Acousmatic sound is sound that is heard without an originating cause being seen. The word acousmatic, from the French acousmatique, is derived from the Greek word akousmatikoi (ἀκουσματικοί), which referred to probationary pupils of the philosopher Pythagoras who were required to sit in absolute silence while they listened to him deliver his lecture from behind a veil or screen to make them better concentrate on his teachings. The term acousmatique was first used by the French composer and pioneer of musique concrète Pierre Schaeffer.  In acousmatic art  one hears sound from behind a "veil" of loudspeakers, the source cause remaining unseen. More generally, any sound, whether it is natural or manipulated, may be described as acousmatic if the cause of the sound remains unseen. The term has also been used by the French writer and composer Michel Chion in reference to the use of off-screen sound in film.  More recently, in the article Space-form and the acousmatic image (2007), composer and academic Prof. Denis Smalley has expanded on some of Schaeffers' acousmatic concepts.  Since the 2000s, the term acousmatic has been used, notably in North America to refer to fixed media composition and pieces.
In 1955, Jérôme Peignot and Pierre Schaeffer were the first to use the term acousmatic to define the listening experience of musique concrète.  In his 1966 publication Traité des objets musicaux Schaeffer defined the acousmatic as: Acousmatic, adjective: referring to a sound that one hears without seeing the causes behind it (Schaeffer 1966: 91). Schaeffer held that the acousmatic listening experience was one that reduced sounds to the field of hearing alone. The concept of reduction (epoché), as used in the Husserlian phenomenological tradition, underpinned Schaeffer's conceptualization of the acousmatic experience. In this sense, a subject moves their attention away from the physical object responsible for auditory perception and toward the content of this perception. The purpose of this activity is to become aware of what it is in the field of perception that can be thought of as a certainty. This reductive procedure redirects awareness to hearing alone.  Schaeffer remarked that: Often surprised, often uncertain, we discover that much of what we thought we were hearing, was in reality only seen, and explained, by the context (Schaeffer 1966: 93).
Schaeffer derived the word acousmatique from akousmatikoi (hearers), a term used in the time of Pythagoras to refer to his uninitiated students. According to historical records followers of Pythagoras underwent a three-year probationary period, directly followed by a five-year period of "silence", before being admitted to Pythagoras' inner circle as mathêmatikoi (learned). The use of silence related to the protocols of rituals connected with the mystery-like instruction and religious ceremonies of the Pythagorean order. These ceremonies took place behind a veil or curtain with only those who had passed the five-year test being allowed to see their teacher face to face; the remaining students partaking acousmatically.    More recent research suggests that the Pythagorean "veil" itself was a euphemism for the figurative language with which Pythagoras taught, and the actual practice of speaking occluded by either a veil or the dark likely never occurred. 
According to the French film sound theorist Michel Chion (1994), in cinema, the acousmatic situation can arise in two different ways: the source of a sound is seen first and is then "acousmatized", or the sound is initially acousmatic with the source being revealed subsequently. The first scenario allows association of a sound with a specific image from the outset, Chion calls this visualised sound (what Schaeffer referred to as direct sound). In this case it becomes an "embodied" sound, "identified with an image, demythologized, classified". In the second instance the sound source remains veiled for some time, to heighten tension, and is only later revealed, a dramatic feature that is commonly used in mystery and suspense based cinema; this has the effect of "de-acousmatizing" the initially hidden source of the sound (Chion 1994, 72). Chion states that "the opposition between visualised and acousmatic provides a basis for the fundamental audiovisual notion of offscreen space" (Chion 1994, 73).
Musique concrète is a type of music composition that utilizes recorded sounds as raw material. Sounds are often modified through the application of audio signal processing and tape music techniques, and may be assembled into a form of montage. It can feature sounds derived from recordings of musical instruments, the human voice, and the natural environment as well as those created using synthesizers and computer-based digital signal processing. Compositions in this idiom are not restricted to the normal musical rules of melody, harmony, rhythm, metre, and so on. The technique exploits acousmatic sound, such that sound identities can often be intentionally obscured or appear unconnected to their source cause.
Pierre Henri Marie Schaeffer was a French composer, writer, broadcaster, engineer, musicologist, acoustician and founder of Groupe de Recherche de Musique Concrète (GRMC). His innovative work in both the sciences—particularly communications and acoustics—and the various arts of music, literature and radio presentation after the end of World War II, as well as his anti-nuclear activism and cultural criticism garnered him widespread recognition in his lifetime.
Electroacoustic music is a genre of popular and Western art music in which composers use technology to manipulate the timbres of acoustic sounds, sometimes by using audio signal processing, such as reverb or harmonizing, on acoustical instruments. It originated around the middle of the 20th century, following the incorporation of electric sound production into compositional practice. The initial developments in electroacoustic music composition to fixed media during the 20th century are associated with the activities of the Groupe de recherches musicales at the ORTF in Paris, the home of musique concrète, the Studio for Electronic Music in Cologne, where the focus was on the composition of elektronische Musik, and the Columbia-Princeton Electronic Music Center in New York City, where tape music, electronic music, and computer music were all explored. Practical electronic music instruments began to appear in the early 20th century.
In musique concrete and electronic music theory the term sound object is used to refer to a primary unit of sonic material and often specifically refers to recorded sound rather than written music using manuscript or a score. It was coined by Pierre Schaeffer in his publication Traité des objets musicaux (1966).
Acousmatic music is a form of electroacoustic music that is specifically composed for presentation using speakers, as opposed to a live performance. It stems from a compositional tradition that dates back to the introduction of musique concrète in the late 1940s. Unlike musical works that are realised using sheet music exclusively, compositions that are purely acousmatic often exist solely as fixed media audio recordings.
Michel Chion is a French film theorist and composer of experimental music.
Andrew Lewis is a British composer known mainly for his acousmatic music, that is, electroacoustic music heard only over loudspeakers, though he also composes some chamber and orchestral music.
Founded in 1986, La Communauté électroacoustique canadienne / The Canadian Electroacoustic Community (CEC) is Canada's national electroacoustic / computer music / sonic arts organization and is dedicated to promoting this progressive art form in its broadest definition: from "pure" acousmatic and computer music to soundscape and sonic art to hardware hacking and beyond.
Francis Dhomont is a French composer of electroacoustic / acousmatic music.
François Bayle is a composer of Electronic Music, Musique concrète. He coined the term Acousmatic Music.
Experimental music is a general label for any music or music genre that pushes existing boundaries and genre definitions. Experimental compositional practice is defined broadly by exploratory sensibilities radically opposed to, and questioning of, institutionalized compositional, performing, and aesthetic conventions in music. Elements of experimental music include indeterminate music, in which the composer introduces the elements of chance or unpredictability with regard to either the composition or its performance. Artists may also approach a hybrid of disparate styles or incorporate unorthodox and unique elements.
Live electronic music is a form of music that can include traditional electronic sound-generating devices, modified electric musical instruments, hacked sound generating technologies, and computers. Initially the practice developed in reaction to sound-based composition for fixed media such as musique concrète, electronic music and early computer music. Musical improvisation often plays a large role in the performance of this music. The timbres of various sounds may be transformed extensively using devices such as amplifiers, filters, ring modulators and other forms of circuitry. Real-time generation and manipulation of audio using live coding is now commonplace.
Spectromorphology is the perceived sonic footprint of a sound spectrum as it manifests in time. A descriptive spectromorphological analysis of sound is sometimes used in the analysis of electroacoustic music, especially acousmatic music. The term was coined by Denis Smalley in 1986 and is considered the most adequate English term to designate the field of sound research associated with the French writer, composer, and academic, Pierre Schaeffer.
Natasha Barrett is a British contemporary music composer specialising in electroacoustic art music. Her compositional aesthetics are derived from acousmatic issues. In addition to acousmatic concert music, she composes for instruments, live electronics, sound installations, multi-media works, real-time computer music improvisation, has made soundscapes for exhibitions, and music for contemporary dance and theater. Since 2000 her work has been influenced by spatialisation as a musical parameter, and the projection of 3-D sound-fields. She currently lives in Norway.
Patrick Ascione was a French composer of electroacoustic and acousmatic music.
The Studio d'Essai, later Club d'Essai, was founded in 1942 by Pierre Schaeffer, played a role in the activities of the French resistance during World War II, and later became a center of musical activity.
Denis Dufour is a composer of art music.
Nicolas Vérin is a French composer and professor of music. His many influences, from jazz to electronics, from American to French music, give him an unusual style, apart from the main trends of French contemporary music, combining energy and subtleness.
Sound diffusion is a performance practice in the field of acousmatic music. According to composer and theorist Denis Smalley, it describes the "projection and the spreading of sound in an acoustic space for a group of listeners" during a concert. These concerts can be seen as acoustic recitals without performers, where sound is exclusively generated by loudspeakers. In many cases, the sound diffusion is performed by the composer themselves, whose task it is to integrate and interpret the music within the concert space.
Elsa Justel is an Argentine composer of instrumental and electroacoustic music, and video-artist.