# Frequency (Auditory)

Technical definition of the range of sounds audible to humans.

Humans can detect sound waves with frequencies that vary from approximately 20 to 20,000 Hz. Probably of greatest interest to psychologists are the frequencies around 500-2,000 Hz, the range in which sounds important to speech typically occur. Humans are most responsive to sounds between 1,000 and 5,000 Hz, and are not likely to hear very low or very high frequencies unless they are fairly intense. For example, the average person is approximately 100 times more sensitive to a sound at 3,000 Hz than to one at 100 Hz. People can best differentiate between two similar pitches when they are between 1,000 and 5,000 Hz.

The relationship between frequency and pitch is predictable but not always simple. That is, as frequency increases, pitch becomes higher. At the same time, if the frequency is doubled, the resulting sound does not have a pitch twice as high. In fact, if one listens to a sound at a given frequency, then a second sound at twice the frequency, the pitch would have increased by one octave in pitch. Each doubling of frequencies involves a one-octave change, for example, the Middle C note on a piano has a frequency of 261.2; the C note one octave higher is 522.4, a change of 261.2 Hz. The next C note on the piano has a frequency of 1046.4 Hz, or a change of 523.2 Hz.

When an individual hears a complex sound consisting of many different wavelengths, such as a human voice, music, and most sounds in nature, the ear separates the sound into its different frequencies. This separation begins in the inner ear, specifically the basilar membrane within the cochlea. The basilar membrane is a strip of tissue that is wide at one end and narrow at the other. When the ear responds to a low frequency sound, the entire length of the basilar membrane vibrates; for a high frequency sound, the movement of the membrane is more restricted to locations nearer the narrow end. Thus, a person can hear the different frequencies (and their associated pitches) as separate sounds.

The ability to hear declines with age, although the loss is greatest for high frequency sounds. At age 70, for example, sensitivity to sounds at 1,000 Hz is maintained, whereas sensitivity to sounds at 8,000 Hz is markedly diminished. As many as 75 percent of people over 70 years of age have experienced some deterioration in their hearing.