Sound, Origin of Sound, Media That Transmit Sound

 Sound

If a tree fell in the middle of a deep forest hundreds of kilometers away from any living being, would there be a sound? Different people will answer this question in different ways. "No," some will say, "sound is subjective and requires a listener. If there is no listener, there will be no sound." "Yes," others will say, "a sound is not something in a listener's head. A sound is an objective thing." Discussions like this one often are beyond agreement because the participants fail to realize that they are arguing not about the nature of sound but about the definition of the word. Either side is correct, depending on the definition taken, but investigation can proceed only when a definition has been agreed upon. Physicists, such as the two shown above, usually take the objective position and define sound as a form of energy that exists whether or not it is heard, and they go on from there to investigate its nature.

Origin of Sound

Most sounds are waves produced by the vibrations of material objects. In a piano, a violin, and a guitar, the sound is produced by the vibrating strings; in a saxophone, by a vibrating reed; in a flute, by a fluttering column of air at the mouthpiece. Your voice results from the vibration of your vocal chords.

In each of these cases, the original vibration stimulates the vibration of something larger or more massive, such as the sounding board of a stringed instrument, the air column within a reed or wind instrument, or the air in the throat and mouth of a singer. This vibrating material then sends a disturbance through the surrounding medium, usually air, in the form of longitudinal waves. Under ordinary conditions, the frequency of the vibrating source and the frequency of the sound waves produced are the same.

We describe our subjective impression about the frequency of sound by the word pitch. Frequency corresponds to pitch: A high-pitched sound from a piccolo has a high frequency of vibration, while a low-pitched sound from a foghorn has a low frequency of vibration. The ear of a young person can normally hear pitches corresponding to the range of frequencies between about 20 and 20,000 hertz. As we grow older, the limits of this human hearing range shrink, especially at the high-frequency end. Sound waves with frequencies below 20 hertz are infrasonic, and those with frequencies above 20,000 hertz are called ultrasonic. We cannot hear infrasonic and ultrasonic sound waves.

Media That Transmit Sound

Most sounds that we hear are transmitted through the air. However, any elastic substance-whether solid, liquid, gas, or plasma-can transmit sound. Elasticity is the ability of a material that has changed shape in response to an applied force to resume its initial shape once the distorting force is removed. Steel is an elastic substance. In contrast, putty is inelastic.¹ In elastic liquids and solids, the atoms are relatively close together, respond quickly to one another's motions, and transmit energy with little loss. Sound travels about four times faster in water than in air and about fifteen times faster in steel than in air.

Relative to solids and liquids, air is a poor conductor of sound. You can hear the sound of a distant train more clearly if your ear is placed against the rail. Similarly, a watch placed on a table beyond hearing distance can be heard if you place your ear to the table. Or click some rocks together under water while your ear is submerged. You'll hear the clicking sound very clearly. If you've ever been swimming in the presence of motorized boats, you probably noticed that you can hear the boats' motors much more clearly under water than above water. Liquids and crystalline solids are generally excellent conductors of sound-much better than air. The speed of sound is generally greater in solids than in liquids, and greater in liquids than in gases. Sound won't travel in a vacuum, because the transmission of sound requires a medium. If there is nothing to compress and expand, there can be no sound.