Timbre

An Elusive Property of Sound

Timbre is a notoriously elusive property of sound. In modern usage, the word was first coined during the French Enlightenment (18th century), and it did not exist as a concept in Western music theory until approximately this time. German physicist Hermann von Helmholtz was the first to systematically investigate timbre (1867), influentially determining that the sensation of musical tone is the product of periodically vibrating frequencies (the note itself, or “fundamental,” plus overtones). This definition of timbre as dependent on the spectrum of a given sound has expanded since the 1960s to include a range of other acoustic attributes. Today, in addition to spectral characteristics, timbre is known to depend on temporal properties such as envelope (attack, decay, steady-state, and release) and flux (changes over time), as well as transients specific to each sound producer (e.g., the noise of the violin bow or the “blatty” attack of the trombone). For example, a piano does not sound like a piano when played backward or with altered transients. Originating in Helmholtz's work, spectra are typically divided into harmonic and inharmonic varieties: harmonic spectra contain most of their energy in the harmonic series (integer multiples of the fundamental frequency, e.g., 440 Hz, 880 Hz, or 1,320 Hz), while the overtones of inharmonic spectra are less regularly dispersed across the spectrum, and can result in sensory dissonance. Most pitched musical sounds are largely harmonic.

The physical qualities of timbre lend themselves to visualization in two primary formats. The spectrum (produced by Fourier analysis) shows the relative strength of acoustic energy across the entire frequency range of the signal, and the spectrogram (or spectrograph) maps these same two variables plus time.

The acoustic components of timbre do not relate in any direct, one-to-one way with actual human perception. In addition to physical definitions, therefore, the topic is often investigated using the methods of experimental psychology in order to determine the relationship between timbre as a percept, and timbre as an acoustic phenomenon. Psychoacousticians tend to approach the topic from two different orientations: as a multistage act of information processing involving perception, representation, memory, and interpretation; and through the lens of ecological psychology, which [Page 1128]views perception as the immediate and direct consequence of subject/world interaction.

Timbre is often treated as a category (i.e., the sound-producing object, such as a piano or a voice), a continuous scale (i.e., varying degrees of a particular timbral quality, such as “brightness” or “harshness”), or a hybrid of the two. Experimental methods for assessing the properties of timbre perception include discrimination tests (either/or), psychophysical rating scales, similarity ratings, matching tasks, classification, and identification. Using these methods, researchers since the mid 1970s have mapped the perceptual similarity/dissimilarity of common musical timbres onto three orthogonal dimensions. The most salient feature of timbre, according to most studies, is “brightness” (or “nasality” or “sharpness”), which acoustically corresponds to greater high-frequency energy; for example, the oboe is “bright,” while the French horn is not.

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