Sound reproduction, audio measurements & fidelity: the 10 basic rules

"Thou shalt not worship false idiots."

November 20, 2024

Many areas in life have their own "ten commandments": a list of principles, thinking strategies presented as commandments - such as Richard Feynman’s 10 Commandments for Science and Life. Despite the confusing title, these tips can help us get rid of dogmas, bad ideas, flawed methods and reasoning. Anyway, it would be fun to collect the most important ground rules covering the science and logic behind sound reproduction and audio measurements:

Audio science (science of sound reproduction) is an applied science and not a fundamental science. However, as other applied fields it has roots in fundamental sciences. - Any claim about amplifiers, DACs, audio formats or loudspeakers is a claim about hearing, physics or signal behavior.
Though hearing is a complex process, it doesn't mean that testing the limits of hearing requires extremely complex tools. Any claim about fidelity can be tested with quite simple methods.
An audible difference is only a difference and not the final explanation ("trust your ears" nonsense). By testing with music we can only make claims about the sound and not about the technology or the real cause. The latter requires special experiments.
Blind test is a solution to only one problem: eliminates the possibility of self-deception arising from perception (unreliable auditory memory, visual confirmation bias). Blind test doesn't provide a solution for how to avoid false positive results and if the test is free of errors, correlation still doesn't imply causation. (Rejecting blind test is a nonsense, but trying to prove "everything" with blind tests is also a nonsense. Blind test is a secondary tool.)
Goal is to understand the "mechanism" and not just generating test results. Any fool can do a study filled with statistical analysis (or similar nonsense), the point is to understand. ("Any fool can know the point is to understand.") Real advances in audio technology come from a deeper understanding of signal behavior, human hearing and sound sources (from loudspeakers to musical instruments). The difference between conventional microphone measurements, dummy head measurements and human hearing (two-ear averaging, critical band averaging, precedence effect) is another important subject.
Scientific models are the key to the measurements. Objective assessment of audio fidelity requires a three-level analysis (model):
- an accurate model of the system being measured (system behavior, causes and diagnosis of signal changes in amplifiers, cables, speakers, rooms...),
- a hearing model (how we hear, what we can hear, differences between age groups),
- characteristics of audio signals, sound soruces.
Measurements can be only as good as the models.
It is not the purity of the signal that matters... In an audio system, the goal is to preserve and transmit the signal in such a way that accumulated errors cannot be heard or low enough to not affect playback fidelity.
Thresholds should be based on worst-case (or a collection of "worst-cases"). Identification and analysis of these special cases is important.
What we can hear is determined by the Absolute Thresholds of Hearing (ATH) and auditory masking (and adaptation - a shift in the non-masked threshold). Not only the audibility of compex tones, but even the audibility of nonlinear distortion, noise or resonances is related to masking and ATH.
Audio fidelity is mainly determined by frequency response, nonlinear distortion and noise (necessary for dynamic range). Phase response, group delay response is secondary - almost irrelevant - , since group delay response of DACs and amplifiers is very far from the audible threshold, and simple conventional loudspeaker designs meet group delay threshold criteria (note: group delay is the correct measurement, phase is impractical).

That's all.

Csaba Horváth