Sound Quality Measurements

Tech Note
If we aimed for 0.005% THD and 0.05% THD is measured, we know that
something is off as the product doesn’t work as intended. Said
measurement result is needed in order to seek out the problem and correct
it, therefore it is useful in this specific case. Yet what it does not provide is
any indication of sound quality, none at all.
3. Measurements don’t track sound quality
Whole audio systems’ (headphones based and full-sized stereo) frequency response measurement is
the only ‘tool’ designers currently have that shed light on audible quality. There is no other which
would reliably correlate with what most of us would call ‘good sound’. And even though its
usefulness has been proven to a reasonable degree, there are often exceptions. Therefore even this
measurement is far from perfect.
There are strong indications that time-domain’s performance impact is far greater than previously
thought. Yet its measurements methods aren’t standardised thus far and research with proper
correlation has generally low statistical significance.
Past frequency and time-domain performance, as long as - for example - harmonic and
intermodulation distortion is lower than its audibility limit (something that is incidentally not
captured by the common ‘THD&N’ measurement), the distortion itself is inaudible.
Similar qualifications of "as low as it's low enough" apply to noise and pretty much any other current
audio measurement. So once a reasonable basic level of performance has been established in terms
of traditional audio measurements, improving these parameters – ceteris paribus - does not provide
improved sound quality.
As to what is audible, this is a complex multi-dimensional picture and contains material for several
doctorates and a professorship (at least).
# To illustrate the limits of distortion audibility and the utter uselessness of THD as a
measure of quality please consider this.
For a 100Hz tone played at 100dB (arguably very loud) 2nd harmonic distortion lower
than 3% and 3rd harmonic distortion lower than 1% will be completely inaudible. Yet if