Psychoacoustics is the study of subjective human perception of sounds. Effectively, it is the study of acoustical perception. Psychoacoustic modeling has long-since been an integral part of audio compression. It exploits properties of the human auditory system to remove the redundancies inherent in audio signals that the human ear cannot perceive. More powerful signals at certain frequencies 'mask' less powerful signals at nearby frequencies by de¬sensitizing the human ear's basilar membrane (which is responsible for resolving the frequency components of a signal). The entire MP3 phenomenon is made possible by the confluence of several distinct but interrelated elements: a few simple insights into the nature of human psychoacoustics, a whole lot of number crunching, and conformance to a tightly specified format for encoding and decoding audio into compact bit streams.
The basic idea is to eliminate information that is inaudible to the ear. This type of compression is often referred to as perceptual encoding. To help determine what can and cannot be heard, compression algorithms rely on the field of psychoacoustics, i.e., the study of human sound perception. Waves vibrating at different frequencies manifest themselves differently, all the way from the astronomically slow pulsations of the universe itself to the inconceivably fast vibration of matter (and beyond). Somewhere in between these extremes are wavelengths that are perceptible to human beings as light and sound. Just beyond the realms of light and sound are sub- and ultrasonic vibration, the infrared and ultraviolet light spectra, and zillions of other frequencies imperceptible to humans (such as radio and microwave). Our sense organs are tuned only to very narrow bandwidths of vibration in the overall picture. In fact, even our own musical instruments create many vibrational frequencies that are imperceptible to our ears. Frequencies are typically described in units called Hertz (Hz), which translates simply as "cycles per second." In general, humans cannot hear frequencies below 20Hz (20 cycles per second), nor above 20kHz
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