Letter S

SAMPLING RATE OR FREQUENCY (Fs): Defines the rate in Hertz, which is used to digitize an audio signal during the sampling process. In a typical digital recorder the input signal is first filtered to remove any frequencies that cannot be accurately represented digitally. Then the instantaneous amplitude of the signal waveform is sampled at a rate at least twice as high as the highest frequency which remains after filtering (at least 40,000 times per second in the case of 20 kHz, for example). The sampling process results in a steady stream of instantaneous signal-waveform values (1/40,000 of a second or 25 microseconds apart for the instance quoted). The standard sampling rates for digital recording are 44.1 and 48 kHz.

SOUND PRESSURE: The atmosphere exerts pressure on all objects in it. When a vibrating body moves in air, it creates slight disturbances of the ambient atmospheric pressure. The amplitude of these pressure variations (that is, their maximum displacement from the ambient atmospheric pressure) is called the sound pressure variation, whereas the effective pressure variation is 0.707 the maximum value. The oscillating variations in sound pressure (called the waveform of the sound) propagate in the form of a sound wave.

SOUND WAVE: Alternations of sound pressure or particle displacement following one another in cycle of compression and rarefaction through a medium such as air. On striking the ear, it may be heard as sound.

STANDING WAVES: A wave phenomenon resulting from the interference of sound waves of the same frequency and kind traveling in opposite directions. Such waves are characterized by the absence of propagation and the existence of nodes or partial nodes and antinodes or loops, which have a fixed distribution in space.