When a vibrating tuning fork is held close to the ear the sound is heard quite loudly, but the loudness falls off rapidly as the fork is moved away. Since the prongs are of very small area, we may regard the fork as an approximately point source of energy,
and allowing for auenuation (page 307), the loudness of the sound is inversely proportional to the square of the distance from the source.
In this respect a point source of sound may be compared with a point source of electromagnetic waves (page 306). Both obey an inverse square law. If, however, the stem of the fork is pressed against a table top the sound can be heard clearly all over the room. Under these conditions the table is set into forced vibration and then acts as a large or extended source. It may thus be considered as equivalent to a large number of point sources, all of which contribute to the loudness of the sound at any point in the room. It is also noticed that, although the sound is louder, it does not last so long. The amplitude of the table top is very much less than that of the fork, but its large area transfers energy to the air at a much greater rate than the small area of the prongs.
In the next section we shall consider the special case of forced vibration, in which the body set in vibration happens to have the same natural frequency as that of the body causing the vibration.