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General Asylum General audio topics that don't fit into specific categories. |
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General Asylum General audio topics that don't fit into specific categories. |
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In Reply to: Do measurements matter? - linked article posted by 13th Duke of Wymbourne on January 23, 2015 at 15:39:56:
The ear/brain of listening humans is the only thing that can define whether or not a piece of gear is good sounding or not. Measurements are just numbers that have to be correlated with human perception.
That is why an amp can product 0.0001% THD and still sound sterile and lifeless...it is not the AMOUNT of distortion that is important it is the quality of that distortion and how it impacts the listener. It could also suggest that other distortions that are unpleasant are lurking somewhere that are not covered by this THD measurement.
Speakers have lots of materials related distortion that sits below a perfect looking frequency response curve but for sure has a serious impact on the overall sound quality...driver materials, cabinet materials, bass loading designs etc. all contribute. Crossover parts as well generate different levels and qualities of distortion.
There have been some attempts to correlate what we hear with what we measure and one of the first was by British BBC engineer D.E.L. Shorter when it was noticed that sound quality and the measurements of tube amps were not going hand in hand. At this time the push/pull tube Class AB tube amp with negative feedback had become the standard and sound quality no longer correlated with THD as it is likely to do with no feedback, Class A SETs...although there are lots of reasons those won't correlate so well with a simple measurement either.
Cheever did some nice historical summary in his 2001 master's thesis and also the introduction of a new metric based on how the ear/brain actually perceives (to the best of current understanding) distortion. His metric is actually also SPL related...so the sound level matters too.
Geddes published a couple of good papers on the subject and came up with a metric that had a good correlation (r = 0.9) between listener preferences and different types of distortions that gave a low to high Gedlee number (some of the worst were actually some of the lowest THD).
He even states that they observed NO or even a slightly NEGATIVE correlation between THD and sound quality!! Think about it.
Norman Crowhurst wrote many papers on the subject of negative feedback but the most interesting was one where he pointed out that increasing the negative feedback ends up generating what amounts to a false noise floor that is actually modulated by the signal. This will never show up in a normal measurement but it has the effect of masking low level information that is vital for sonic realism. Why is that different from real noise? Real noise is not correlated with the signal and it has been shown that you can effectively hear music signal correlated sounds BELOW the threshold of the background noise. Now if that "noise" is correlated with the signal then the brain can no longer distinguish between noise and signal because both are correlated and that information gets lost to us.
Finally, Nelson Pass's white paper on what is happening with negative feedback application is interesting because it shows that feedback tends to concentrate distortion rather than really getting rid of it.
So, measurement do matter and SHOULD be used in ways mentioned in the articles above to use what we know about human perception to make better sounding gear. However, this is not being done at all by most designers who have stumbled upon one or maybe two of these ideas but nearly none who are applying these ideas systematically.
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Follow Ups
- They do matter but only when they correlate with subjective results - morricab 09:07:05 01/25/15 (2)
- RE: They do matter but only when they correlate with subjective results - rick_m 12:18:33 01/25/15 (1)
- RE: They do matter but only when they correlate with subjective results - morricab 12:47:42 01/25/15 (0)
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