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Tone clusters
Multiple adjacent tones played simultaneously

A tone cluster is a musical chord comprising at least three adjacent tones in a scale. Prototypical tone clusters are based on the chromatic scale and are separated by semitones. For instance, three adjacent piano keys (such as C, C♯, and D) struck simultaneously produce a tone cluster. Variants of the tone cluster include chords comprising adjacent tones separated diatonically, pentatonically, or microtonally. On the piano, such clusters often involve the simultaneous striking of neighboring white or black keys.

The early years of the twentieth century saw tone clusters elevated to central roles in pioneering works by ragtime artists Jelly Roll Morton and Scott Joplin. In the 1910s, two classical avant-gardists, composer-pianists Leo Ornstein and Henry Cowell, were recognized as making the first extensive explorations of the tone cluster. During the same period, Charles Ives employed them in several compositions that were not publicly performed until the late 1920s or 1930s. Composers such as Béla Bartók and, later, Lou Harrison and Karlheinz Stockhausen became proponents of the tone cluster, which feature in the work of many 20th- and 21st-century classical composers. Tone clusters also play a significant role in the work of free jazz musicians such as Cecil Taylor and Matthew Shipp.

In most Western music, tone clusters tend to be heard as dissonant. Clusters may be performed with almost any individual instrument on which three or more notes can be played simultaneously, as well as by most groups of instruments or voices. Keyboard instruments are particularly suited to the performance of tone clusters because it is relatively easy to play multiple notes in unison on them.

In standard Western classical music practice, all tone clusters are classifiable as secundal chords—that is, they are constructed from minor seconds (intervals of one semitone), major seconds (intervals of two semitones), or, in the case of certain pentatonic clusters, augmented seconds (intervals of three semitones). Stacks of adjacent microtonal pitches also constitute tone clusters.

In tone clusters, the notes are sounded fully and in unison, distinguishing them from ornamented figures involving acciaccaturas and the like. Their effect also tends to be different: where ornamentation is used to draw attention to the harmony or the relationship between harmony and melody, tone clusters are for the most part employed as independent sounds. While, by definition, the notes that form a cluster must sound at the same time, there is no requirement that they must all begin sounding at the same moment. For example, in R. Murray Schafer's choral Epitaph for Moonlight (1968), a tone cluster is constructed by dividing each choir section (soprano/alto/tenor/bass) into four parts. Each of the sixteen parts enters separately, humming a note one semitone lower than the note hummed by the previous part, until all sixteen are contributing to the cluster.

Tone clusters have generally been thought of as dissonant musical textures, and even defined as such. As noted by Alan Belkin, however, instrumental timbre can have a significant impact on their effect: "Clusters are quite aggressive on the organ, but soften enormously when played by strings (possibly because slight, continuous fluctuations of pitch in the latter provide some inner mobility)." In his first published work on the topic, Henry Cowell observed that a tone cluster is "more pleasing" and "acceptable to the ear if its outer limits form a consonant interval." Cowell explains, "the natural spacing of so-called dissonances is as seconds, as in the overtone series, rather than sevenths and ninths....Groups spaced in seconds may be made to sound euphonious, particularly if played in conjunction with fundamental chord notes taken from lower in the same overtone series. Blends them together and explains them to the ear." Tone clusters have also been considered noise. As Mauricio Kagel says, "clusters have generally been used as a kind of anti-harmony, as a transition between sound and noise." Tone clusters thus also lend themselves to use in a percussive manner. Historically, they were sometimes discussed with a hint of disdain. One 1969 textbook defines the tone cluster as "an extra-harmonic clump of notes.

Notation and execution

In his 1917 piece The Tides of Manaunaun, Cowell introduced a new notation for tone clusters on the piano and other keyboard instruments. In this notation, only the top and bottom notes of a cluster, connected by a single line or a pair of lines, are represented. This developed into the solid-bar style seen in the image on the right. Here, the first chord—stretching two octaves from D2 to D4—is a diatonic (so-called white-note) cluster, indicated by the natural sign below the staff. The second is a pentatonic (so-called black-note) cluster, indicated by the flat sign; a sharp sign would be required if the notes showing the limit of the cluster were spelled as sharps. A chromatic cluster—black and white keys together—is shown in this method by a solid bar with no sign at all. In scoring the large, dense clusters of the solo organ work Volumina in the early 1960s, György Ligeti, using graphical notation, blocked in whole sections of the keyboard.

The performance of keyboard tone clusters is widely considered an "extended technique"—large clusters require unusual playing methods often involving the fist, the flat of the hand, or the forearm. Thelonious Monk and Karlheinz Stockhausen each performed clusters with their elbows; Stockhausen developed a method for playing cluster glissandi with special gloves. Don Pullen would play moving clusters by rolling the backs of his hands over the keyboard. Boards of various dimension are sometimes employed, as in the Concord Sonata (c. 1904–19) of Charles Ives; they can be weighted down to execute clusters of long duration. Several of Lou Harrison's scores call for the use of an "octave bar", crafted to facilitate high-speed keyboard cluster performance. Designed by Harrison with his partner William Colvig, the octave bar is

a flat wooden device approximately two inches high with a grip on top and sponge rubber on the bottom, with which the player strikes the keys. Its length spans an octave on a grand piano. The sponge rubber bottom is sculpted so that its ends are slightly lower than its center, making the outer tones of the octave sound with greater force than the intermediary pitches. The pianist can thus rush headlong through fearfully rapid passages, precisely spanning an octave at each blow.

In jazz

Scott Joplin wrote the first known published composition to include a musical sequence built around specifically notated tone clusters.

Tone clusters have been employed by jazz artists in a variety of styles, since the very beginning of the form. Around the turn of the twentieth century, Storyville pianist Jelly Roll Morton began performing a ragtime adaptation of a French quadrille, introducing large chromatic tone clusters played by his left forearm. The growling effect led Morton to dub the piece his "Tiger Rag". In 1909, Scott Joplin's deliberately experimental "Wall Street Rag" included a section prominently featuring notated tone clusters.

The fourth of Artie Matthews's Pastime Rags (1913–20) features dissonant right-hand clusters. Thelonious Monk, in pieces such as "Bright Mississippi" (1962), "Introspection" (1946) and "Off Minor" (1947), uses clusters as dramatic figures within the central improvisation and to accent the tension at its conclusion. They are heard on Art Tatum's "Mr. Freddy Blues" (1950), undergirding the cross-rhythms. By 1953, Dave Brubeck was employing piano tone clusters and dissonance in a manner anticipating the style free jazz pioneer Cecil Taylor would soon develop. The approach of hard bop pianist Horace Silver is an even clearer antecedent to Taylor's use of clusters. During the same era, clusters appear as punctuation marks in the lead lines of Herbie Nichols. In "The Gig" (1955), described by Francis Davis as Nichols's masterpiece, "clashing notes and tone clusters depic[t] a pickup band at odds with itself about what to play." Recorded examples of Duke Ellington's piano cluster work include "Summertime" (1961) and ...And His Mother Called Him Bill (1967) and This One's for Blanton!, his tribute to a former bass player, recorded in 1972 with bassist Ray Brown. Bill Evans' interpretation of “Come Rain or Come Shine” from the album Portrait in Jazz (1960), opens with a striking 5-tone cluster.

In jazz, as in classical music, tone clusters have not been restricted to the keyboard. In the 1930s, the Jimmie Lunceford Orchestra's "Stratosphere" included ensemble clusters among an array of progressive elements. The Stan Kenton Orchestra's April 1947 recording of "If I Could Be With You One Hour Tonight," arranged by Pete Rugolo, features a dramatic four-note trombone cluster at the end of the second chorus. As described by critic Fred Kaplan, a 1950 performance by the Duke Ellington Orchestra features arrangements with the collective "blowing rich, dark, tone clusters that evoke Ravel." Chord clusters also feature in the scores of arranger Gil Evans. In his characteristically imaginative arrangement of George Gershwin's "There's a boat that's leaving soon for New York" from the album Porgy and Bess, Evans contributes chord clusters orchestrated on flutes, alto saxophone and muted trumpets as a background to accompany Miles Davis' solo improvisation. In the early 1960s, arrangements by Bob Brookmeyer and Gerry Mulligan for Mulligan's Concert Jazz Band employed tone clusters in a dense style bringing to mind both Ellington and Ravel. Eric Dolphy's bass clarinet solos would often feature "microtonal clusters summoned by frantic overblowing." Critic Robert Palmer called the "tart tone cluster" that "pierces a song's surfaces and penetrates to its heart" a specialty of guitarist Jim Hall's.

Clusters are especially prevalent in the realm of free jazz. Cecil Taylor has used them extensively as part of his improvisational method since the mid-1950s. Like much of his musical vocabulary, his clusters operate "on a continuum somewhere between melody and percussion." One of Taylor's primary purposes in adopting clusters was to avoid the dominance of any specific pitch. Leading free jazz composer, bandleader, and pianist Sun Ra often used them to rearrange the musical furniture, as described by scholar John F. Szwed:

When he sensed that [a] piece needed an introduction or an ending, a new direction or fresh material, he would call for a space chord, a collectively improvised tone cluster at high volume which "would suggest a new melody, maybe a rhythm." It was a pianistically conceived device which created another context for the music, a new mood, opening up fresh tonal areas.[105]

As free jazz spread in the 1960s, so did the use of tone clusters. In comparison with what John Litweiler describes as Taylor's "endless forms and contrasts," the solos of Muhal Richard Abrams employ tone clusters in a similarly free, but more lyrical, flowing context. Guitarist Sonny Sharrock made them a central part of his improvisations; in Palmer's description, he executed "glass-shattering tone clusters that sounded like someone was ripping the pickups out of the guitar without having bothered to unplug it from its overdriven amplifier." Pianist Marilyn Crispell has been another major free jazz proponent of the tone cluster, frequently in collaboration with Anthony Braxton, who played with Abrams early in his career. Since the 1990s, Matthew Shipp has built on Taylor's innovations with the form. European free jazz pianists who have contributed to the development of the tone cluster palette include Gunter Hampel and Alexander von Schlippenbach.

Don Pullen, who bridged free and mainstream jazz, "had a technique of rolling his wrists as he improvised—the outside edges of his hands became scarred from it—to create moving tone clusters," writes critic Ben Ratliff. "Building up from arpeggios, he could create eddies of noise on the concise Cecil Taylor outbursts." In the description of Joachim Berendt, Pullen "uniquely melodized cluster playing and made it tonal. He phrases impulsively raw clusters with his right hand and yet embeds them in clear, harmonically functional tonal chords simultaneously played with the left hand." John Medeski employs tone clusters as keyboardist for Medeski, Martin, and Wood, which mixes free jazz elements into its soul jazz/jam band style.

Like jazz, rock and roll has made use of tone clusters since its birth, if characteristically in a less deliberate manner—most famously, Jerry Lee Lewis's live-performance piano technique of the 1950s, involving fists, feet, and derrière. Since the 1960s, much drone music, which crosses the lines between rock, electronic, and experimental music, has been based on tone clusters. On The Velvet Underground's "Sister Ray," recorded in September 1967, organist John Cale uses tone clusters within the context of a drone; the song is apparently the closest approximation on record of the band's early live sound. Around the same time, Doors keyboardist Ray Manzarek began introducing clusters into his solos during live performances of the band's hit "Light My Fire."

Kraftwerk's self-titled 1970 debut album employs organ clusters to add variety to its repeated tape sequences. In 1971, critic Ed Ward lauded the "tone-cluster vocal harmonies" created by Jefferson Airplane's three lead singers, Grace Slick, Marty Balin, and Paul Kantner. Tangerine Dream's 1972 double album Zeit is replete with clusters performed on synthesizer. In later rock practice, the D add9 chord characteristic of jangle pop involves a three-note set separated by major seconds (D, E, F♯), the sort of guitar cluster that may be characterized as a harp effect. The Beatles' 1965 song "We Can Work It Out" features a momentarily grating tone cluster with voices singing A sharp and C sharp against the accompanying keyboard playing a sustained chord on B to the word "time." The Band's 1968 song "The Weight" from their debut album Music from Big Pink features a dissonant vocal refrain with suspensions culminating in a 3-note cluster to the words "you put the load right on me."

The sound of tone clusters played on the organ became a convention in radio drama for dreams. Clusters are often used in the scoring of horror and science-fiction films. For a 2004 production of the play Tone Clusters by Joyce Carol Oates, composer Jay Clarke—a member of the indie rock bands Dolorean and The Standard—employed clusters to "subtly build the tension", in contrast to what he perceived in the cluster pieces by Cowell and Ives suggested by Oates: “Some of it was like music to murder somebody to; it was like horror-movie music”.

Use in other music

In traditional Japanese gagaku, the imperial court music, a tone cluster performed on shō (a type of mouth organ) is generally employed as a harmonic matrix. Yoritsune Matsudaira, active from the late 1920s to the early 2000s, merged gagaku's harmonies and tonalities with avant-garde Western techniques. Much of his work is built on the shō's ten traditional cluster formations. Lou Harrison's Pacifika Rondo, which mixes Eastern and Western instrumentation and styles, mirrors the gagaku approach—sustained organ clusters emulate the sound and function of the shō. The shō also inspired Benjamin Britten in creating the instrumental texture of his 1964 dramatic church parable Curlew River. Its sound pervades the characteristically sustained cluster chords played on a chamber organ. Traditional Korean court and aristocratic music employs passages of simultaneous ornamentation on multiple instruments, creating dissonant clusters; this technique is reflected in the work of twentieth-century Korean German composer Isang Yun.

Several East Asian free reed instruments, including the shō, were modeled on the sheng, an ancient Chinese folk instrument later incorporated into more formal musical contexts. Wubaduhesheng, one of the traditional chord formations played on the sheng, involves a three-pitch cluster. Malayan folk musicians employ an indigenous mouth organ that, like the shō and sheng, produces tone clusters. The characteristic musical form played on the bin-baja, a strummed harp of central India's Pardhan people, has been described as a "rhythmic ostinato on a tone cluster."

Among the Asante, in the region that is today encompassed by Ghana, tone clusters are used in traditional trumpet music. A distinctive "tongue-rattling technique gives a greater vibrancy to...already dissonant tonal cluster[s].... [I]ntentional dissonance dispels evil spirits, and the greater the clangor, the greater the sound barrage.

Wendy Carlos used essentially the exact reverse of this methodology to derive her Alpha scale, Beta scale and Gamma scale; they are the most consonant scales one can derive by treating tone clusters as the only type of triad that really exists, which is paradoxically an anti-harmonic monistic method.