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Article

Echo  

Murray Campbell and Mary Térey-Smith

The repetition of sound after a short time interval. In addition to the applications discussed below the term is used for a signal-processing device (also known as a delay) that produces a slightly delayed playback of sounds either by a tape loop or by digital delay; see Electric guitar §2 .

See also Organ stop .

Natural echoes arise from the reflection of a sound wave by a solid surface, such as a wall or cliff. For the echo to be perceived as distinct from the original sound, the extra path length travelled by the reflected sound wave must have a minimum value of around 17 metres, corresponding to a minimum time interval of 50 milliseconds between direct and reflected sounds.

The reverberant sound field in a concert hall is created by multiple reflections of sound waves. In a well-designed hall, the direct sound reaching a member of the audience is followed by a series of reflections within a time interval of around 35 milliseconds. These ‘early delayed arrivals’ are not heard as separate echoes; because of the ‘precedence effect’ they are perceived as a reinforcement of the direct sound. Subsequent reflections blend smoothly into the reverberation. A concave surface, focussing sound waves into a particular part of the hall, can give rise to an audible echo; a ‘flutter echo’ can arise from successive reflections between parallel walls....

Article

Mark Lindley

Tunings of the scale in which some or all of the concords are made slightly impure in order that few or none will be left distastefully so. Equal temperament, in which the octave is divided into 12 uniform semitones, is the standard Western temperament today except among specialists in early music. This article traces the history of temperaments in performing practice and in relation to the main lines of development in the history of harmony; for additional technical and historical details see Tuning, Pythagorean intonation, Just intonation, Microtone, Mean-tone, Well-tempered clavier, Equal temperament and Interval, especially Table 1.

Since the 15th century, tempered tuning has characterized keyboard music and in Western culture the art music of fretted instruments such as the lute. Its prevalence is due mainly to the fact that the concords of triadic music – octaves, 5ths and 3rds – are in many cases incommensurate in their pure forms. Three pure major 3rds (e.g. A♭–C–E–G♯) fall short of a pure octave by approximately one fifth of a whole tone (lesser diesis); four pure minor 3rds (G♯–B–D–F–A♭) exceed an octave by half as much again (greater diesis); the circle of twelve 5ths, if the 5ths are pure, does not quite cumulate in a perfect unison; and, most important of all in the context of Renaissance and Baroque music, the whole tone produced by subtracting a pure minor 3rd from a pure 4th (C–F–D) is about 11% smaller than the whole tone produced by subtracting a pure 4th from a pure 5th (C–G–D). These discrepancies are summarized in ...