UNIT VI SECTION 8 ACOUSTICS IS A PROBLEM OF MODERN CIVILIZED SOCIETY Introduction. One can scarcely hear himself talk in many factories where machinery of all kinds is creating a ceaseless din and clatter of squeaks, rumblings, rattles, and hammerings. Modern warfare is characterized by the sounds of airplane motors, machine guns, exploding bombs, and heavy gunfire. The noise of streetcars, automobile horns, truck and bus gears, automobile exhausts, traffic signals, the blare of radio loud-speakers, and the cries of newsboys are so commonplace that a city dweller may find it difficult to adjust himself to the relative quiet of the countryside. While one may become so well adjusted to noises that he cannot study without a radio playing, there is no question concerning the nervous tension that is created by the continuous stimulation of the nerves of the ear by unnecessary and disturbing sounds which are called noise. Even music may be disturbing to frayed nerves. One might almost say that one's social maturity can be judged by the amount of unnecessary noise that he makes. It is only common sense that the more noise there is, the louder one will have to talk and the more penetrating automobile horns will have to be. Progress, as far as sound in its relation to society is concerned, is represented by campaigns, laws, and methods of reducing unnecessary sounds, rather than the development of more and louder sounds. The radio loudspeaker has given man a marvelous method of amplifying sound, but the radio loud-speaker may become a public nuisance when employed by people, young or old, who are thoughtless of the sound sensitivities of other people. This Section deals with the control of sound, which is the problem of the science of acoustics. The Intensity of Sound Waves May Be Increased by Resonance. The intensity or loudness of a sounding body may be increased by This principle can be illustrated by striking a use of a sounding board. 460
ACOUSTICS 461 tuning fork and then holding it against the table top. The table top is caused to vibrate and thus becomes a vibrating medium of greater intensity because it has a greater surface to agitate the air against it. Inasmuch as the energy of vibration is greater, it must last for a shorter time, the vibrations dying out much faster than when the tuning fork is held in the hand. The original vibrating body is called the generator, while the sounding board is called the resonator. Thus the strings in a piano make up the generator, while the sounding board is the resonator. There are two distinct kinds of resonators. One kind has no vibration frequency of its own and responds to all of the vibrations of the generator. Such vibrations are called forced vibrations. A second type of resonator possesses a natural frequency and strengthens sounds of its own pitch only. Such vibrations are called sympathetic vibrations or resonance. A tuning fork is usually mounted in a hollow box whose size is such that the air column will vibrate in resonance with the fork. 256 VPS 256 VPS 15 Y ±) B Sympathetic vibrations. Just as a child in a swing can be made to swing farther by applying a gentle impulse at the proper time, so one tuning fork can be caused to vibrate by the vibrations of another fork in tune with it. If the loud pedal of a piano is held down so that all of the strings are free to vibrate, that string will vibrate whose natural frequency corresponds to the frequency of a briefly sung note. Sound Waves May Be Canceled by Interference. Mufflers used to absorb or minimize sound energy from internalcombustion engine exhausts depend on the cancellation of sound waves by interference.^ In one type of muffler the length of the closed tube is one fourth of the wave length to be canceled. Waves entering the tube are reflected and return to the main tube 90 degrees out of phase, thus canceling the incoming sound wave. In a second type of muffler one path is longer than the other by one half of the wave length to be canceled, and the waves uniting at the outlet cancel each other. Such muf- ' Interference may be explained by referring to a boy in a swing. If a person gives the swing a push each time it reaches the end of a swing, the swing will swing farther and farther with each push; but if a person gives the swing a push when it is not at the end of the swing, the amplitude of the swing will be decreased. A push equal to the force of the swing, but in the opposite direction, when applied as the swing comes to the position closest to the ground, will stop the motion entirely. Sound waves are canceled in the same way by other sound waves which are out of phase with the incoming sound waves.