Granular synthesis
Granular synthesis is a sound synthesis method that operates on the microsound time scale.

It is based on the same principle as sampling. However, the samples are split into small pieces of around 1 to 100 ms in duration. These small pieces are called grains. Multiple grains may be layered on top of each other, and may play at different speeds, phases, volume, and frequency, among other parameters.
At low speeds of playback, the result is a kind of soundscape, often described as a cloud, that is manipulatable in a manner unlike that for natural sound sampling or other synthesis techniques. At high speeds, the result is heard as a note or notes of a novel timbre. By varying the waveform, envelope, duration, spatial position, and density of the grains, many different sounds can be produced.

Both have been used for musical purposes: as sound effects, raw material for further processing by other synthesis or digital signal processing effects, or as complete musical works in their own right. Conventional effects that can be achieved include amplitude modulation and time stretching. More experimentally, stereo or multichannel scattering, random reordering, disintegration and morphing are possible.

History
Greek composer Iannis Xenakis is known as the inventor of the granular synthesis technique.
The composer Iannis Xenakis (1960) was the first to explicate a compositional theory for grains of sound. He began by adopting the following lemma: "All sound, even continuous musical variation, is conceived as an assemblage of a large number of elementary sounds adequately disposed in time. In the attack, body, and decline of a complex sound, thousands of pure sounds appear in a more or less short interval of time 
  
    
      
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  ." Xenakis created granular sounds using analog tone generators and tape splicing. These appear in the composition Analogique A-B for string orchestra and tape (1959).
Curtis Roads was the first to implement granular synthesis on a computer in 1974. Twelve years later, in 1986, the Canadian composer Barry Truax implemented real-time versions of this synthesis technique. "Granular synthesis was implemented in different ways by Truax."

Microsound
This includes all sounds on the time scale shorter than musical notes, the sound object time scale, and longer than the sample time scale.  Specifically, this is shorter than one tenth of a second and longer than 10 milliseconds, which includes part of the audio frequency range (20 Hz to 20 kHz) as well as part of the infrasonic frequency range (below 20 Hz, rhythm).These sounds include transient audio phenomena and are known in acoustics and signal processing by various names including sound particles, quantum acoustics, sonal atom, grain, glisson, grainlet, trainlet, microarc, wavelet, chirplet, fof, time-frequency atom, pulsar, impulse, toneburst, tone pip, acoustic pixel, and others. In the frequency domain they may be named kernel, logon, and frame, among others.Physicist Dennis Gabor was an important pioneer in microsound. Micromontage is musical montage with microsound.
Microtime is the level of "sonic" or aural "syntax" or the "time-varying distribution of...spectral energy".

Related software
Csound – comprehensive music software including granular synthesis (overview of granular synthesis opcodes)
Max/MSP – graphical authoring software for real-time audio and video
Pure Data (Pd) – graphical programming language for real-time audio and video
SuperCollider – programming language for real time audio synthesis
ChucK - strongly-timed computer music programming language
EmissionControl2 - granular sound synthesizer

Related hardware
Mutable Instruments Clouds – a digital, open source eurorack synthesizer module which has four factory set modes, the first and default being a granular processor
Make Noise Morphagene – a eurorack synthesizer module built around microsound, or granular synthesis, in addition to Musique Concrète-inspired sound on sound audio manipulation

See also
Digital signal processing
Micromontage audio montage on the time scale of microsounds
Texture synthesis, analogous process for images

References
Bibliography
Articles
"Granular Synthesis" by Eric Kuehnl
"The development of GiST, a Granular. Synthesis Toolkit Based on an Extension of the FOF Generator" by Gerhard Eckel and Manuel Rocha Iturbide
Searching for a global synthesis technique through a quantum conception of sound by Manuel Rocha Iturbide
Further articles on Granular Synthesis
Bencina, R. (2006) "Implementing Real-Time Granular Synthesis", in Greenbaum & Barzel (eds.), Audio Anecdotes III, ISBN 1-56881-215-9, A.K. Peters, Natick. online pdf

Books
Miranda, E. R. (2002). Computer Sound Design: Synthesis Techniques and Programming. Oxford: Focal Press. ISBN 0-240-51693-1.
Roads, Curtis (2001). Microsound. Cambridge: MIT Press. ISBN 0-262-18215-7.
Wilson, Scott (2011). The SuperCollider Book. Cambridge: The MIT Press. ISBN 978-0-262-23269-2.
Iturbide, Manuel Rocha (1999). "Doctoral Thesis: Les techniques granulaires dans la synthèse sonore". ArteSonoro.net. University of Paris VIII.

Discography
Curtis Roads (2004). CD with Microsounds. MIT Press. ISBN 0-262-18215-7. Contains excerpts of nscor and Field (1981). Microsounds at Discogs.
nscor (1980), New Computer Music (1987) Wergo 2010–50 at Discogs (list of releases)
Iannis Xenakis. Analogique A-B (1959), on Alpha & Omega at Discogs and Music For Strings at Discogs

External links
Granular Synthesis Resource Web Site