Wavelengths And Frequencies

A vibrating object radiates sound waves outward from the source equally in all directions until they hit a surface that either reflects or absorbs them (Fig. 50-1). The sound waves have peaks and valleys, similar to the waves in water. The distance between the peak of one wave and the peak of the next is called the wavelength. Wavelengths of sounds we can hear vary from more than 15.25 meters (50 ft) for very low pitches to less than 25 mm (1 in.) for very high pitches.

Whether we perceive a sound as high or low depends on its frequency. The peaks in sound waves will pass a stationary point at different rates. A higher pitched sound has peaks that pass at a higher frequency (more frequently), while the peaks of a lower pitched sound pass at a lower frequency (less frequently). The frequency with

Sound pressure waves radiate in all directions.

Sound pressure waves radiate in all directions.

which these peaks pass a given point is measured as the number of cycles completed per second. A sound wave's frequency is measured in hertz (Hz). One hertz equals one cycle per second, so a wave whose peaks pass at 50 cycles per second has a frequency of 50 Hz.

As we've already noted, high-pitched sounds have higher frequencies, and high frequencies correspond to short wavelengths. Bass notes have lower frequencies; low frequency and long wavelengths go together. Frequency is an important variable in how a sound is transmitted or absorbed, and must be taken into account in designing the acoustics of a building.

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