The term acoustics refers to the physics of sound waves as they are transmitted through air. Acoustics is important to architects and engineers who design and build concert halls, where sound must propagate from a stage or speaker system to a large number of people. Engineers consider acoustics in the construction of speaker enclosures. Some knowledge of acoustics is helpful if you want to set up a high-fidelity (hi-fi) sound system.

The sound spectrum
Sound consists of molecular vibrations at audio frequency, ranging from about 20 Hz to 20 kHz. Young people can hear the full range of audio frequencies; older people lose hearing sensitivity at the upper and lower extremes. An elderly person might only hear sounds from 50 to 5000 Hz.

In music, audio frequencies are divided into three broad, vaguely defined parts, called bass (pronounced “base”), midrange, and treble. The bass frequencies start at 20 Hz and extend to 150 or 200 Hz. Midrange begins at this point, and extends to 2000 or 3000 Hz.

Treble consists of the audio frequencies higher than midrange. As the frequency increases, the wavelength becomes shorter. In air, sound travels at about 1100 ft/s, or 335 m/s. The relationship between the frequency f of a sound wave in hertz, and the wavelength wft in feet, is as follows:  wft 1100/f

In the metric system, the relationship between f in hertz and wm in meters is given by wm 335/f

The metric formula is also valid for frequencies in kilohertz and wavelengths in millimeters. When using the formulas, be sure you don’t get the units confused.

A sound of 20 Hz has a wavelength of 55 ft, or 16.8 m, in the air. A sound of 1000 Hz produces a wave measuring 1.1 ft (13 in), or 33.5 cm. At the extreme treble end of the audio spectrum, a wave is just 0.055 ft (two-thirds of an inch), or 17 millimeters, long at 20 kHz. In substances other than air, such as water or metal, the above formulas do not apply. There is also some change in the speed of sound with changes in air pressure and humidity.

Sound waves 55 ft long behave much differently than waves measuring less than an inch, and both of these act differently than a wave 13 in in length. Acoustics engineers must take this matter into consideration when designing sound systems.

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