"Total Aural Dissonance" - the proper metric?

[jackinnj]

Cheever recommends a "figure of merit" -- Total Aural Dissonance -- achieved by a dynamic intermodulation measurement -- as being more consistent with what the ear actually hears (and creates due to the self-harmonics in the ear.)

I read Cheever's Master's thesis (linked as a 1 meg PDF here: http://w3.mit.edu/cheever/www/cheever_thesis.pdf and was wondering whether anyone on DIYAUDIO wants to offer a critique? Cheever lays the blame at high negative feedback, btw.

[Prune]

There is another metric proposed here, that has been weighted specifically to represent human perception of distortion:
http://www.gedlee.com/distortion_perception.htm
(check out the PowerPoint prensentations and sample wavs)

[SY]

Cheever's thesis has been discussed to death. Exceedingly unimpressive. He goes wrong on page 1 and heads off from there. If I were on his committee, I'd have failed him.

[jackinnj]

Originally posted by SY
Cheever's thesis has been discussed to death. Exceedingly unimpressive. He goes wrong on page 1 and heads off from there. If I were on his committee, I'd have failed him.

Well, you have higher standards than MIT ! -- but even Richard Feynam had problems with MIT (and talk about things to do in the snow at Cornell with the faculty wives.)

[SY]

Jack, I was honestly astonished that they passed this thesis. No knock on MIT, it's a great school. But even the best let something slip through every once in a while.

To be fair, my own thesis was pretty terrible. I cringe when I read it these days.

[jackinnj]

i guess that there was some discussion on RAHE (rec.audio.high.end), and I don't really pay that much attention to the high end audio magazines.

i know from my wife's work in molecular biology (getting her PhD at age 55) the work which goes into a thesis or peer-review journal article is astounding -- it was so much simpler 30 years ago when I got my masters. she's been getting home every night ~10:00 pm -- maybe I should check to see if she has a paramour! javascript:smilie('')

[jcx]

Some people here can't read - cheever's thesis was a master’s thesis submitted to University of New Hampshire

it's still embarrassing to some of us that Sussman gave it any attention but the cheever thesis pdf is merely hosted on one of mit's machines, hardly an endorsement by the institute

[SY]

jcx, thanks, I really missed that one.

[Prune]

Can anyone comment on the proposal in the link I posted?

[CeramicMan]

I think there may be some merit in the theory that some distortion can be masked if the non-linearity that causes it is similar to the way that human hearing is non-linear. (Brain-wave: that would explain the seemingly ludicrous and drunken notion of there being audible differences when changing the "absolute polarity" of loudspeakers, etc....)

I've done a few subjective and rather unscientific experiments before on this using a modular soft-synth, and I compared:

-a distorted sine wave with harmonics that decay nearly exponentially as frequency increases, where the 2nd harmonic is at around -40dB

with

-a distorted sine wave where 2nd harmonic is still the loudest distortion component, but this time it's at around -58dB, and the higher harmonics decay more gradually, forming a flatter "distortion floor".

The first sine wave actually sounded cleaner. I could still hear the harmonics, but there wasn't any obvious un-sine-wave-like distortion. It sounded close to a pure sine wave, like a whistle tone approximates a sine wave.

The second sine wave sounded like a purer sine wave except that it was marred by a trumpet-like sound playing the same tone in the distance.

I wouldn't want an amplifier that produces either of those 2 distortions though, because IMD was still obvious when playing more than one note. I don't think we should go back to the days of valves amps either. My cheap-n-nasty sound card had easily enough resolution for the above experiments, so there!

After reading that thesis for hours and hours, I felt a bit ripped off at the end because there weren't any real ideas on how to improve the sound quality of amplifiers, just a supposedly improved method of critiqueing other people's designs. Plus, I'm not fully convinced about being able to use aural masking to hide distortion. Surely the distortion products would therefore form new harmonics in our ears that would then be audible?

CM