Audio enthusiasts are often presented with technical specifications and measurements extoling the virtues of one make/model of equipment over another. A common metric is total harmonic distortion (THD) which is often expressed as a percentage. THD basically tells us what percentage of the audio coming out is a not in the original source material going in but is in fact being “manufactured” as it were by the playback audio equipment usually as dwindling multiples of the original frequencies in the music.
Manufacturers and audio engineers will often use THD as an indicator of overall sound quality. As is, equipment item A has lower THD than equipment item B, it therefore follows that equipment item A sounds better than item B. While this “can” be true, it is most often not true in any practical sense.
The problem with using THD as an objective measure of sound quality is that below a certain threshold level, human beings simply can not discern different levels of distortion. Sure, it’s there. You can measure it with the right equipment. Except you can’t hear it (below a certain level). Which raises a fair and interesting question. If you can’t hear something, does it matter that it’s there?
For example, let’s say equipment item A has 0.01% THD while equipment item B has 0.1% THD. If I then tell you that your average normal human being can’t discern any distortion – AT ALL!! – at levels below 1%, can you reasonably expect to benefit from equipment item A having better measurements than item B?
A brief compilation of studies into the ability of humans to perceive distortion in audio playback is listed in the table below. A fair summary of these findings is that distortion (total harmonic distortion) levels below ~1% are not audible.
| Study/Report Author & Date | Subject | Results/Conclusion | Misc. Notes |
|---|---|---|---|
| James Moir/1981 | Just detectible distortion (JDD) levels | JDD "level can be no lower than 1%" | Assumed non-linear distortion was the primary type of distortion involved. JDD levels dropped as listeners learned to recognize distortion. |
| D.E.L Shorter/1950 | Sound quality of systems with known quantities of harmonic distortion | Just perceivable distortion values of 0.8% to 1.3% | Multiplying harmonic amplitudes by n^2/4 (n = harmonic order) before rms summing produced better correlation between objective measurement and subjective assessment. |
| P.A Fryer/1979 | Listening tests for intermodulation distortion | 2% - 4% distortion detectable in piano music, 5% in other types of test signals | Distorted test material using 1st order IM distortion products |
| Von Braunmühl & Weber/1937 | Distortion sensitivity at selected frequency bands | 1% - 2% at frequencies > ~ 500 Hz | Noted that at lower frequencies JDD levels could go much higher |
| Harry F Olson/1940 | Just detectible distortion (JDD) levels | JDD level of .7% using 40 Hz to 14 kHz bandwidth test system | Restricting system to 4 kHz doubled the JDD level |
Information in the table above was sourced from the following online post: https://www.audioholics.com/room-acoustics/human-hearing-distortion-audibility-part-3