Document kyZxEGYnymwb9z0m9Q796L7J

112 ROSS A. MC FARLAND creased, communication by speech becomes progressively more difficult and sub ject to errors; and (IB) strong frequency components near the frequencies of speech sounds are more disruptive than very high or low audible frequency components.97. Special training in speaking and listening can yield very significant improvements in communications under noisy conditions.98 For instance, the avoidance of words i of low intelligibility in the presence of noise may yield valuable improvements in | communications. When workmen must communicate frequently in noisy places, | a set of gestures and hand signals often develops spontaneously. However, it should be realized that precise understanding of the signals may well be lacking. They, may. have been established without any special forethought and may be totally inadequate when certain circumstances involving hazards arise. Wherever hand signals are used, it is advisable to review the system critically with a view to determining the dangers of misunderstanding.2 The degree of speech communication possible in the presence of noise of given intensities and frequencies has been determined by Rosenblith, Stevens et al., and should be useful for setting maximum permissible noise levels for certain jobs or'j areas (see Figure 12). At the 55 decibel level (SC-55) conversation is intelligible | at a distance of several feet with a normal voice. At the 65 decibel level, however, communication is difficult, while at 75 decibels communication is marginal.99 It; should be noted that the speech communication level expressed in decibels varies with the frequency, or "pitch," of the sound. At low frequencies a given background noise is less detrimental to communication than it is in higher frequencies. In; addition,Interrupted noise oFdiicon{in'u6us"tbhWIfFmuclT'morTallnoytef't] are steady noises. For a complete discussion of noise and its physiological effect;, upon man in industry see Chapter XVIII. Vibration may be frequently linked with noise in many industrial j obs since; the two often have common origins. The specific effects are not too well known^ although it has been definitely established that vibration adversely influences7; binocular acuity, especially at frequencies between 25 and 40 and between 60| and 90 cycles per second. This may be due to the attempt of the eyes to 'follow;! movements of vibrating instruments or because the eyeball is set into resonanc^f with the vibration. Certain reflex actions of the body may be diminished at somel| frequencies, while others may be completely depressed at different amplitudes.24 |n: additipn,, ife-jg^tswjTthat people exposed to -vibration for long periods ot`iil$. frequently cqmpiain of headaches, exhaustion, and fatigue. Figure 13 shows tfi subjective responses as determined by three investigators.93 Since the human body;' - SrSrStevena-andH. Davis, Hearing: Its Psychology and Phymlogv..Miiey.,NswJXfid im " G. L. Dragert, J. C. Kelley, M. W. Buck, and T. D. Hanley, Training manual for portable; interphone trainer, Special Devices Center Tech. Rept. 104-S-1S, Port Washington, N. Y., ri.'dS " W. A. Rosenblith, K. N. Stevens, and the Staff of Bolt, Beranek, and Newman, Hand-.; book pf acoustic noise control: Volume II. Noise and man, WADC Tech. Rept. SB-104, Wright; Air Development Center, Wright-Patterson Air Force Base, Ohio, 1953. HUMAN ENGINEERING AND INDUSTRIAL SAFETY 113 sensitive to the various types of vibration encountered in some industrial and is usually much less capable of tolerating this vibration than is the ore of the machine or environment, it is essential that operators' physiolimits of tolerance'be clearly recognized. However, the reaction to vibration at different frequencies and amplitudes. In cases where different vibrations ised of varying frequencies and amplitudes are superimposed, the ones with gher values should be chosen in determining comfort or safety criteria since 'will be the more unpleasant, that is, the higher the frequency the lower the \ \ ^\ \\ Nso \\ \ \6o \ N S\ \to ,.6 30 4^-75 SC - 65 SC -55 .SC-45 SC - 35 SC - 25 150 300 600 1200 2400 4800 300 6oo 1200 2400 4800 10000 f Frequency band cpe communication criteria (SC). If the sound pressure-levels of the environ- exceed the SC-45 curve at the various frequency bands, relaxed conversation will normal vice at a distance of 10 feet. At the SC-55 level communication is ^gormal voice at 3 feet, with a raised voice at 6 feet, and with a .very loud vdice |$SCf65 a raised voice is necessary at 2 feet, and a very loud voice at 4 feetvA.t fejgation is minimal, requiring a very loud voice at 1 foot and Bhouting at 2 to 3 Lophblith, Stevens el a/*) x