What Is Sound?
From what has already been said, you will probably answer that sounds are waves in the air, which produce the sensation of hearing. This is correct, but sound is not limited to vibrations of the air. Other elastic substances can be made to vibrate in the same way, and the waves so produced when conveyed to our ears, produce the sensation of hearing. If you put your ear under water and then strike two stones together in the water you will hear a sound as readily as you would in air. Sound waves may be transmitted by solid bodies also, and some of these are better for this purpose than air or liquids. Perhaps you have tried the experiment of placing your ear against one of the steel rails on a railroad track to listen for the coming of a distant train. If you have tried this, you know that a sound that is too faint, or is made too far away, to be heard through the air, can easily be heard through the rail.
In view of the fact that other substances than air can be thrown into waves that will affect the sense of hearing, we may define sound as vibrations in any elastic object, that produces the sensation of hearing.
The definition is sometimes called the physical definition of sound, in contradistinction to the physiological definition of sound which is given as the sensation produced when vibrations in elastic substances are conveyed to our ears. You will see then that sound when referring to the physical definition is what makes sound known in the physiological definition. The term sound alone, without qualifications, may have either meaning, and therefore statements concerning sound may be misleading, unless we are exact in explaining the sense in which the word is used.