6.1 THE PHYSICS OF SOUND AND HEARING Sound Production and Propagation Reflection, Dispersion, Absorption, and Refraction Wave Character Energy Relationships in Sound The Hearing Mechanism Hearing Impairment Audiometry Principles Audiometric Practices Hearing Aids 6.2 NOISE SOURCES Typical Range of Noise Levels Characteristics of Industrial Noise Industrial Noise Sources Mining and Construction Noise Transportation Noise Urban Noise Specific Noise Sources 6.3 THE EFFECTS OF NOISE Reactions to Noise Auditory Effects PTS 467 Acoustic Trauma Damage-Risk Criteria Psychological Effects of Noise Pollution Speech Interference Annoyance Sleep Interference Effects on Performance Acoustic Privacy Subjective Responses 6.4 NOISE MEASUREMENTS Basic Definitions and Terminology Frequency Sensitivity and Equal Loudness Characteristics Objective and Subjective Values Weighting Networks Frequency Analysis of Noise Speech Interference and Noise Criteria (NC) Curves Vibration and Vibration Measure- ment Measuring Noise Background Corrections Instruments for Measuring Noise Impact and Impulse Magnitudes Monitoring Devices (Noise Dosi- meters) Field Measurements Practical Problems 6.5 NOISE ASSESSMENT AND EVALUATION Workplace Noise Noise Dosimeters Sound Level Meters Community Noise Noise Rating Systems Instrumentation 6 Noise Pollution David H.F. LiuԽHoward C. Roberts ©1999 CRC Press LLC Traffic Noise Prediction Plant Noise Survey 6.6 NOISE CONTROL AT THE SOURCE Source-Path-Receiver Concept Noise-Level Specifications Process Substitution Machine Substitution Systems Design Control of Noise Source by Design Reducing Impact Forces Reducing Speeds and Pressures Reducing Frictional Resistance Reducing Radiating Area Reducing Noise Leakage Isolating and Damping Vibrating Elements Control of Noise Source by Redress Balancing Rotating Parts Reducing Frictional Resistance Applying Damping Materials Sealing Noise Leaks Performing Routine Maintenance 6.7 NOISE CONTROL IN THE TRANSMISSION PATH Acoustical Separation Absorbent Materials Acoustical Linings Physical Barriers Barriers and Panels Enclosures Isolators and Silencers Vibration Isolators and Flexible Couplers Mufflers and Silencers 6.8 PROTECTING THE RECEIVER Work Schedules Equipment and Shelters ©1999 CRC Press LLC Sound can be defined as atmospheric or airborne vibra- tion perceptible to the ear. Noiseis usually unwanted or undesired sound. Consequently, a particular sound can be noise to one person and not to others, or noise at one time and not at other times. Sound loud enough to be harmful is called noise without regard to its other characteristics. Noise is a form of pollution because it can cause hearing impairment and psychological stress. This section introduces the subject of sound in engi- neering terms and includes appended references which pro- vide detailed back-up material. It includes the general prin- ciples of sound production and propagation, a description of the ear and its functions, a description of the effects of noise on the hearing apparatus and on the person, and an introduction to hearing measurement and hearing aids. Sound Production and Propagation Audible sound is any vibratory motion at frequencies be- tween about 16 and 20,000 Hz; normally it reaches the ear through pressure waves in air. Sound is also readily transmissible through other gases, liquids, or solids; its ve- locity depends on the density and the elasticity of the medium, while attenuation depends largely on frictional damping. For most engineering work, adiabatic conditions are assumed. Sound is initially produced by vibration of solid objects, by turbulent motion of fluids, by explosive expansion of gases, or by other means. The pressures, amplitudes, and velocities of the components of the sound wave within the range of hearing are quite small. Table 6.1.1 gives typical values; the sound pressures referenced are the dynamic ex- cursions imposed on the relatively constant atmospheric pressure. In a free field(defined as an isotropic homogeneous field with no boundary surfaces), a point source°of sound produces spherical (Beranek 1954) sound waves (see Figure 6.1.1). If these waves are at a single frequency, the instantaneous sound pressure (P r,t ) at a distance r and a time t is P r,t ϭ[(V2 ෆ P)/r] cos[ (t Ϫ r/t)] dynes/cm 2 6.1(1) where the term v2 ෆ P ෆ denotes the magnitude of peak pres- sure at a unit distance from the source, and the cosine term represents phase angle. In general, instantaneous pressures are not used in noise control engineering (though peak pressures and some non- sinusoidal pulse pressures are, as is shown later), but most sound pressures are measured in root-mean-square (RMS) values—the square root of the arithmetic mean of the squared instantaneous values taken over a suitable period. The following description refers to RMS values. For spherical sound waves in air, in a free field, RMS pressure values are described by P r ϭP o /r dynes/cm 2 6.1(2) where P r denotes RMS sound pressure at a distance r from the source, and P o is RMS pressure at unit distance from the source. (Meters in metric units, feet in English units.) Acoustic terminology is based on metric units, in general, though the English units of feet and pounds are used in engineering descriptions. A few other terms should be defined, and their mathe- matical relationships noted. Sound intensity I is defined as the acoustic power W passing through a surface having unit area; and for spher- ical waves (see Figure 6.1.1), this unit area is a portion of a spherical surface. Sound intensity at a distance r from a source of power W is given by I r ϭW/4 r 2 watts/cm 2 6.1(3) Sound intensity is also given by I r ϭP 2 o /r 2 c watts/cm 2 6.1(4) where is the adiabatic density of the medium, and c is the velocity of sound in that medium. Similarly, the fol- lowing equation gives the sound pressure if the sound is radiated uniformly: P r ϭ(1/r)W ෆ c ෆ /4 ෆ ෆ 6.1(5) If the radiation is not uniform but has directivity, the term c is multiplied by a directivity factor Q. To the noise- control engineer, the concept of intensity is useful princi- pally because it leads to methods of establishing the sound power of a source. The term c is called the acoustic impedance of the medium; physically it represents the rate at which force can be applied per unit area or energy can be transferred per unit volume of material. Thus, acoustic impedance can be expressed as force per unit area per second (dynes/ cm 2 /sec) or energy per unit volume per second (ergs/cm 3 / sec). Table 6.1.1 shows the scale of mechanical magnitudes represented by sound waves. Amplitude of wave motion at normal speech levels, for example, is about 2 ϫ10 Ϫ6 cm, or about 1 micro inch; while amplitudes in the lower part of the hearing range compare to the diameter of the hydrogen atom. Loud sounds can be emitted by a vibrat- ©1999 CRC Press LLC 6.1 THE PHYSICS OF SOUND AND HEARING ing partition even though its amplitude is only a few mi- cro inches. REFLECTION, DISPERSION, ABSORPTION, AND REFRACTION Sound traversing one medium is reflected when it strikes an interface with another medium in which its velocity is different; the greater the difference in sound velocity, the more efficient the reflection. The reflection of sound usu- ally involves dispersion or scattering. Sound is dispersed or scattered when it is reflected from a surface, when it passes through several media, and as it passes by and around obstacles. Thus, sound striking a building as plane waves usually is reflected with some dis- persion, and plane waves passing an obstacle are usually somewhat distorted. This effect is suggested by Figure 6.1.1. The amount of dispersion by reflection depends on the relationship between the wavelength of the sound and the contour of the reflecting surface. The absorption of sound involves the dissipation of its mechanical energy. Materials designed specifically for that purpose are porous so that as the sound waves penetrate, the area of frictional contact is large and the conversion of molecular motion to heat is facilitated. Sound waves can be refracted at an interface between media having different characteristics; the phenomenon can be described by Snell’s law as with light. Except for events taking place on a large scale, refraction is usually distorted by dispersion effects. In the tracking of seismic waves and undersea sound waves, refraction effects are im- portant. In engineering noise analysis and control, reflection, re- fraction, and dispersion have pronounced effects on di- rectivity patterns. WAVE CHARACTER Since sound is a wave motion, it can be focussed by re- flection (and less easily by refraction), and interference can ©1999 CRC Press LLC FIG. 6.1.1 Sound sources. A point source at S produces a calculable intensity at a. The sound waves can set an elastic membrane or partition (like a large window) at W into vibration. This large source can produce roughly planar sound waves, which are radiated outward with little change in form but are distorted and dispersed as they pass the solid barrier B. TABLE 6.1.1 MECHANICAL CHARACTERISTICS OF SOUND WAVES RMS Sound RMS Sound Sound RMS Sound Particle Particle Pressure Pressure Velocity Motion at Level (dynes/cm 2 ) (cm/sec) (1,000 Hz cm) (dB 0.0002 bar) Threshold of hearing 0000.0002 00000.0000048 000.76 ϫ 10 Ϫ9 000 0000.002 00000.000048 007.60 ϫ 10 Ϫ9 020 Quiet room 0000.02 00000.00048 076.00 ϫ 10 Ϫ9 040 0000.2 00000.0048 760.00 ϫ 10 Ϫ9 060 Normal speech at 3Ј 0002.0 00000.048 007.60 ϫ 10 Ϫ6 080 Possible hearing impairment 0020.0 00000.48 076.00 ϫ 10 Ϫ6 100 0200 00004.80 760.00 ϫ 10 Ϫ6 120 Threshold of pain 2000 00048.0 007.60 ϫ 10 Ϫ3 140 Incipient mechanical damage 0020 ϫ 10 3 00480 076.00 ϫ 10 Ϫ3 160 0200 ϫ 10 3 04800 760.00 ϫ 10 Ϫ3 180 Atmospheric pressure 2000 ϫ 10 3 48000 007.60 200 occur, as can standing wave patterns. These effects are im- portant in noise control and in auditorium acoustics. Another wave–motion phenomenon, the coincidence ef- fect, affects partition behavior. When two wave forms of the same frequency are su- perimposed, if they are inphase, they add and reinforce each other; while if they are of opposing phase, the resul- tant signal is their difference. Thus, sound from a single source combined with its reflection from a plane surface can produce widely varying sound levels through such in- terference. If reflective surfaces are concave, they can fo- cus the sound waves and produce high sound levels at cer- tain points. Dispersion often partially obscures these patterns. Sound from a single source can be reinforced by re- flection between two walls if their separation is a multiple of the wavelength; this standing-wave pattern is described by Figure 6.1.2. These phenomena are important in auditorium design, but they cannot be ignored in noise control work. Reinforcement by the addition of signals can produce lo- calized high sound levels which can be annoying in them- selves and are also likely to produce mechanical vibra- tions—and thus new, secondary noise sources. Random noise between parallel walls is reinforced at a series of frequencies by the formation of standing waves; this reinforcement partially accounts for the high noise level in city streets. ENERGY RELATIONSHIPS IN SOUND The magnitudes most used to describe the energy involved in sound or noise are sound pressure and sound power. Pressure, either static (barometric) or dynamic (sound vi- brations), is the magnitude most easily observed. Sound pressure is usually measured as an RMS value—whether this value is specified or not—but peak values are some- times also used. From the threshold of hearing to the threshold of pain, sound pressure values range from 0.0002 to 1000 or more dynes per square centimeter (Table 6.1.1). To permit this wide range to be described with equal resolution at all pressures, a logarithmic scale is used, with the decibel (dB) as its unit. Sound pressure level (SPL) is thus defined by SPL ϭ20 log 10 (P/P ref ) dB 6.1(6) where P is measured pressure, and P ref is a reference pres- sure. In acoustic work this reference pressure is 0.0002 dynes/cm 2 . (Sometimes given as 0.0002 microbars, or 20 micronewtons/meter 2 . A reference level of 1 microbar is sometimes used in transducer calibration; it should not be used for sound pressure level.) Table 6.1.2 lists a few rep- resentative sound pressures and the decibel values of sound pressure levels which describe them. This logarithmic scale permits a range of pressures to be described without using large numbers; it also repre- sents the nonlinear behavior of the ear more convincingly. A minor inconvenience is that logarithmic quantities can- not be added directly; they must be combined on an en- ergy basis. While this combining can be done by a math- ematical method, a table or chart is more convenient to use; the accuracy provided by these devices is usually ad- equate. Table 6.1.3 is suitable for the purpose; the procedure is to subtract the smaller from the larger decibel value, find the amount to be added in the table, and add this amount to the larger decibel value. For example, if a 76 dB value is to be added to an 80 dB value, the result is 81.5 dB (80 plus 1.5 from the table). If more than two values are to be added, the process is simply continued. If the smaller of the two values is 10 dB less than the larger, it adds less than 0.5 dB; such a small amount is usually ignored, but if several small sources exist, their combined effect should be considered. The sound power of a source is important; the magni- tude of the noise problem depends on the sound power. Sound power at a point (sound intensity) cannot be mea- sured directly; it must be done with a series of sound pres- sure measurements. The acoustic power of a source is described in watts. The range of magnitudes covers nearly 20 decimal places; again a logarithmic scale is used. The reference power level normally used is 10 Ϫ12 watt, and the sound power level (PWL) is defined by PWL ϭ10 log 10 (W/10 Ϫ12 ) dB 6.1(7) or, since the power ration 10 Ϫ12 means the same as Ϫ120 dB, the following equation is also correct: PWL ϭ10 log 10 W ϩ120 dB 6.1(8) In either case, W is the acoustic power in watts. ©1999 CRC Press LLC FIG. 6.1.2Reflection of sound waves. If the distance d between two parallel walls is an integral number of wavelengths, standing waves can occur. Interaction between direct waves from a source S and the reflected waves can produce interference. Sound power levels are established through sound pres- sure measurements; in a free field, sound is radiated spher- ically from a point source, thus PWL ϭSPL ϩ20 log 10 r ϩ0.5 dB 6.1(9) For precise work, barometric corrections are required. In practical situations, a directivity factor must often be in- troduced. For example, if a machine rests on a reflecting surface (instead of being suspended in free space), reflec- tion confines the radiated sound to a hemisphere instead of a spherical pattern, with resulting SPL readings higher than for free-field conditions. Actual sound power values of a source, in watts, can be computed from PWL values using Equation 6.1(8). In all cases, the units should be stated when sound pres- sure or sound power values are listed (dynes/cm 2 , watts), and the reference levels should be made known when sound pressure levels or sound power levels are listed (0.0002 dynes/cm 2 , and 10 Ϫ12 watt). The Hearing Mechanism Sound reaches the ear usually through pressure waves in air; a remarkable structure converts this energy to electri- cal signals which are transmitted to the brain through the auditory nerves. The human ear is capable of impressive performance. It can detect vibratory motion so small it ap- proaches the magnitude of the molecular motion of the air. Coupled with the nerves and brain, the ear can detect frequency differences and combinations, magnitude, and direction of sound sources. It can also analyze and corre- late such signals. A brief description of the ear and its func- tioning follows. Figure 6.1.3 shows the anatomical division of the ear. The external human ear (called the auricle or the pinna) and the ear opening (the external auditory canal or mea- tus) are the only parts of the hearing system normally vis- ible. They gather sound waves and conduct them to the eardrum and inner drum. They also keep debris and ob- jects from reaching the inner ear. The working parts of the ear include the eardrum and organs which lie behind it; they are almost completely sur- rounded by bone and are thus protected. The sound transducer mechanism is housed in the mid- dle ear (Figure 6.1.4). The eardrum or tympanic membrane is a thin, tough membrane, slightly oval in shape and a lit- tle less than 1 cm in mean diameter; it vibrates in response to sound waves striking it. The vibratory motion is trans- mitted through three tiny bones, the ossicles (the malleus, the incus, and the stapes; or the hammer, anvil, and stir- rup), to the cochlea; it enters the cochlea at the oval win- dow. ©1999 CRC Press LLC TABLE 6.1.2REPRESENTATIVE SOUND PRESSURES AND SOUND LEVELS Sound Pressure Sound Level Source and Distance (dynes/cm 2 ) (decibels 0.0002 bar) Saturn rocket motor, close by 1,100,000.06 195 Military rifle, peak level at ear 20,000.06 160 Jet aircraft takeoff; artillery, 2500Ј 2000.06 140 Planing mill, interior 630.06 130 Textile mill 63.06 110 Diesel truck, 60Ј 6.06 90 Cooling tower, 60Ј 2.06 80 Private business office .06 50 Source Acoustic Power of Source Saturn rocket motor 30,000,000watts Turbojet engine 10,000watts Pipe organ, forte 10watts Conversational voice 10microwatts Soft whisper 1millimicrowatt TABLE 6.1.3ADDITION OF DECIBEL VALUES Difference Between the Amount to be Added to Two Decibel Values the Higher Level 0 3.0 1 2.5 2 2.0 3 2.0 4 1.5 5 1.0 6 1.0 7 1.0 8 0.5 9 0.5 10 00. The ossicles are in an air-filled space called the middle ear; close to the middle ear are small muscles which act on them and on the tympanum. The principal function of the ossicles seems to be to achieve an impedance match between the external auditory canal and the fluid-filled cochlea. The principal function of the middle-ear muscles seems to be to control the efficiency of the middle ear by controlling tension of the eardrum and the mechanical ad- vantage of the ossicles as a lever system. The middle ear is connected through the Eustachian tube with the nasal passages so that it can accommodate to atmospheric pres- sures; without this connection, changing atmospheric pres- sure would apply a steady force to the eardrum and pre- vent its free vibration. The cochlea or cochlear canal functions as a transducer; mechanical vibrations enter it; electrical impulses leave it through the auditory nerve. The cochlea is a bone shaped like a snail, coiled two and one-half times around its own axis (Figure 6.1.3). It is about 3cm long and 3mm in di- ameter at its largest part. It is divided along most of its length by the cochlea partition, which is made up of the basilar membrane, Reissner’s membrane, and the organ of Corti. A cross section through the cochlea (Figure 6.1.5) re- veals three compartments: the scala vestibuli, the scala me- dia, and the scala tympani. The scala vestibuli and the scala tympani are connected at the apex of the cochlea. They are filled with a fluid called perilymph in which the scala media floats. The hearing organ (organ of Corti) is housed in the scala media. The scala media contains a different fluid, endolymph, which bathes the organ of Corti. The scala media is triangular in shape and is about 34 mm in length (Figure 6.1.5). Cells grow up from the basi- lar membrane. They have a tuft of hair at the end and are attached to the hearing nerve at the other end. A gelati- nous membrane (tectoral membrane) extends over the hair cells and is attached to the limbus spiralis. The hair cells are embedded in the tectoral membrane. ©1999 CRC Press LLC FIG. 6.1.3Anatomical divisions of the ear. (© Copyright 1972 CIBA Pharmaceutical Company, Division of CIBA-GEIGY Corporation. Reproduced, with permission, from Clinical Symposia,illustrated by Frank H. Netter, M.D. All rights reserved.) Vibration of the oval window by the stapes causes the fluids of the three scala to develop a wave-like motion. The movement of the basilar membrane and the tectoral membrane in opposite directions causes a shearing motion on the hair cells. The dragging of the hair cells sets up elec- trical impulses which are transmitted to the brain in the auditory nerves. The nerve endings near the oval and round windows are sensitive to high frequencies. Those near the apex of the cochlea are sensitive to low frequencies. Another structure of the inner ear is the semicircular canals, which control equilibrium and balance. Extremely high noise levels can impair one’s sense of balance. The ear has some built-in protection; since it is almost entirely surrounded by bone, a considerable amount of me- chanical protection is provided. The inner-ear mechanism offers some protection against loud noises. The muscles of the middle ear (the tensor tympanus and stapedius) can re- duce the ear’s sensitivity to frequencies below about 1000 Hz when high amplitudes are experienced; this reaction is called the aural reflex. For most people, it is an involun- tary muscular reaction, taking place a short time after ex- posure—0.01 second or so. For sounds above the thresh- old of pain, the normal action of the ossicles is thought to change; instead of acting as a series of levers whose me- chanical advantage provides increased pressure on the eardrum, they act as a unit. Neither of these protective re- actions operates until the conditions are potentially dam- aging. HEARING IMPAIRMENT With the exception of eardrum rupture from intense ex- plosive noise, the outer and middle ear are rarely damaged by noise. More commonly, hearing loss is a result of neural damage involving injury to the hair cells (Figure 6.1.6). Two theories are offered to explain noise-induced injury. The first is that excessive shearing forces mechanically damage the hair cells. The second is that intense noise stim- ulation forces the hair cells into high metabolic activity, which overdrives them to the point of metabolic failure and consequent cell death. Once destroyed, hair cells can- not regenerate. AUDIOMETRY PRINCIPLES Audiometry is the measurement of hearing; it is often the determination of the threshold of hearing at a series of fre- quencies and perhaps for the two ears separately, though more detailed methods are also used. Audiometric tests are made for various reasons; the most common to determine the extent of hearing loss and for diagnosis to permit hear- ing aids to be prescribed. In modern society a gradual loss in hearing is normal and occurs with increasing age; Figure 6.1.7 shows this condition. These curves show the average loss in a num- ber of randomly selected men and women (not selected solely from noisy occupations), and these data are accepted as representing typical presbycusis conditions. (Presbycusis refers to the normal hearing loss of the elderly.) For all persons tested, the effect increases with age and is more pronounced at high frequencies than at low. Men normally show the effect to a greater degree than women. In the last decade or so, women have experienced more presbycusis than formerly. Experts disagree as to whether noise is the predominant factor; but evidence shows that presbycusis and other processes of aging take place faster when noise levels and other social stresses are high. Another term, sociocusis,is being used to describe the hearing loss from exposure to the noises of modern society. ©1999 CRC Press LLC FIG. 6.1.4The sound transducer mechanism housed in the middle ear. (Adapted from an original painting by Frank H. Netter, M.D., for Clinical Symposia,copyright by CIBA-GEIGY Corporation.) FIG. 6.1.5Cross section through the cochlea. ©1999 CRC Press LLC FIG. 6.1.6 Various degrees of injury to the hair cells. FIG. 6.1.7 Normal presbycusis curves. Statistical analysis of audiograms from many people show normal losses in hearing acuity with age. Data for men are represented by solid lines; those for women by dotted lines. Group surveys—of young men at college entrance ex- aminations, for example—show increasing percentages of individuals whose audiograms look like those of men many years older. This indication is almost invariably of noise- induced hearing loss. If the audiogram shows losses not conforming to this pattern (conductive losses), more care- ful checking is indicated; such an audiogram suggests a congenital or organic disorder, an injury, or perhaps ner- vous damage. Group surveys are valuable in locating in- dividuals who are experiencing hearing damage without realizing it; it is often not recognized until the subject be- gins to have difficulty in conversation. By this time irre- mediable damage occurred. Such tests are easily made us- ing a simple type of audiometer. As a part of a hearing–conservation program—either a public health or an industrial program—regular audio- metric checks are essential. For this purpose, checking only threshold shift at several frequencies is common. The great- est value of these tests is that they are conducted at regu- lar intervals of a few months (and at the beginning and the termination of employment) and can show the onset of hearing impairment before the individual realizes it. A valuable use of the screening audiometric test is to determine temporary threshold shifts (TTS). Such a check, made at the end of a work period, can show a loss of hear- ing acuity; a similar test made at the beginning of the next work period can show if the recovery is complete. The amount and duration of TTS is somewhat proportional to the permanent threshold shift (PTS) which must be ex- pected. Certainly if the next exposure to noise occurs be- fore the ear has recovered from the last, the eventual re- sult is permanent hearing impairment. ©1999 CRC Press LLC FIG. 6.1.8 Recording of audiometer data. Typical forms for recording audiometric data are either like the simplified table or like the audiometric curve. More data are nor- mally included than are shown here. [...]... 2828 565 6 11,312 22 ,61 4 14 18 22 28 35 45 56 71 90 112 140 179 224 280 353 448 560 7 06 897 1121 1401 1794 2242 2803 3531 4484 560 5 7 062 8 968 11,210 14,012 17,9 36 16. 5 20.5 25.5 31.5 40.5 50.5 63 .5 80.5 100.5 125.5 160 .5 200.5 250.5 315.5 400.5 500.5 63 0.5 800.5 1000.5 1250.5 160 0.5 2000.5 2500.5 3150.5 4000.5 5000.5 63 00.5 8000.5 10,000.5 12,500.5 16, 000.5 20,000.5 18 22 28 35 45 56 71 90 112 140 179... 28 35 45 56 71 90 112 140 179 224 280 353 448 560 7 06 897 1121 1401 1794 2242 2803 3531 4484 560 5 7 062 8 968 11,210 14,012 17,9 36 22,421 a ISO Recommendation 266 and USAS SL 6- 1 960 listed sets of preferred numbers which were recommended for use in acoustic design Most filters are now designed on this basis The tables give octave-band and one-third octave-band filter characteristics, using preferred numbers... ERIC HERRING Date 7-1 1-8 3 Time 0910 ID No 4 4-5 0-FGT Operator C NEMO Location BOOTH 33 Age 23 Hearing Level ISO R 389, 1 964 ANSI 1 969 Remarks SPENT WEEKEND AS JUDGE AT "BATTLE OF HARD ROCK BANDS" LEFT dB -1 0 Audiometer B & K 1800 RIGHT HERTZ dB 500 1000 2000 3000 4000 60 00 8000 500 1000 2000 3000 4000 60 00 8000 1000 -1 0 0 0 10 10 20 20 30 30 40 40 50 50 60 60 70 70 80 80 90 90 FIG 6. 3.1 An audiogram... Figure 6. 4.3 TABLE 6. 4.2 BAND PASS FILTER DATA; PREFERRED NUMBER SERIESA Octave Band Frequencies (Hz) Low Band Edge 000,22 000,44 000,88 00,1 76 00,352 00,7 06 0,1414 0,2828 0, 565 6 11,312 Center Frequency 31.5 63 .5 125.5 250.5 500.5 1000.5 2000.5 4000.5 8000.5 16, 000.5 One-third Octave Band Frequencies (Hz) High Band Edge Low Band Edge Center Frequency High Band Edge 44 88 1 76 353 7 06 1414 2828 565 6 11,312... transmission at one-half the lower band-edge frequency (or at twice the upper) is at least 30 dB below the pass-band transmission; and it is at least 50 dB below passband transmission at one-fourth the lower (or four times the upper) edge of the pass-band frequency In computing loudness from measured data, one-tenth, one-third, one-half, and full octave-band analyzers are used In describing the noise-transmission... to rotational speed The entire pattern is quite com- FIG 6. 2.1 Octave-band spectra of noises 1 Large motor-generator set (SPL 79 dBC); 2 150-HP blower, measured at inlet (SPL 102 dBC); 3 Jet aircraft in process of landing, at 200 meters altitude (SPL 101 dBC); 4 High-pressure reducing valve (SPL 91 dBC); 5 100-HP centrifugal pump (SPL 93 dBC); 6 600-HP diesel engine at 100 feet (SPL 112 dBC) Noise levels,... Motorcycles, road speed (50Ј) Motorcycles, accelerating (50Ј) Dump trucks, road speed (50Ј) Tractor-trailer, road speed (50Ј) Chicago subway platform Chicago subway car New York subway platform Diesel freight train (500Ј) as high as 100–117 100–105 1100– 96 1100–70 100–1 06 1100–90 1100–92 1100–88 160 –100 1 166 –72 1175–91 1 165 –87 175–100 1178–90 1100–95 100–110 195–110 100–110 –111180 a dBC dBC dBC dBC dBA dBA dBA... weightings are TABLE 6. 4.1 COMPARISON OF NONEQUIVALENT NOISE UNITS Loudness Levela (phons) 140 125 120 100 80 60 40 20 a Description Loudness (sones) Sound Level (dBA) Perceived Noise Level (PNdB) Threshold of pain Automobile assembly line Jet aircraft Diesel truck Motor bus (50Ј) Low conversation Quiet room Leaves rustling 1,024.25 362 .25 2 56. 25 64 .25 16. 25 4.25 1.25 0.25 140 125 120 100 80 60 40 20 153 138... obtained in the same manner, including weighting TABLE 6. 4.3 BACKGROUND NOISE CORRECTIONS Subtraction of Noise Values in Decibels Total Noise Minus Background Noise (dB) Correction (dB) 00.25 00.25 00.50 01.00 01.50 02.00 02.50 03.00 03.50 04.00 04.50 05.00 06. 00 07.00 08.00 09.00 10.00 ϱ 12.50 09 .60 06. 80 05.40 04.45 03 .60 03.00 02 .60 02.30 02.00 01 .65 01.35 01.00 00.75 00.55 00.45 Procedure: 1 Measure... assumes no noise TABLE 6. 5.1 OSHA HEARING CONSERVATION TABLEa A-Weighted Sound Level 80 dB* 85† 90‡ 95 100 105 110 115 120 125§ 130§ Important Levels: *Measuring Threshold †Hearing Conservation Begins-50% Dose ‡Eight-Hour Criteria Level §Minimum Upper Range a ϭ Table G-16a (Abbreviated) Duration (Hours) 32 16 8 4 2 1 0.5 0.25 0.125 0. 063 0.031 LTL—Low threshold level, when set at 80 dB, measures only . 1,100,000. 06 195 Military rifle, peak level at ear 20,000. 06 160 Jet aircraft takeoff; artillery, 2500Ј 2000. 06 140 Planing mill, interior 63 0. 06 130 Textile mill 63 . 06 110 Diesel truck, 60 Ј 6. 06 90 Cooling. LLC 500 1000 60 00 40003000 2000 1000 500 800 060 00400030002000 80001000 -1 0 0 10 20 30 40 50 60 70 80 90 dB LEFT HERTZ RIGHT dB -1 0 0 10 20 30 40 50 60 70 80 90 Hearing Level ISO R 389, 1 964 ANSI 1 969 Name. 000. 76 ϫ 10 Ϫ9 000 0000.002 00000.000048 007 .60 ϫ 10 Ϫ9 020 Quiet room 0000.02 00000.00048 0 76. 00 ϫ 10 Ϫ9 040 0000.2 00000.0048 760 .00 ϫ 10 Ϫ9 060 Normal speech at 3Ј 0002.0 00000.048 007 .60 ϫ