Document zzd1mrpXQ7B85GXnGGryeXog0

622 JEROME R. COX, JR. It is thus incumbent on industrial managements to recognize the trend t< more noisy occupations and toward compensation for industrial hearing 16 understand the medical, legal, and engineering developments relative to ind noise; and to examine their own operations in the light of these development! I. Fundamentals of Noise Noise is frequently defined as any unwanted sound. This, however' functional definition and not a physical one. The question of whether a soA wanted or not depends not only on the individual, but also on the circum^f Let us momentarily bypass the subjective portion of the problem and examl physical nature of sound. A. WHAT IS SOUND? Sound is a traveling disturbance, which usually appears as variations" pressure and density of the atmosphere. A source of sound that is famili physically simple is the tuning fork. When struck, the tines vibrate to a setting the surrounding air into motion. The relative motion of the air in nil ing regions causes pressure fluctuations above and below atmospheric; These pressure fluctuations, known as compressions (regions of increased1'pL and rarefactions (regions of reduced pressure), travel outward from the';ffb: all directions and produce a sound wave (see Figure 1). yi Figure 1. A schematic representation of a sound wave generated by a tuning fS^ani . the Bound pressure against distance at one instant of time. (After Pietrashnja6' B. PURE TONES _The-SOund-pr-oduced-by-a-tuning-fork-is-a-simple-tone7T>ure fact, it is often called a pure tone. The pressure variations for such a'ton the sine wave pattern shown in Figure 1. This figure shows the pressure! .,'i t J`A. C. Pietrasanta, Some fundamental principles of noise control, Refng.'Erfqjt 100 (April, 1954); 62, 56, 96, 98,102 (May, 1954). te'iv it, to-" NOISE AND THE CONSERVATION OF HEABING 623 jj!(upfa)|mstant. Since the entire sound wave is travelling away from the Taffikpeed of about 1100 feet per second (the velocity .of sound at freezing j7- feet per second, and it increases about one foot per second for ^SllFahrenheit increase in temperature), the pressure pattern an instant iw-ioffiBHow the sine wave displaced slightly to the right. |lBl) C. FREQUENCY Ue^the^tuning fork smaller, the vibrations would be more rapid. More Jte to and fro vibrations of the tines, would occur in a second. The ' juffigR^ofithe sound, which is the number of cycles per second (c.p.s.), would, iMigher. D. WAVE LENGTH effime this higher frequency sound wave has advanced the same distance [shown in Figure 1, more cycles of compression and rarefaction of the ((would have been emitted by the source. In order that this, increased ftfluist'Eif' ||||ycles exist in the same space, the distance from the peak of one fid the peak of the next must be smaller. This distance between corn- called a wave length and is designated by the symbol X (lambda). Its |tb the frequency, /, and the speed of sound, c, is given by the equation: W fX = c (1) BMef fr a frequency of 1100 c.p.s. the wave length is one loot; 110 c.p.s. igth would be 10 feet. In both cases the frequency times the wave uSl to the speed of sound, and this relationship holds no matter what E. SOUND PRESSURE !&gg|to see that these two related quantities, frequency and wave length, wr comPletely tne produced by the tuning fork. The fork may "'"'"iJy and produce a faint sound, or firmly and produce a loud sound. Both ^_in|haVe exactly the same frequency and wave length since the tines will ^^te^^aji.ff-frc)-t;he-8ame-number--6Ftimes in-ai`second:'The~haTdef''t'he'f6rk'is the greater the distance the tines travel during each cycle. This K^^^hSp'tance travelled by the tines will cause greater pressure fluctuations ^^^araflVelow atmospheric pressure. Thus the harder the tines ard'struck, the i^^l(al)il('Pdieight,of ihe;,sound wave shown.in Figure 1, ________________ m 55?f!P?lerence 'between the atmospheric pressure and the actual pressure mnnKff'lRRefaction or compression resulting from a sound wave is called the A convenient way to measure this sound pressure is in fractions of ^^^Mrljvis a unit of pressure equal to atmospheric pressure (bar comes from ISJiujvClrOek word as does the word barometer, the instrument that measures