How much tweeter distortion is audible?

[smellygas]

After a stimulating discussion in another thread here, I became curious. At what point is harmonic distortion audible? I know with THD, people say the threshold for audibility is about 1% (or -40dB below the fundamental).

However, different distortion products (e.g. 3rd order vs. 2nd order) are more noxious than others, and so should have different thresholds for audibilty. Also, I would guess that we are more sensitive at different frequencies than others, especially in light of the equal-loudness phon curve. Does anybody have additional information on what levels of various orders of HD at various frequencies are audible?

SG

[smellygas]

I'm thinking that we don't really know. I did stumble across a paper in the AES journal, where the author (Toole) cited another author that claimed a target THD <0.05% was desirable, provided higher-order harmonics were not dominant. 0.05%, though, is very low (-66dB).

Does anyone know? Because there area lot of people (i.e. Zaph, Linkwitz, others) out there that publish tweeter distortion plots, and people here seem to be able to determine that one tweeter is better than the next based on distortion measurements. How can this be done if nobody knows what is audible and what is not?

SG

[Iain McNeill]

I think it's pretty much impossible to predict the ability of a piece of music to mask the audibility of a particular harmonic distortion component generated in playback due to that piece of music. That's why we hear these "nasties" intermittently, only on certain passages.

In tweeters the problem of THD quickly becomes mute. I can't hear the 3rd harmonic of 6KHz while the 2nd harmonic is musical and probably acceptable/preferred. What is probably worst is high THD at frequencies just below the XO freq where the tweeter is still contributing. The harmonics below f3 may well be louder than the fundamental put out by the tweeter (mid driver is dominant here of course but are tweeter harmonics audible?)

In the absence of psychoacoustics & perception knowledge it behooves us to learn all we can to reduce distortion mechanisms.

[Brett]

I agree completely with your post Iain.

Buried somewhere in the Geddes threads, and probably on his site, there are references to tests he did on the audibility of THD using compression drivers. IIRC the levels were very high before becoming audible; something like 10% at HF I think. The lower end of the band would seem to be most likely to be audibly problematic to me, again depending on the driver, xover etc..

Found it here.
Geddes theory page

[smellygas]

Originally posted by Brett
I agree completely with your post Iain.

Buried somewhere in the Geddes threads, and probably on his site, there are references to tests he did on the audibility of THD using compression drivers. IIRC the levels were very high before becoming audible; something like 10% at HF I think. The lower end of the band would seem to be most likely to be audibly problematic to me, again depending on the driver, xover etc..

Found it here.
Geddes theory page

Woh, these were some very interesting papers from Geddes. Thanks for the link! I don't fully understand the math, but he basically showed that there is only a very weak correlation between IMD/THD and listener perception. He didn't include the actual % or dB levels of distortion, which makes it difficult for me to interpret. However, what this likely means is that simple measurement of IMD or THD levels does NOT correlate very well with audible problems with the sound. He does, however, create his own composite formula for quanitfying the amount of distortion present (not strictly HD or IMD) that correlates VERY well with listener preference (we're talking R^2 of 0.9). This tells me that increasing distortion is definitely objectionable, but increasing HD or IMD does NOT necessariliy correlate with objectioable sound. Maybe someone else can interpret the article better than me. Interesting.

SG

[Pan]

Originally posted by Iain McNeill

In tweeters the problem of THD quickly becomes mute.

Nope becasue THD is only a sign of nonlinearity and the result of feeding a sinewave into the system. Music contains plenty of spectral components and other distortion products than what you see with sine waves.

I can't hear the 3rd harmonic of 6KHz while the 2nd harmonic is musical and probably acceptable/preferred.

If you play a 5kHz and 6kHz component you will not only see partials at 10, 15, 12, 18kHz but also IMD products at 1kHz an also some sidebands 1kHz above and below the 10-18kHz series.


/Peter

[Pan]

I skimmed thru some of Geddes texts:

Nonlinearities that occur at low signal levels
will be more audible than those that occur at
higher signal levels.

This is not correct since the ability to hear distortion decreases both as the signal level is decreased and increased.. meaning that there is a range in the middle of our hearing around 80dB SPL or so where we are most sensitive to distortion.

To get a rough idea of what can be heard, play sinewaves on your speakers and headphones. THD levels at 0.5% is easily heard as unpleasant coloration on a sinewave.

On the Klippel website there is some interesting tests that can be done as well.

To me and many ohers speakers with low distortion sounds better than high distortion and that should be read as 0.1% is low while 0.5% is high for medium SPL's.

A speaker with 10% distortion would make me press "paus" or get out of the room.


/Peter

[Iain McNeill]

Originally posted by Pan
To get a rough idea of what can be heard, play sinewaves on your speakers and headphones. THD levels at 0.5% is easily heard as unpleasant coloration on a sinewave.

I disagree. We don't listen to sine waves and music will have MUCH more masking ability than a single tone. The percentage values that you can hear with sine waves won't have any relevance to distortion audibility in music.

BTW, generating a specific % THD is pretty difficult unless you're just mixing in a single harmonic component. I'd really like to have a box that added a certain amount of distortion to a music signal, with control over the harmonic ratios as well. Kinda like a precision fuzz box

[gedlee]

Originally posted by smellygas


Woh, these were some very interesting papers from Geddes. Thanks for the link! I don't fully understand the math, but he basically showed that there is only a very weak correlation between IMD/THD and listener perception. He didn't include the actual % or dB levels of distortion, which makes it difficult for me to interpret. However, what this likely means is that simple measurement of IMD or THD levels does NOT correlate very well with audible problems with the sound. He does, however, create his own composite formula for quanitfying the amount of distortion present (not strictly HD or IMD) that correlates VERY well with listener preference (we're talking R^2 of 0.9). This tells me that increasing distortion is definitely objectionable, but increasing HD or IMD does NOT necessariliy correlate with objectioable sound. Maybe someone else can interpret the article better than me. Interesting.

SG

Let me interpret since I did the work. The "weak correlation" of THD and IMD to perception was NEGATIVE - you forgot to mention that. This means that, as a metric of perception, one should increase the THD to make it sound better. This is of course absurd, but it's the absurdity of THD and IMD measures that is the culprit not the results of the test.

The THD and IMD numbers are indeed shown in all the graphs. They are the X-axis.

Its NOT the level of the harmonics that matter but where the nonlinearity occurs - at low levels or high levels and the order, 2nd, sixth, etc. Low level nonlinearity is by far the most insidious especially if high order - like crossover distortion in an amp. This is why an amp with extremely low levels of THD can still sound terrible. But loudspeakers, on the other hand, tend to have nonlinearities that increase with level and are likely very low level like second or third. This makes them fairly benign. In fact, for the most part, nonlinearity in a loudspeaker (as long as its not broke) is a non-issue. In a test of compression drivers we had twenty five people evaluate distortion levels up to 25% and statistically noone could detect it at those levels.

Its hard to make blanket statements about the audibility of nonlinearities in specific cases, but for the most part nonlinearity is a major concern in electronics, but so much so in loudspeakers.
There are, of course, going to be exceptions to this.

[gedlee]

Originally posted by Iain McNeill
BTW, generating a specific % THD is pretty difficult unless you're just mixing in a single harmonic component. I'd really like to have a box that added a certain amount of distortion to a music signal, with control over the harmonic ratios as well. Kinda like a precision fuzz box

I have a MathCAD program that will do that, but you need MathCAD to run it.

Maybe I can post some examples if we can agree on what to post. (Agreement on this topic is all but nonexistant.)