Timbre is defined negatively, as whatever differs between equal-pitch, equal-loudness sounds

Evidence

There is no subjective rating scale against which timbre judgments can be made, unlike pitch and loudness which can, on average, be reliably rated by listeners on scales from “high” to “low.” The commonly quoted American National Standards Institute formal definition of timbre reflects this: “Timbre is that attribute of auditory sensation in terms of which a listener can judge two sounds similarly presented and having the same loudness and pitch as being dissimilar” (ANSI, 1960). In other words, two sounds that are perceived as being different but which have the same perceived loudness and pitch differ by virtue of their timbre (consider the examples on track 62 of the accompanying CD). The timbre of a note is the aspect by which a listener recognizes the instrument which is playing a note when, for example, instruments play notes with the same pitch, loud­ ness and duration. — David Howard, Jamie Angus, coustics and Psychoacoustics, 1451

There is no subjective rating scale against which timbre judgments can be made, unlike pitch and loudness which can, on average, be reliably rated by listeners on scales from “high” to “low.” The commonly quoted American National Standards Institute formal definition of timbre reflects this: “Timbre is that attribute of auditory sensation in terms of which a listener can judge two sounds similarly presented and having the same loudness and pitch as being dissimilar” (ANSI, 1960). — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1425

Categorization

• DEFINE: The ANSI definition is the canonical formal definition of timbre — a negative definition (dissimilarity) rather than positive attributes.
• QUESTION: Since timbre has no single subjective scale, how do psychoacoustic studies actually quantify and compare timbre across listeners?
• XREF: Connects to the pitch and loudness scales established in earlier chapters, which timbre conspicuously lacks. — connects to: Timbre, unlike pitch and loudness, has no objective scale — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1481; Timbre lacks a one-dimensional subjective scale unlike pitch and loudness — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1425; Timbre reduces to a weighted ratio of fundamental and resolved versus unresolved harmonics (+3 more)
• corroborated: 3 independent passages support this claim — David Howard, Jamie Angus, coustics and Psychoacoustics, 1451; David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1425

Broader Connections

• Timbre served as a residual basket for non-loudness non-pitch sensations #corroborates (0.90) — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1339
• Timbre lacks a one-dimensional subjective scale unlike pitch and loudness #expands (0.85) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1425
• Timbre reduces to a weighted ratio of fundamental and resolved versus unresolved harmonics #contradicts (0.80)
• Timbre is conditioned entirely by the strength and presence of overtones #contradicts (0.80) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1426
• Each instrument’s timbre is defined by its unique recipe of harmonic partials #qualifies (0.75)