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If two TONEs are about 15 Hertz or less apart, INTERFERENCE will result from their similar though not exactly identical frequencies. Gradually they will move out of PHASE until at 180° destructive interference results, producing diminished loudness. When they move back into phase, constructive interference will produce increased loudness. Thus, beats are a form of AMPLITUDE MODULATION. As two frequencies are brought closer together, the beats will gradually slow down and disappear when they become identical.
Sound at frequencies above the audible range, namely above 20 kHz, audible only to various non-human species. Because of its very short WAVELENGTH in the megahertz range, ultrasound is used as a safe alternative to X-ray photography in medical diagnosis. Ultrasound scanning is sometimes called sonography.
The ability to judge PITCH absolutely, without reference to another pitch or frequency. Also called absolute pitch. Perfect pitch appears to be an innate ability of some individuals, whereas related skills, such as remembered pitch or relative pitch which use other pitches as references, can be learned through musical practice.
The tonal QUALITY or TIMBRE of a sound. The term is usually used in a subjective, descriptive manner, often with such adjectives as 'full' or 'rich'.