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(Greek: akouein = to hear) Commonly, the study of SOUND and its behaviour in various media and environments, including the effects of ABSORPTION, DIFFRACTION, INTERFERENCE, REFLECTION, and REFRACTION.
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.
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.
The difference in SOUND PRESSURE LEVEL between the saturation or overload level and the BACKGROUND NOISE level of an acoustic or electroacoustic system, measured in DECIBELs. This RANGE may be expressed as a SIGNAL-TO-NOISE RATIO for maximum output.