05.02.2011 09:05 AM
Acoustical Wavelengths

When considering the acoustics of a space, whether a control room, studio, or auditorium, it’s often helpful to think in terms of sound wavelengths, rather than frequency. Of course the two are related by the speed of sound (wavelength = speed of sound / frequency) but understanding the physical size of acoustical waves can help in room design and geometry, choosing appropriate acoustical room materials, treatments and placement, and in troubleshooting acoustical problems.

The speed of sound varies with such factors as temperature and relative humidity, and it may be necessary in some cases to calculate it based on these specific environmental factors. Note that as the speed of sound changes, so will the sound wavelength for a given frequency (another good reason to think in terms of wavelengths). This is important, for example, in sound systems designed to work in outdoor arenas where temperatures vary at different seating levels, and at different times of the day.

But for many situations, using a typical value of 1,130 feet per second for the speed of sound is a good number to start with.

Using that value, a sound wave of 1130 Hz has a wavelength of one foot. The generally accepted frequency range for audible sound spans from 20 Hz to 20000 Hz. For an acoustical sound wave this translates to a wavelength range of 56.5 feet at the low end to 0.0565 feet (0.678 inches) at the highest frequency.

This is quite a wide range, and can be divided into three main parts—the longest wavelengths, the shortest wavelengths, and wavelengths in between. There are formulas for approximating when one part morphs into the next, but it comes down to the wavelength of sound compared with the physical dimensions of a space, or objects in a space. This in turn determines how a particular sound wavelength “behaves” in that space and how it should be treated.

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