The Speed of Sound in Air
Determinations of the speed of sound over long distances have usually been of the flash and bang type. One of two observers, separated by a distance of some miles, fires a gun; the other starts a stop-watch as soon as he sees the flash, and stops the watch as soon as he hears the sound. This gives a reading for the time of travel of the sound as the speed of light is so great that the time for the flash of light to travel is relatively negligible. The distance between the observation posts is divided by the time of travel and gives the speed of sound in air. The method is liable to error due to the action of wind, but this can be eliminated by the method of reciprocal timing; a gun is fired from each of the observation posts so that timekeepers at each post can observe the times of travel of the sound in opposite directions. Several pairs of such timings are made and the average result of them is found.
Another source of error is known as the personal equation of the observer. There is a time lag between seeing the flash and starting the watch, and also between hearing the sound and stopping the watch; if the latter is larger than the former, the recorded time is longer than the true time; the difference is known as the personal equation of the observer.
The first accurate determination of the speed of sound in air was carried out by the French Academy in 1783 over a distance of about 18 miles, thereby considerably reducing the effect of any personal equation.
The method of reciprocal timing was used and the velocity was found to be 337 metres/sec, at 6° C, or 332 metres/sec, when reduced to 0° C. A repetition of the determination in 1822 gave, as an average result, 331 metres/sec. (1090 ft./sec.) at 0° C.
More recent methods have used microphones to detect the arrival of the sound. In one such method, two microphones are arranged in line with the source of sound; each is connected in an electric circuit containing a sensitive galvanometer.
A recording is made of the galvanometer movement on a moving photographic film, and the time interval is determined from the spacing of the two marks on the film. A similar method can be used with hydrophones (microphones designed for use under water) for the determination of the speed of sound in water, which iV about four times the speed in air.
The speed of sound in all gases increases with rise of temperature, the speed being proportional to the square root of the absolute temperature, so that:
The increase in speed is therefore not uniform, but it is very nearly so if t is not greater than 50; for air the. increase is almost exactly 2 ft./sec. each Centigrade degree rise in temperature.
Factors which affect the speed of sound are the elasticity and density of the medium. The greater the elasticity, the greater is the force which a compressed part of the medium exerts on the surrounding parts; the lower the density, the more rapidly does the medium react to the force due to elasticity. Though metals have a much higher density than gases they have a very much higher elasticity, and the speed of sound in them is greater.
flash and bang вспышка и звук
ft/sec. = foot per second фут/сек.
metres/sec. = metres per second метры/сек.
0° С - Zero Centigrades 0° Цельсия
reciprocal обоюдный, ответственный
square root квадратный корень
taut туго натянутый, упругий
time lag отставание во времени
velocity скорость, быстрота
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