Human Hearing

Human hearing operates over a wide range of frequency and intensity (Fig. 1), and within this range humans have a remarkable capacity to detect, discriminate, and locate sounds. A young listener hears sounds ranging in frequency from about 20 to 20,000 Hz, and over a considerable portion of this range a change in frequency as little as 0.15% is detectable. This same listener detects sounds that move the eardrum over a distance no greater than the diameter of a hydrogen atom, and yet hearing remains quite clear as sound pressure level is then raised by a factor of 106 or more. Within this dynamic range of 120 decibels (dB), a change in sound intensity of 1 or 2 dB is easily detected. Listeners also detect with uncanny accuracy the location of a sound in space and discriminate between two speakers located within a few degrees of each other on the horizontal plane.

Periodic envelope fluctuations, some highly complex, are common and important information-bearing features of natural sounds including speech. The even more complex process of speech communication, which in order to develop properly requires hearing ability, is normally acquired very early in life and shortly thereafter is carried out effortlessly even in environments filled with competing sounds.

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