Many of my compositions, especially my recent electroacoustic works, feature what I call defining sounds -- pitch or rhythmic motives, sonorities or chords that are associated with a unique or particular timbre and that recur prominently in varied form and different contexts during a work. These defining sounds are intended to impart a particular "energy level," quality of movement and expressive "character" or "tone" to the work. I have been asked on several occasions to define this term more fully, and to illustrate how these defining sounds are created and combined, and so am providing a summary (perhaps in more detail than anyone wishes) and 19 mp3 audio examples here.

In my two-movement composition Diaspora for soprano and computer generated sounds the most prominent of these defining sounds within the computer part consist of progressions of vocal-like sonorities or "chords" that are derived from generic vocal samples, such as this source bass choir tone : example 1

Several steps were involved in processing these rather bland source vocal tones and transforming them into timbral/pitched thematic material with a more distinctive and memorable quality of expression.

(1) First the samples were run through a phase vocoder algorithm, which, among other things, I used to eliminate some of the frequencies (harmonics) present within the original tones. Often, the fundamental frequency, and the higher frequencies, are eliminated, leaving only six, seven or eight surviving frequencies from the middle of the original spectrum. Sometimes, the remaining frequencies also are shifted by a constant factor (e.g. by adding 100 hertz to each frequency), which alters the original harmonic or inharmonic ratios between these frequencies. (Note that that shifting all of the frequencies in a spectrum by a constant amount, as here, changes the timbre, producing a result quite different from shifting all frequencies by a constant ratio, which produces a simple pitch transposition.) The result, to my ears, is somewhat like a "shadow" of the source sound, retaining some of the recognizable qualities of the original tone but giving it a new musical definition or "feel."

Here are two variants of the source bass choir tone that were created in this fashion. In example 2 the original sample remains recognizable. In example 3, however, upward frequency shifting of the harmonics creates a thinner, "more airy" timbre.

A salient feature of both examples is an ambiguous or paradoxical aural quality that I find intriguing. Because some of the harmonics of the source vocal tone have been removed, and the amplitude and/or frequency relationships of the remaining partials have been recast, the frequencies no longer fuse as readily into a single perceived tone, and we are more aware of the individual frequency components within the two processed variants. One might hear or interpret either of these variants as a timbre or as a "chord." Jean-Claude Risset first explored such "timbre/chord" aural paradoxes forty years ago.

(2) The next step in the creation of the defining vocal sounds within Diaspora was to mix together groups of two or three processed variants like the two above, usually with pitch transpositions, in order to fashion more complex or richer composite chord/timbres, like this example 4.

(3) Finally, during the composition of Diaspora, I combined groups of two, three or more of these composite timbral/pitch sonorities, such as the fourth example above, to form progressions, like this example 5 and this example 6. These fourth and fifth examples exemplify the way in which the defining vocal sounds are heard in Diaspora. To my ears these progressions have a hollow, drifting or rootless, tenuous or rarefied quality that defines one of the principal affective musical qualities of the composition.

In addition to the hollow vocal-like progressions discussed above several other groups of defining sounds are employed in Diaspora. The contrasts, similarities, interplays and movement between these collections is vital to the structure and expressive content composition. Some of the defining sounds are employed in both movements, delineating elements that characterize the piece as a whole, while others are used only (or else primarily) in one of the movements to underscore contrasts in structure and expressive quality between the two movements.

• Some of these defining gestures are always associated with a particular timbre, with the color and the pitch and/or rhythmic material inextricably linked. In the second movement of Diaspora this rumbling low tone, derived from an analysis and multiple, varied resynthesis of a low piano tone run through resonant band pass filters, recurs several times, usually on the pitch a0, the pitch b0, the pitch c-sharp 1 or the pitch f1 (the lowest A, B, C-sharp and F on the piano): example 7 These rather "cavernous" or "yawning" low tones sometimes are layered alongside, and become associated with, another defining sound of the piece: dense, low pitched tremolo (rapidly rearticulated) chords. example 8 illustrates this pairing.
• In other cases the timbre or mix of associated timbres may remain constant while the pitch and or rhythmic structure undergoes variation or development. An assemblage of interlocking twittering percussive sounds lights up several times within both movements, sometimes as a foreground element and sometimes as a background layer. example 9 is an excerpted from the first movement; example 10 occurs in the second movement.
• In still other cases, a defining idea may become associated with two, three or more timbres or timbral mixes, and may change in pitch, rhythm, articulation and other parameters while still retain a recognizable identity, creating a group of gestures related by certain common qualities. A series of dense, sharply articulated but rather "hollow" chords, sometimes taking the form of guillotine-like "slices," as in example 11 and sometimes including a flam, upbeat or "double hit," as in example 12, occur at widely-spaced, irregular intervals in the first movement, then more frequently and prominently as one of the principal defining sounds of the second movement. Several types of variants, "echos" and "aftershocks" of these hammerblow punctuations pervade the second movement:
  1. hollow, low-pitched "thuds," which may sound like bass drum strokes or have a more metallic quality, like a rapidly strummed washboard: example 13 concatenates six of these thunks, which occur as background punctuations at various points within the second movement
  2. clangorous (but still hollow-sounding) chords that sound somewhat like a cross between chime and piano tones. example 14 concatenates three of these chime-like sonorities, which occur at three different points within the middle of the second movement
  3. the high-to-low pitched "slicing" or "guillotine-like" quality of examples 10 and 11 spawns another group of variants, such as the downward percussive sweep in example 15.
• Counterbalancing the downward sweeps illustrated in example 15 are ascending arpeggi generally associated with a rather diaphonous, glassy timbre: example 16
• The intertwining qualities of the ascending arpeggios in example 16 are mirrored in another defining sound of the second movement, in which isolated words from the poetic text of the piece are layered into wave-like, often alliterative rhythmic patterns. example 17 offers two illustrations of these pyramiding wordplays, the first employing spoken words, the second employing whispered voicings.
  Another related idea consists of arpeggiated runs associated with a metallic harp-like timbre: example 18

Terms such as "associated" (or "paired with") and "vairants" have appeared frequently within this discussion, as have subjective descriptive terms such as "hollow,""dense" and "metallic." The defining sounds in Diaspora often are varied and developed simultaneously in groups of two or three ideas rather than individually, and then are further varied or developed in new combinations ("associations") at later points in the work. My intention is that through these changing associations (or linkings) common or similar elements and relationships among these defining sounds become increasingly apparent over time, and clarify the defining ideas (the structure and qualities of expression) of the work as a whole.

The following 30 second excerpt from the computer part to the second movement illustrates this "coming together," layering and simultaneous development of several of the defining sounds discussed above: example 19
Included within this passage is the following assemblage of defining sounds:

• vocal "timbre/chord" progressions (heard in examples 5 and 6) : 0 to 10 seconds and 23 to 29 seconds
• twittering percussive sounds (examples 9 and 10) : 0 to 22 seconds
• hollow, low pitched thuds (example 13) : 7", 10", 19", 23" and 26"
• low pitched tremolo chords (example 8) : 9" through 12"
• pyramiding wordplays (example 17), in this case through layered repetitions of the word "pulsing" : 13" through 18" (background element)
• metallic harp (example 18) : 13" through 18" (background element)

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