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

"Without debating, life would be a mistake." - Oscar Wilde after many debates

November 20, 2024

Audio measurements, problems related to audio fidelity or sound reproduction can be a mess for "newbies". Without a compass it's easy to get lost in the noisy jungle of flawed reasoning, false analogies and junk philosophies (not to mention marketing frauds). The following is a list of the most important ground rules covering the science and logic behind sound reproduction and audio measurement:

1. Audio science is an applied science

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.

2. Audible difference... is only a difference

An audible difference is only a difference and not the final answer ("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.

We can also add that the analytical and focusing ability of our hearing system is very limited, especially when compared to vision. Separating one part of an image from the rest is a simple task, but separating one part of the music from the rest - e.g. an instrument in an orchestra - can be extremely difficult. Comparing minor audible differences is a difficult task as well.

3. Goal is to understand... (signal behavior, human hearing, sound sources)

Goal is to understand the "mechanism" and not just generating test results. Any fool can do a study filled with statistical analysis "oompa loompa", the point is to understand. ("Any fool can know the point is to understand.", said by a physicist who promoted a weird hairstyle and developed an even more weird theory about energy, milk and coffee.)

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 is another important subject (two-ear averaging, critical band averaging, precedence effect).

4. Blind test is a solution to only one problem!

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 tests shouldn't be used as a primary tool, only as a secondary tool to support auditory measurements.)

Blind testing methodology is the most misunderstood topic in audio. On both sides. Moreover, the exclusive use of tests can lead not only to misconceptions but also to superficial knowledge.

5. Scientific models are the key to measurements

Scientific models are the key to 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.

6. What we can hear...

What we can hear is determined by the Absolute Thresholds of Hearing (ATH) and auditory masking. Not only the audibility of compex tones, but even the audibility of nonlinear distortion, noise or resonances is related to masking and ATH. (In fact, there is a third mechanism: adaptation or compression, a shift in the non-masked threshold.)

7. "Purity" of the signal is irrelevant

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.

The shape of the signal itself is also irrelevant (square wave response, impulse response). We can't assess fidelity by looking at the waveform.

8. Audio measurements & fidelity

(A short summary.)

There are two main reasons why the measured performance is important. First of all, audio fidelity measurements describe to what extent an audio system can reproduce the original performance (live, electronic). Secondly, there is a strong correlation between "preference" ("good sound") and audio measurements, though higher preference does not necessarily mean better measured fidelity, especially when the differences become so small that belief can mess up perception and the wire drama begins...

What are the audio fidelity related measurements or "parameters"? Audio fidelity is mainly determined by frequency response, nonlinear distortion curves and noise (necessary for expressing dynamic range). Time domain measurements (phase response, group delay response) are 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).

9. Thresholds & "worst-case" thresholds

Audibility thresholds for audio parameters (noise, distortion...) varies according to the harmonic and temporal properties of the audio signal. There always exists a particular signal with the lowest threshold, which can be identified as "worst-case". Identification and analysis of these special cases is important.

For example, nonlinear distortion is best heard with pure tones and two-tone tones. Detection is more difficult with complex time-varying signals and pulses. In contrast, time-related errors are more audible in pulses or pulse series. (Is it a coincidence that measurement signals are worst-case signals? At least the basic ones.)

10. Fidelity of the playback system, the format and the recording are different things!

Audiophiles tend to confuse these concepts, especially the fidelity of the format with the fidelity of the (actual) recording. This results in many misconceptions: analog is better than digital, FLAC is better than streaming, just to name some.

Csaba Horváth

This article was last modified on January 28, 2025 (sub-headings, some reorganization).