How much distortion can we hear?

[maylar]

Or... are THD numbers actually meaningful?

Given that the current technology produces amps that are way below 0.1% THD, what significance is there to this spec in the real world?

Can humans hear the difference between 0.05% and 0.1%? Does it matter if the distortion content is even or odd harmonics?

[Duo]

It seems to me that distortion figures and audible distortion are two entirely different creatures co-existing in a world of purists and realists. Then there are also those persons who don't care.

Interesting to me is the fact that almost every distortion figure I see is obtained during maximum power output and definitely doesn't clearly define the distortion over the complete audio bandwidth of the system.

Personally, I've listened to all kinds of amplifiers that sounded good and bad with absolutely no relation to their ratings.

Some real pieces of junk have sounded better than some more expensive stuff.

[udip]

Originally posted by maylar
Can humans hear the difference between 0.05% and 0.1%? Does it matter if the distortion content is even or odd harmonics?

Human's ear is very sensetive, and can detect subtle changes.
Even harmonics are much more pleasant to the ears and sometimes even considered as "warmth" added to the sound.
odd harmonics are less natural and considered "harsh".
Tubes, for example, have much worst distortion figures
than transistors but many people prefer tubes.
Why is that? Because tube distortion is mostly even harmonics.


Just my opinion...

regards,

Udi.

[smoking-amp]

Set the volume on your system to a confortable level. Assuming you have a nominal 8 Ohm speaker, connect an 8000 Ohm resistor in series with the speaker. Whats left is about 0.1%, can you still hear it? Try 80,000 Ohms for 0.01%.

Don

[jcx]

I think there really is pretty good info on what constitutes CLEARLY AUDIBLE distortion - mp3 and other compression schemes are based on tossing out stuff that is perceptually masked by the music signal - a reasonable assumption is that since the presence or absence of signal in masked frequency components doesn’t matter then similar levels of distortion at the masked frequencies also won't matter – a distortion component is very likely to be audible if a good perceptual coder decides to use bits to encode it

the lesson from these perceptual models is that harmonics of a signal frequency component are likely to be masked - especially when most musical instruments produce harmonic series of their fundamentals anyway

so strictly speaking "harmonic" distortion is not very audible and THD # don't predict distortion audibility and serious audio engineers have known this from day one so much of the controversy is clouded by debates misusing a very uninformative #

engineers designing amplifiers for audio and radio have known since the beginnings of the field that intermodulation distortion components seriously degrade channel quality - intermodulation components of complex signals contain sum and difference frequency components of all of the possible orders

many harebrained theories have be proposed that sound reasonable in light of harmonic structure in music and masking but utterly fail to do the math for Intermodulation Products, it is impossible to have only “harmonic” distortion of a complex signal

there is a link between harmonic and intermod - a nth order harmonic of a single tone is evidence of a nth order distortion mechanism (some part of the system is producing a signal proportional to v**n ) - the intermod products for 2 tones acted on by a nth order nonlinearity include all possible frequencies formed by a*f1 +/- b*f2 where a,b assume all possible integer pairs that add up to n

the # of intermod frequencies increases rapidly with the order of the nonliearity causing the distortion and the # signal components, energy in the IMD products greatly exceeds the "harmonic" components at n*f1, n*f2 - for 70+ yrs engineers have explored "weighting factor" formulas to multiply the easily measured harmonic distortion components by rapidly increasing functions of n in attempts to get # that correlate somewhat with perceived quality; they largely failed to get much beyond the general conculusion that high order distortion is worse that lower oreder distortion

the current approach that appears to have some (still modest) success compares the undistorted and distorted signals’ properties after sending them through a complex model of the 1st layer of perceptual coding to account for masking of all of the distortion products, harmonic and intermod - nobody is offering absolute distortion audibility indexes, they just rank relative levels, also this is still quite poor in resolution compared to the judgments of subtleties you will find some audiophiles claiming to perceive (but just see them run when you mention abx/double bilnd testing)

at moderate levels of distortion THD is useless, presumably at Halcro’s sub ppm levels it could put a bound on the likely IMD

distortion shouldn’t be oversimplified and designers pursuing “low distortion” should really be talking about IMD with complex signals, but THD and minutiae of the “harmonic distortion structure” are likely to continue to fuel debates for some time

[janneman]

Originally posted by smoking-amp
Set the volume on your system to a confortable level. Assuming you have a nominal 8 Ohm speaker, connect an 8000 Ohm resistor in series with the speaker. Whats left is about 0.1%, can you still hear it? Try 80,000 Ohms for 0.01%.

Don

Don,

That's an interesting way to roughly check it (it doesn't account for masking). Just out of curiosity, have you done it? What's your experience with this, can you still hear it?

Jan Didden

[ingrast]

I wholeheartedly agree with jcx. Depending on how you look at it, THD may mean lots or next to nothing.

Extremely low THD is the hallmark of a fairly linear system, barring slew rate hard limits exposed with higher frequency test tones. In this condition it may well be expected IM to be low.

THD on the other hand is a bundled quantity resulting from the sum of spurious powers distributed within the audio band. Completely different spectral structures may lead to the same number though the actual listening results will be very different.

The bottom end in my view is THD can be considered a valid quality parameter as long as it is a very low figure (probably much better than 0.01%), up to 20 KHz.

Rodolfo

[PMA]

Too many phenomena still unexplained - and will be left unexplained regardless this discussion .

[rdf]

Great reply jcx, any links to references for further study?

[maylar]

Thanks for sharing your thoughts on this. The reason I ask is that it's common to see on other forums a comment like, "You can't hear less than 0.1% distortion anyway, so who cares if the spec is .02 or .08?". I'm just wondering if there's any truth to that.

Still others claim that speakers have 10 times the distortion of amplifiers anyway, so amp distortion is insignificant.

Good point about IMD, jcx. It's one spec that doesn't get much attention and probably should.

Too many phenomena still unexplained - and will be left unexplained regardless this discussion.

No doubt you're correct.