also, is harmonic distortion necessarily bad?  Isn't tube sound added harmonics which gives it a different timbre?

 
Usually, high harmonic distortion also implies high intermodulation distortion, which can be bad even if it is low order.
 

  I've read that more harmonic content there are, noticibly unpleasant the sound becomes.  Does higher order become unpleasant because it's in the high frequencies? 

 
Higher order harmonics are further away from the fundamental, and are therefore not masked as easily. Additionally, higher order harmonics of relatively low frequencies (< 1 kHz) are more audible because the sensitivity of hearing increases up to about 3-3.5 kHz.
 
In the case of intermodulation between a low and a high frequency tone (for example, loud distorted bass "rattling" the midrange or treble), higher order non-linearity creates sidebands further away from the high frequency sound, and also around its harmonics, which again reduces the effectiveness of masking.
 
Intermodulation between two high frequency tones is different, however, because in that case the second order distortion product appearing at the difference of the frequencies (e.g. 1 kHz from 16 and 17 kHz) can have high audibility due to being in the midrange, and is not masked by the original tones either.

 

[esldude]

I seem to recall, that with test tones the lowest at which THD is discernible is right around .1% which is only possible around 3-4 khz.  The limit rises at other frequencies.  As already mentioned IMD can be more obvious, but I seem to recall it also is around .1%.  Now with music the level at which detection is possible is higher due to masking, and a host of other reasons. 

 

[bigshot]

.1% is the absolute worst case scenario, which rarely exists in the real world. Below 1% on the average is a much more meaningful number when it comes to listening to music on home stereos. More than that for speakers.

 

[SilverEars]

  I seem to recall, that with test tones the lowest at which THD is discernible is right around .1% which is only possible around 3-4 khz.  The limit rises at other frequencies.  As already mentioned IMD can be more obvious, but I seem to recall it also is around .1%.  Now with music the level at which detection is possible is higher due to masking, and a host of other reasons. 

Yeah, found a good article on masking. Looks like THD doesn't tell you enough to know perceptable distortion. Wish they provide Gm values. So would HD from tubes be masked?
 
http://en.goldenears.net/index.php?mid=KB_Columns&document_srl=1335
 

From the article:

 

There is a couple that I should mention on this subject: Mr.Earl Geddes and Ms.Lidia Lee, who have conducted research on the topic. If you are interested, please take a look at: Sound Quality Perception

 

The below diagram represents the envelopes inside which the harmonics are rendered inaudible due to the masking effect of the signal itself. You'll see that lower order harmonics are relatively inaudible as they are inside this envelope, whilst the higher order harmonics relatively stand out as they occur outside the masking zone.

The diagram beneath shows similarly the masking envelopes, but for low level signals; the masked area shrinks dramatically with the reduction in signal, thus exposing the harmonics.

 

For this reason, THD and IMD (which fail to reflect masking) are not the best indicators of how much distortion is actually heard - this necessitates a new metric that has been weighed (so as to make more audible portions have more weight) to better reflect human hearing.

If you want to read more on this topic, try the following:
AES presentations on Distortion Perception I  and  Distortion Perception II

In these papers, the following formula is suggested as the new metric for harmonic distortion, intended to add weighting to the commonly used THD and IMD measurements:

 

This might appear random due to the math, but when the harmonics data were translated using the formula, the Gm value was found to have a close correlation with the perceived sound, where below 1 was inaudible and 3 represented barely audible.
The GedLee Principals concluded that
* THD and IMD have no correlation to the subjectively perceived distortion in a nonlinear system.
* This study offers strong support for the contention that distortion metrics must include some form of masking model.
* A new metric, Gm , is proposed which has been shown to have a very high level of correlation to the subjective perception of distortion in a nonlinear system.

 

[castleofargh]

IMD: day 5, humans still call me harmonic because I'm at 2khz. I'm so crafty I sound just like a 2khz anything. cover op lvl99.