Page 28 - Master Handbook of Acoustics

FIGURE 1-6 Pressure variations of sound waves are superimposed on prevailing barometric
pressure. (A) An instantaneous view of the compressed and rarefied regions of a sound wave in air.
(B) The compressed regions are very slightly above and the rarefied regions very slightly below
atmospheric pressure.

Speed of Sound

The speed of sound in air is about 1,130 ft/sec (344 m/sec) at normal temperature and pressure. This
is about 770 mi/hr (1,240 km/hr). In the field of aerodynamics, this speed is known as Mach 1.0
(technically, it is air speed relative to the local speed of sound). This speed is not particularly fast in
relation to familiar things. For example, a Boeing 787 jetliner has a cruising speed of 561 mi/hr
(Mach 0.85). The speed of sound is dramatically slower than the speed of light (670,616,629 mi/hr).

It takes sound about 5 seconds to travel 1 mile. You can gauge the distance of a thunderstorm by
counting the time between the sight of the lightning strike and the sound of its thunder; if you count to
5, the storm is about a mile away. The speed of sound in the audible range is not appreciably affected
by intensity of the sound, its frequency, or by changes in atmospheric pressure.
Sound will propagate at a certain speed that depends on the medium, and other factors. The more
dense the molecular structure, the easier it is for the molecules to transfer sound energy; compared to

air, sound travels faster in denser media such as liquids and solids. For example, sound travels at
about 4,900 ft/sec in fresh water and about 16,700 ft/sec in steel. Sound also travels faster in air as
temperature increases (an increase of about 1.1 ft/sec for every degree Fahrenheit). Finally, humidity