Hearing perception is a funny and tricky area. The type of distortion
will factor into the amount you can stand or detect. I know that odd
harmonics are less obnoxious, often dsirablee compared to even mode
harmonics..
I have indeed built distortion circuits to cause either odd or even type
of harmonic distortion, primarily for use with electric guitars but also
tested to a lesser extent on other audio signals such as mixed music and
voice-only signals.
There is a lot of "conventional wisdom" that odd harmonic distortion is
supposed to be worse than even harmonic distortion. What I have found is
that lower order even is worse than lower order odd. (Mash down peaks
of one side and "expand" the other side, or [lesser] do nothing but mash
down peaks of one side, sounds to me worse than mashing down peaks in a
symmetric manner.)
It appears to me that the intermodulation distortion is worse when
harmonic distortion of a sinewave is towards the 2nd harmonic than when
harmonic distortion is towards the 3rd.
As for audibility of distortion of a single frequency sinewave signal -
to a fair extent audibility of harmonic distortion increases with harmonic
number until the harmonics approach ultrasonic frequencies. Especially,
distortion harmonics at frequencies below 2 KHz will be less audible than
ones in or a little above the human hearing frequency response hump
centered around 2.7-2.8 KHz.
Also, when a distortion test is on audibility of specific harmonics from
distortion of a pure sine wave as opposed to testing for audible
intermodulation distortion, 2nd harmonic "passes-as-least-offensive".
2nd harmonic is same-note 1-octave-up from fundamental.
Audibility of harmonics in a distortion test on a pure sine wave largely
increases as harmonic number increases (as long as the harmonic is not
ultrasonic or approaching ultrasonic), and also is less if it is a whole
number of octaves from the fundamental, second-least if it is a "fifth"
above (1.5 times higher in frequency) a whole number of octaves above the
fundamental.
However, when there is significant harmonic distortion towards 2nd
harmonic as opposed to higher harmonics, ratio of audible offensiveness
from IM distortion to ausible offensiveness of harmonic distortion appears
to me to be maximized.
So, best have a look at the wave form and not just a % THD
meter for this one. Non harmonics related would even be worse and more
disrubing to human perception.
- Don Klipstein (
[email protected])