Audio Quality Measurement - Unquantifiable?

Unquantifiable?

Many audio components are tested for performance using objective and quantifiable measurements, e.g., THD, dynamic range and frequency response. Some take the view that objective measurements are useful and often relate well to subjective performance, i.e., the sound quality as experienced by the listener. An example of this is the work of Toole on loudspeakers. He has shown that the performance of loudspeakers, as assessed in listening tests, are linked to objective measurements of loudspeaker performance. In Toole's work, listening tests were designed to eliminate any potential biases in results. Tests of this sort are called blind (or controlled) tests.

Some argue that because human hearing and perception are not fully understood, listener experience should be valued above everything else. This tactic is often encountered in the "high-end audio" world, where it is used to sell amplifiers with poor specifications. The usefulness of blind listening tests and common objective performance measurements, e.g., THD, are questioned. For instance, crossover distortion at a given THD is much more audible than clipping distortion at the same THD, since the harmonics produced are at higher frequencies. This does not imply that the defect is somehow unquantifiable or unmeasurable; just that a single THD number is inadequate to specify it and must be interpreted with care. Taking THD measurements at different output levels would expose whether the distortion is clipping (which increases with level) or crossover (which decreases with level).

Whichever the view, it should be noted that some measurements have been traditionally used, despite having no objective value. For example, THD is an average of a number of harmonics equally weighted, even though research performed decades ago identifies that lower order harmonics are harder to hear at the same level, compared with higher order ones. In addition, even order harmonics are said to be generally harder to hear than odd order. A number of formulas that attempt to correlate THD with actual audibility have been published, however none have gained mainstream use.

It is claimed that subtle changes in sound quality are easier to hear in non-blind tests than blind tests. Objective performance measurements are said not to fit in with ordinary listener experience. Writing in Stereophile magazine, John Atkinson recalls his experience of an amplifier that performed well objectively and in blind listening tests, but did not sound good in actual use.