Guitarplayer_PR wrote:
"There's a problem: I don't think brightness and warmth are opposites at all"
...which is the problem in a nutshell.
In the end I think all you can do is use measurements to look for things that correlate with 'brightness' and 'warmth' as you understand them, and then work from there.
The best example I know of that sort of study was published by Dunnewald back in May of '91 in the 'Journal' of the Catgut Acoustical Society, Vol 1, No. 7 (Series 2), pp 1-5. Subjective judgements of careful recordings were matched with objective measurements of the spectra of a large number of violins of varying quality. The qualities used were 'harsh vs clear' and 'nasal vs un-nasal', and these were defined at the outset using tonal examples. Evaluations of the spectra were done using a computer, which divided the frequency range from 190-6400 Hz into seven bands.
The only absolute quantity that was a good predictor of quality was the level of the Helmholtz resonance in the objective test: if it didn't reach an average of 18 dB in the anechoic chamber with a fixed drive power it was not a good fiddle, no matter what else it did. Otherwise the differences were relative; much more output in the 'B' band (650-1300) than the average of A,C, and D (low, and up to 2850 above the B band) resulted in an 'nasal' sound, and so on. The correlations proved to be in good agreement with the judgement of the market: 8.4% of 'factory' instruments met the objective criteria for a 'good' sound as defined in this study, while 30.7% of 'master instruments before 1800', and 92.5% of 'old Italian violins' did.
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