UNIT VI SECTION 7 SOUND IS PRODUCED BY VIBRATIONS IN MATTER Introduction. The sense of hearing is next in importance to the sense of sight in enabhng us to receive impressions from a distance. In this Section we shall consider the nature and characteristics of sound waves, which are produced by vibrations in matter. Sound Waves Are Longitudinal or Compressional Waves. Light waves are transverse to the direction in which the wave travels, whereas sound waves are longitudinal, i.e., they lie along the direction in which the wave travels. Sound waves are compressional waves. The elasticity of the original layer of air compressed by the sound source makes it expand, thus passing on the state of compression to the next layer of air beyond. Sounds Are Transmitted by Waves in Matter. Any stimulus which will produce the sensation of hearing is called a sound. This sensation is produced by vibration of the eardrum, which, in turn, is caused to vibrate by a wave motion of some medium, usually the air. The compressional waves in this medium itself are produced by the vibration of some body. The air is not essential for the transmission of vibrations to the ear. Any medium which can transmit compressional waves will serve. A swimmer can hear better under water than above water, because water carries vibrations faster and with greater intensity than air. If the ear is placed on a railroad track, the sound of an approaching train can be heard, because solids like wood or steel transmit vibrations. Vibratory motion is the commonest of all motions. Automobiles, bridges, and buildings may be caused to vibrate all too easily. Nearly any material is more or less elastic and will resist any attempt to deform it, thus producing vibrations when the deforming force is released. Sounds may be produced by blows, as with a hammer, by explosions, by compression, or by any other type of deformation, such as a pull or a twist. 448
SOUND PRODUCED BY VIBRATIONS IN MATTER 449 The singing of teakettles shortly before the boiling-point is reached is due to the fact that the condensation and collapse of steam globules as they rise into the colder water near the surface set up vibrations within the liquid. This process is called cavitation. Sound waves differ from electromagnetic waves in that the former are transmitted by matter, while the latter are transmitted through empty space. The fact that sound waves require matter for their transmission can be readily demonstrated by ringing a bell in a jar which can be evacuated. As the air is removed, the intensity of the sound decreases until it can no longer be heard. It is important to remember that waves are disturbances that travel in a medium but that the material of the medium itself does not travel. Sound Waves Travel Much More Slowly than Light Waves. The thunderclap is heard some time after a distant flash of lightning The jet of steam from a distant steamer is seen before the sound is seen. of the whistle is heard. Other common observations lead to the conclusion that an appreciable time is required for sound to travel from one point to another. The velocity of sound in dry air is 1085 feet per second at 0° C. and increases at the rate of 2 feet per second per degree C. rise in temperature. Corrections for both the temperature and moisture content of air must be made in calculating the speed of sound in air, because the velocity imparted to molecules depends upon their masses rather than their number. Slight changes in barometric pressure do not afTect the speed of sound because they merely determine the number of molecules present. Temperature affects the speed of sound in air because it changes the natural speed of the molecules. It thus becomes evident that the presence of water vapor changes the density of the atmosphere by introducing molecules of different mass, while changes in the density of the atmosphere due to barometric changes are the result of changing the number of molecules present in a given volume. The velocity of sound in water is more than four times as great as that in air, and in steel it is nearly fifteen times as great as that in air. Sound waves, like light waves, may be reflected. Echoes are reflected sound waves. Thus the distance between two objects may be determined by noting the time required for a given sound to return to its source as an echo. The combined effect of wind, humidity, and temperature on the passage of sound through air often causes soundlocating instruments to make serious errors; for example, three trained anti-aircraft units in England on May 23, 1917, reported that airships were nearly overhead, though in reality they were 25 miles distant.