Intensity
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-210
2025-11-16
24
The intensity of a sound depends upon three things: the amplitude of the vibration producing it, the area, and the distance at which the sound is heard.
- Amplitude. - When a tuning fork is struck, the energy which it can impart to the air will depend upon the extent of its vibrations. If these are slight, only a weak tone is produced. Strike a harder blow, and the amplitude increases, the energy the fork can give to the air is greater, and the sound is louder. The relation between amplitude and intensity may be readily shown by substituting a tuning fork for the whalebone in the demonstration in $ 190. Make a number of traces on the glass when the fork is sounding at different intensities, and compare them.
- Area. -A small tuning fork, on being put into vibration, sets only a small quantity of air in motion and gives a sound having but little intensity; but if the prongs are broad, the amount of air put in motion is greater and the sound is louder.
- Distance. - Since the sounding body is sending out waves in every direction, the sound wave is the outside of a spherical shell of which the body is the center. The shell of molecules to be vibrated becomes larger and larger as the sound wave passes out from the center, and hence the energy that can be imparted to each air mole- C clue becomes less and less. The intensity of sound depends directly on the amount of this energy.
Suppose a source of sound is at the point A, Fig. 1. Suppose this to be the center of a spherical shell of which the radius is AB: the wave of sound produced by the sounding body will be received all over the surface of the sphere. If now this sphere is replaced by a larger one, of which the radius is AC, the same sound wave will be received over the larger surface and the intensity of the sound on' each unit of area of this surface will be decreased.
The areas of these surfaces are directly proportional to the squares of their radii; hence we may write: the intensity of sound varies inversely as the square of its distance from the sounding body.
If the waves of sound can be kept in one direction, as by being reflected from the inner surface of a tube, the intensity at any point will be greater and they will go much farther. Speaking tubes are made on this principle.
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