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Article

Stephen E. Hefling

Rhythms in which long notes alternate with one or more short notes, so called because the long notes are usually written with the aid of the dot of addition (see Note values). Dotted rhythms are found in mensurally notated music of all periods; this article, however, deals mainly with music of the 17th and 18th centuries, in which it was customary to alter certain sorts of written rhythmic values in performance (see also Notes inégales; for notational meanings of the dot before 1600 see Notation, §III). The principal issue is the degree to which such rhythms sounded uneven, rather than the specific manner of their notation (e.g. the dot may be replaced by a rest or tie).

Dozens of contemporary theoretical and pedagogical sources indicate that the dot was ordinarily equal to one half the value of the note or rest preceding it, just as it is today. But the treatises also present various exceptions. The dot could stand for a tie (...

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....