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Consumer Electronics by S. P. Bali

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FORCED VIBRATIONS

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 approximate point source of energy, and allowing for attenuation 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. 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 vibrations and then acts as a large extended source. ...

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