769 NOISE Noise and sound refer to audible pressure fluctuations in air. Both are characterized by sound level in decibels and fre- quency content in hertz. Although sound is vital for commu- nication, noise is one of our greatest problems. Intentionally generated acoustic signals including speech and music are usually referred to as sound. Noise is a term used to identify unwanted sound, including sound generated as a byproduct of other activities such as transportation and industrial opera- tions. Intrusive sound, including speech and music unwelcome to the hearer, are also considered noise. Thus, the distinction between noise and sound is subjective, and the two terms are often used interchangeably. When a body moves through a medium or vibrates, some energy is transferred to that surrounding medium in the form of sound waves. Sound is also produced by turbulence in air and other fluids, and by fluids moving past stationary bodies. In general, gases, solids and liquids transmit sound. Well-documented effects of noise include hearing damage, interference with communication, masking of warning signals, sleep interruption, and annoyance. Noise detracts from the quality of life and the environment; it con- tributes to anger and frustration and has been implicated as a contributor to psychological and physiological problems. The National Institute for Occupational Safety and Health (NIOSH) named hearing loss as a priority research area, noting that noise-induced hearing loss is 100% prevent- able, but once acquired, it is permanent and irreversible. The Occupational Safety and Health Administration (OSHA) noted that hearing loss can result in a serious disability, and put employees at risk of being injured on the job. The World Health Organization (WHO) notes that noise-induced hearing impairment is the most prevalent irreversible occupational hazard, and estimates that 120 million people worldwide have disabling hearing difficulties. In developing countries, not only occupational noise but also environmental noise is an increasing risk factor for hearing impairment. The European Union (EU) identified environmental noise caused by traffic, industrial and recreational activities as one of the main local environmental problems in Europe and the source of an increasing number of complaints. It is estimated that 20% of the EU population suffer from noise levels that both scientists and health experts consider unac- ceptable. An additional 43% of the population live in ‘gray areas’ where noise levels cause serious daytime annoyance. Estimates of the cost of noise to society range from 0.2% to 2% of gross domestic product. Noise control involves reduction of noise at the source, control of noise transmission paths, and protection of the receiver. Source control is preferred. For example, design of transportation systems and machinery for lower noise output may be the most effective means of noise control. But, after trying all feasible noise source reduction, airborne noise and/ or solid-borne noise may still be objectionable. Interruption of noise transmission paths by means of vibration isolation, source enclosures, sound absorbing materials, or noise barri- ers is then considered. In some industrial situations, excessive noise is still pres- ent after all attempts to control noise sources and transmission paths. Administrative controls—the assignment of employees so that noise exposure in reduced—should then be consid- ered. As a last resort, employees may be required to use per- sonal hearing protection devices (muff-type and insert-type hearing protectors). Communities often resort to ordinances that limit noise levels and restrict hours of operation of noise- producing equipment and activities. Community noise con- trol methods also include zoning designed to separate noise sources from residential and other sensitive land uses. FREQUENCY, WAVELENGTH AND PROPAGATION SPEED Frequency. Audible sound consists of pressure waves with frequencies ranging from about 20 hertz (Hz) to 20,000 Hz, where 1 Hz ϭ 1 cycle per second. Sound consisting essen- tially of a single-frequency sinusoidal pressure wave is called a pure tone. In most cases, noise consists of sound waves arriving simultaneously from a number of sources, and having a wide range of frequencies. A sound wave which has a frequency below the audible range is called infrasound and sound of frequency above the audible range is called ultrasound. Propagation speed. The propagation speed of airborne sound is temperature dependent. It is given by: c ϭ 20.04[TЈ ϩ 273.16] 1/2 (1.1) where c ϭ propagation speed, i.e. the speed of sound, (m/s) T Ј ϭ air temperature (ЊC). At an air temperature of T Ј ϭ 20ЊC (68ЊF), the propaga- tion speed is c ϭ 343 m/s (approx). Sound waves propagate at a different speed in solids and liquids. The propagation speed for axial waves in a steel rod is about 5140 m/s. Note that C014_003_r03.indd 769C014_003_r03.indd 769 11/18/2005 10:46:06 AM11/18/2005 10:46:06 AM © 2006 by Taylor & Francis Group, LLC 770 NOISE c is a wave a propagation speed; it does not represent particle velocity within the medium. Wavelength. If a pure-tone pressure wave could be observed at a given instant, the length of one cycle of the wave in the propagation directly could be identified as the wavelength. Thus, λ ϭ c / f (1.2) where λ ϭ wavelength (m), c ϭ propagation speed (m/s) and f ϭ frequency (Hz). The effectiveness of noise barriers and sound-absorbing materials is dependent on the sound wave- length (thus, effectiveness is frequency-dependent). SOUND PRESSURE AND SOUND PRESSURE LEVEL One standard atmosphere is defined as a pressure of 1.01325 ϫ 10 5 Pa (about 14.7 psi). Typical sound pressure waves rep- resent very small disturbances in ambient pressure. Sound pressure level is defined by Lpp pp pgrms 2 ref 2 rms ref lgϭϭ10 20 1⁄ ⎡ ⎣ ⎤ ⎦ [] ր (2.1) where L p ϭ sound pressure level in decibels (dB), lg ϭ common (base-ten) logarithm, p rms ϭ root-mean-square sound pressure (Pa) and p ref ϭ reference pressure ϭ 20 ϫ 10 −6 Pa. Sound pressure represents the difference between instan- taneous absolute pressure and ambient pressure. For a pure- tone sound wave of amplitude P , p rms ϭ P /2 1/2 . The reference pressure is the nominal threshold of hearing, corresponding to zero dB. Sound pressure may be determined from sound pressure level by the following relationship: pp L L rms ref p P ϭϫ ϭϫ Ϫ 10 2 10 20 100 20 ⁄ [] ⁄ . (2.2) A-WEIGHTING Human hearing is frequency-dependent. At low sound levels, sounds with frequencies in the range from about 1 kHz to 5 kHz are perceived as louder than sounds of the same sound pressure, but with frequencies outside of that range. A-, B- and C-weighting schemes were developed to compensate for the frequency-dependence of human hearing at low, moder- ate and high sound levels. Other weightings are also used, including SI-weighting which relates to speech interference. A-weighting has gained the greatest acceptance; many stan- dards and codes are based on sound levels in A-weighted decibels (dBA). When noise is measured in frequency bands, the weighting adjustment may be added to each measured value. Sound level meters incorporate weighting networks so that weighted sound level is displayed directly. A-weight- ing adjustments are shown in Table 1. Some representati Most values are approximate; actual noise sources produce a wide range of sound levels. EQUIVALENT SOUND LEVEL Sound energy is proportional to mean-square sound pressure. Equivalent sound level is the energy-average A-weighted sound level over a specified time period. Thus, LTt L T eq lg dϭ10 1 10 10 0 ⁄ ( ) ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ ⁄ ∫ (4.1) where L eq ϭ equivalent sound level (dBA), L ϭ instantaneous sound level (dBA) and T ϭ averaging time, often 1 hour, 8 TABLE 1 A-weighting Frequency Hz Adjustment dB 20 Ϫ50.5 25 Ϫ44.7 31.5 Ϫ39.4 40 Ϫ34.6 50 Ϫ30.2 63 Ϫ26.2 80 Ϫ22.5 100 Ϫ19.1 125 Ϫ16.1 160 Ϫ13.4 200 Ϫ10.9 250 Ϫ8.6 315 Ϫ6.6 400 Ϫ4.8 500 Ϫ3.2 630 Ϫ1.9 800 Ϫ0.8 1,000 0 1,250 ϩ0.6 1,600 ϩ1.0 2,000 ϩ1.2 2,500 ϩ1.3 3,150 ϩ1.2 4,000 ϩ1.0 5,000 ϩ0.5 6,300 Ϫ0.1 8,000 Ϫ1.1 10,000 Ϫ2.5 12,500 Ϫ4.3 16,000 Ϫ6.6 20,000 Ϫ9.3 C014_003_r03.indd 770C014_003_r03.indd 770 11/18/2005 10:46:07 AM11/18/2005 10:46:07 AM © 2006 by Taylor & Francis Group, LLC ve sound levels are given in Table 2. NOISE 771 hours, 24 hours, etc. The time period may be identified by the subscript, e.g. L eq24 for a 24 hour averaging time. Integrating sound level meters compute and display equivalent sound level directly. If equivalent sound level is to be determined from a number of representative instanta- neous measurements or predictions, the above equation may be rewritten as follows: LN L i N eq lgϭ10 1 10 10 1 ⁄ ( ) ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ ⁄ = ∑ i . (4.2) If a large number of readings are involved, it is con- venient to incorporate the above equation into a computer program. If the base-10 logarithm is not available on the computer it may be obtained from lg n n 10xx ( ) ( ) ⁄ ( ) ϭ11 (4.3) where ln is the natural (base-e) logarithm. Example Problem: Equivalent Sound Level Considering four consecutive 15-minute intervals, during which representative sound levels are 55, 58, 56 and 70 dBA respectively. Determine equivalent sound level for that hour. Solution: L eq lg 1 4 dBA. ϭϩϩϩ ϭ 10 10 10 10 10 64 5 55 10 58 10 56 10 70 10 ⁄ ( ) () ⁄⁄⁄⁄ . It can be seen that higher sound levels tend to dominate when determining L eq . Note that the mean average sound level (55 ϩ 58 ϩ 56 ϩ 70)/4 ϭ 59.8 has no significance. DAY–NIGHT SOUND LEVEL Day–night sound level takes into account the importance of quiet during nighttime hours by adding a 10 dBA weighting to noise during the period from 10 pm to 7 am. It is given by Lt t L L DN am pm pm am lg 1/24 dϭϩ ϩ 10 10 10 10 7 10 10 7 [] ⎡ ⎣ ⎢ { ⎤ ⎦ ⁄ ∫ ∫ ( 10) 10ր d ⎥⎥ } (5.1) where L DN ϭ day–night sound level and t ϭ time (hours). COMBINING NOISE FROM SEVERAL SOURCES Correlated sound waves. Sound waves with a precise time and frequency relationship may be considered correlated. A sound wave arriving directly from a source may have a precise phase relationship with a reflected sound wave from the same source. The sound level resulting from combining two correlated sound waves of the same frequency depends on the phase relationship between the waves. Reactive muf- flers and silencers are designed to produce reflections that cancel the progressive sound wave. Active noise control is accomplished by generating sound waves out-of-phase with the noise which is to be cancelled. Active noise control systems employ continuous measurement, signal processing, and sound generation. Uncorrelated noise sources. Most noise sources are not correlated with one another. The combined effect of two or more uncorrelated sources is obtained by combining the energy from each at the receiver. To do this, we may add mean-square sound pressures. In terms of sound levels, the result is L L i N i T = ⁄ = ∑ 10 10 10 1 lg (6.1) where L T ϭ total sound level due to N contributions L i (dBA as measured or predicted at the receiver). For two contributions, the total sound level is L L LL T DIF 10 lg 10 lg 1 10 ϭϩ ϭϩ ϩ Ϫ 10 10 10 110 210 1 ⁄⁄ ⁄ ⎡ ⎣ ⎤ ⎦ ⎡ ⎣ ⎤ ⎦ (6.2) where L 1 ϭ the greater sound level and DIF ϭ L 1 Ϫ L 2 , the difference between the two sound levels. The last term in equation 6.2 may be identified as L (add), the quantity to be added to L 1 to obtain total sound level L T . L (add) Values are given to the nearest one-tenth decibel. Although measured and predicted sound levels are often reported to the TABLE 2 A-weighted sound levels Approximate sound level Noise source or criterion 140 Threshold of pain 122 Supersonic aircraft A 120 Threshold of discomfort 112 Stage I aircraft A 110 Leaf blower at operator 105 OSHA 1 hr/da limit B 99 EEC 1 hr/da limit B 90 OSHA and EEC 8 hr/da limit B 70 EPA criterion for hearing conservation C 67 DOT worst hour limit D 65 Daytime limit, typical community ordinance 45 Noise limit for virtually 100% indoor speech intelligibility 35 Acceptable for sleep 0 Threshold of hearing Notes: A: Aircraft measurements 500 ft beyond end of runway, 250 ft to side. Stage 3 aircraft in current use are quieter. B: Criteria for worker exposure (US Occupational Safety and Health Administration and European Economic Community). C: Environmental Protection Agency identified 24-hr equivalent sound level. D: Department of Transportation design noise level for residential use. C014_003_r03.indd 771C014_003_r03.indd 771 11/18/2005 10:46:07 AM11/18/2005 10:46:07 AM © 2006 by Taylor & Francis Group, LLC is tabulated against DIF, the difference in levels, in Table 3. 772 NOISE nearest whole decibel, fractional values are often retained for comparison purposes, and to insure accuracy of intermediate calculations. Note that addition of the contributions of two equal but uncorrelated sources produces a total sound level 3 decibels higher than the contribution of one source alone. If the difference between contributions is 10 or more decibels, then the smaller contribution increases total sound level by less than one-half decibel. If the difference is 20 or more decibels, the smaller contribution has no significant effect; for DIF Ն 20, L (add)Ͻ 1/20. This is an important consideration when evaluating noise control efforts. If sev- eral individual contributions to overall sound level can be identified, the sources producing the highest sound levels effect of combining noise levels. Example Problem: Combining Noise Contributions The individual contributions of five machines are as follows when measured at a given location: 85, 88, 80, 70 and 95 dBA. Find the sound level when all five are operating together. Solution: Using equation 6.1, the result is L T ϭ 10 lg[10 85/10 ϩ 10 88/10 ϩ 10 80/10 ϩ 10 70/10 ϩ 10 95/10 ] ϭ 96.25 dBA. We could use Table 3 instead. Combining the levels in ascending order, the result is 70 ϩ 80 ϭ 80.4 and 80.4 ϩ 85 ϭ 86.3 and 86.3 ϩ 88 ϭ 90.3 and 90.3 ϩ 95 ϭ 96.3 dBA Fractional parts of one dBA are only retained for purposes or illustration. For several sources which contribute equally to sound level at the receiver, total sound level is given by LL n T ϭϩ 1 10 lg (6.3) where L 1 ϭ sound level contribution at the receiver due to a single source and n ϭ the number of sources. Table 3 and related) contributions. SOUND FIELDS The region within one or two wavelengths of a noise source or within one or two typical source dimensions is called the near field. The region where reflected sound waves have a significant effect on total sound level is called the reverber- ant field. Consider an ideal nondirectional noise source which generates a spherical wave. For regions between the near field and the reverberant field, sound intensity is given by IW rϭ ⁄ ⎡ ⎣ ⎤ ⎦ 4 2 p (7.1) TABLE 3 Combining noise from two uncorrelated sources DIF L(add) DIF L(add) 0.0 3.0 5.0 1.2 0.2 2.9 5.5 1.1 0.4 2.8 6.0 1.0 0.6 2.7 6.5 0.9 0.8 2.6 7.0 0.8 1.0 2.5 7.5 0.7 1.2 2.5 8.0 0.6 1.4 2.4 8.5 0.6 1.6 2.3 9.0 0.5 1.8 2.2 9.5 0.5 2.0 2.1 10.0 0.4 2.2 2.0 10.5 0.4 2.4 2.0 11.0 0.3 2.6 1.9 11.5 0.3 2.8 1.8 12.0 0.3 3.0 1.8 12.5 0.2 3.2 1.7 13.0 0.2 3.4 1.6 13.5 0.2 3.6 1.6 14.0 0.2 3.8 1.5 14.5 0.2 4.0 1.5 15.0 0.1 4.2 1.4 15.5 0.1 4.4 1.3 16.0 0.1 4.6 1.3 16.5 0.1 4.8 1.2 17.0 0.1 — — 17.5 0.1 — — 18.0 0.1 — — 18.5 0.1 — — 19.0 0.1 — — 19.5 0.0 L1 ϭ greater sound level, L2 ϭ lower sound level. DIF ϭ L1 Ϫ L2, Combined sound level LT ϭ L1 ϩ L(add). 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Difference in levels 3.5 3 2.5 2 1.5 1 0.5 0 Add to higher level FIGURE 1 Combining noise levels. C014_003_r03.indd 772C014_003_r03.indd 772 11/18/2005 10:46:08 AM11/18/2005 10:46:08 AM © 2006 by Taylor & Francis Group, LLC should be considered first. Figure 1 is a graph showing the Figure 2 show the effect of combining n equal (but uncor- NOISE 773 where I ϭ sound intensity (W/m 2 ), where sound pressure and particle velocity are in-phase, W ϭ sound power of the source (W) and r ϭ distance from the source (m). The above equation is called the inverse square law. Scalar sound intensity level is given by LI Iref Iϭ10lg ⁄ [] (7.2) where L I ϭ scalar sound intensity level and I ref ϭ 10 Ϫ12 W/m 2 . For airborne sound under typical conditions, sound pres- sure level and scalar sound intensity level are approximately equal, from which LL W r pI ഠ ϭϪϪ10 20 109lg lg (7.3) for the spherical wave where L p and L I are expressed in dB. If sound power has been A-weighted, L p and L I are in dBA. When the inverse-square law applies, then sound levels decrease with distance at the rate: 20 lg r. Thus, if sound level is known at one location, it may be estimated at another location. Table 4 and Figure 3 show the distance adjustment to be added to sound level at distance r 1 from the source to obtain the sound level at distance r 2 . MEASUREMENT AND INSTRUMENTATION Sound level meters. The sound level meter is the basic tool for making noise surveys. A typical sound level meter is a hand-held battery-powered instrument consisting of a micro- phone, amplifiers, weighting networks, a rootmean-square rectifier, and a digital or analog sound level display. The A- weighting network is most commonly used. This network so that sound level is displayed in dBA. When measuring out- of-doors, a windscreen is used to reduce measurement error due to wind impinging on the microphone. Integrating sound level meters automatically calculate equivalent sound level. If a standard sound level meter is used, equivalent sound level may be calculated from representative measurements, using the procedure described later. Frequency analysis. The cause of a noise problem may sometimes be detected by analyzing noise in frequency bands. An octave band is a frequency range for which the upper frequency limit is (approximately) twice the lower TABLE 4 Spherical wave attenuation r2/r1 ADJ 0.5 6.0 0.6 4.4 0.7 3.1 0.8 1.9 0.9 0.9 1.0 0.0 1.1 Ϫ0.8 1.2 Ϫ1.6 1.3 Ϫ2.3 1.4 Ϫ2.9 1.5 Ϫ3.5 1.6 Ϫ4.1 1.7 Ϫ4.6 1.8 Ϫ5.1 1.9 Ϫ5.6 2.0 Ϫ6.0 2.1 Ϫ6.4 2.2 Ϫ6.8 2.3 Ϫ7.2 2.4 Ϫ7.6 2.5 Ϫ8.0 2.6 Ϫ8.3 2.7 Ϫ8.6 2.8 Ϫ8.9 2.9 Ϫ9.2 3.0 Ϫ9.5 Distance adjustment based on the inverse-square law. L(r2) ϭ L(r1) ϩ ADJ. Distance ratio r2/r1 Add to sound level at r1 10 5 0 –5 –10 0.5 1.0 1.5 2.0 2.5 3.0 FIGURE 3 Distance adjustment based on the inverse-square law. Number of equal contributions Add to level due to one source 10 8 6 4 2 0 1 2 345678910 FIGURE 2 Combining n equal contributions. C014_003_r03.indd 773C014_003_r03.indd 773 11/18/2005 10:46:09 AM11/18/2005 10:46:09 AM © 2006 by Taylor & Francis Group, LLC electronically adjusts the signal in accordance with Table 1, 774 NOISE limit. An octave band is identified by its center frequency defined as follows: fff cLu ϭ [] 12/ (8.1) where f c ϭ the center frequency, f L ϭ the lower band limit, and f u ϭ the upper band limit, all in Hz. The center frequen- cies of the preferred octave bands in the audible range are 31.5, 63, 125, 250 and 500 Hz and 1, 2, 4, 8 and 16 kHz. The center frequencies of the preferred one-third-octave bands are Real-time analyzers and Fast-Fourier-Transform (FFT) analyzers examine a signal in all of the selected frequency bands simultaneously. The signal is then displayed as a bar- graph, showing the sound level contribution of each selected frequency band. Sound intensity measurement. Vector sound intensity is the net rate or flow of sound energy. Vector sound inten- sity measurements are useful in determining noise source power in the presence of background noise and for location of noise sources. Sound intensity measurement systems uti- lize a two-microphone probe to measure sound pressure at two locations simultaneously. The signals are processed to determine the particle veloc- ity and its phase relationship to sound pressure. Calibration. Acoustic calibrators produce a sound level of known strength. Before a series of measurements, sound measurement instrumentation should be adjusted to the cali- brator level. Calibration should be checked at the end of each measurement session. If a significant change has occurred, the measured data should be discarded. Calibration data should be recorded on a data sheet, along with instrumenta- tion settings and all relevant information about the measure- ment site and environmental conditions. Background noise. When measuring the noise contribu- tion of a given source, all other contributions to total noise are identified as background noise. Let the sound level be measured with the given source operating, and then let back- ground noise alone be measured. The correction for back- ground noise is given by COR 10 lg 1 10 DIF 10 ϭϪ Ϫ ⁄ ⎡ ⎣ ⎤ ⎦ (8.2) where DIF ϭ Total noise level – background noise level, and the noise level contribution of the source in question is given by: L SOURCE Total noise level COR.ϭϩ Background noise corrections are tab plotted in Figure 4. Whenever possible measurements should be made under conditions where background noise is negli- gible. When total noise level exceeds background noise by at least 20 dB, then the correction is less than 1/20 dB. Such ideal conditions are not always possible. Truck noise, for example, must sometimes be measured on a highway with other moving vehicles nearby. If the difference between total noise level and background noise is less than 5 dB, then the contribution of the source in question cannot be accurately determined. HEARING DAMAGE RISK The frequency range of human hearing extends from about 20 Hz to 20 kHz. Under ideal conditions, a sound pressure level of 0 dB at 1 kHz can be detected. Human hearing is less sensitive to low frequencies and very high frequencies. Hearing threshold. A standard for human hearing has been established on the basis of audiometric measurements at a series of frequencies. An individual’s hearing threshold represents the deviation from the standard or audiometric- zero levels. A hearing threshold of 25 dB at 4 kHz, for example, indicates that an individual has “lost” 25 dB in ability to hear sounds at a frequency of 4 kHz (assuming the individual had “normal” hearing at one time). A temporary threshold shift (TTS) is a hearing thresh- old change determined from audiometric evaluation before, and immediately after exposure to loud noise. A measurable permanent threshold shift (PTS) usually occurs as a result of long-term noise exposure. The post-exposure audiometric measurements to establish PTS are made after the subject has been free of loud noise exposure for several hours. A com- pound threshold shift (CTS) combines a PTS and TTS. There is substantial evidence that repeated TTS’s translate into a measurable PTS. Miller (1974) assembled data relating TTS, CTS and PTS resulting from exposure to high noise levels. Occupational Safety and Health Administration (OSHA criteria. OSHA (1981, 1983) and the Noise Control Act (1972) set standards for industrial noise exposure and guidelines for hearing protection. OSHA criteria have resulted in reduced noise levels in many industries and reduced the incidence of hearing loss to workers. However, retrospective studies have shown that some hearing loss will occur with long-term exposure a OSHA-permitted sound levels. The basic OSHA criterion level (CL) is a 90 dBA sound exposure level for an 8 hour day. An exchange rate (ER) of 5 dBA is specified, indicating that the permissible daily Total - background level Background noise correction. 0 –0.5 –1 –1.5 –2 –2.5 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 FIGURE 4 Background noise correction. C014_003_r03.indd 774C014_003_r03.indd 774 11/18/2005 10:46:10 AM11/18/2005 10:46:10 AM © 2006 by Taylor & Francis Group, LLC those listed in the first column of Table 1 (Section 3). ulated in Table 5 and NOISE 775 exposure time is halved with each 5 dBA sound level increase. The threshold level, the sound level below which no contribu- tion is made to daily noise dose, is 80 dBA (threshold level is not to be confused with hearing threshold). When noise expo- sure exceeds the action level (85 dBA) a hearing conservation program is to be implemented. A hearing conservation program should include noise exposure monitoring audiometric testing, employee training, hearing protection and record-keeping. According to OSHA standards continuous noise expo- sure (measurable on the slow-response scale of a sound level meter) is not to exceed 1151 dBA. For sound levels L where 80 Յ L Յ 115 dBA allowable exposure time is given by T L ϭϭ Ϫ Ϫ 82 82 5 ⁄ ⎡ ⎣ ⎤ ⎦ ⁄ ⎡ ⎣ ⎤ ⎦ ⁄ ( ) ⁄ ( CL) ER L90 (9.1) where T ϭ allowable exposure time (hours/day). The result is shown in Table 6. Noise dose. When sound levels vary during the day, noise dose is used as an exposure criterion. Noise dose is given by DCT i N % ϭ ϭ 100 ii ⁄ ∑ 1 where C ϭ actual exposure of an individual at a given sound level (hr), T ϭ allowable exposure time at that level and N ϭ the number of different exposure levels during one day. Noise dose D % should not exceed 100%. As an alternative to moni- toring and calculations, workers may wear dosimeters which automatically measure and calculate daily dose. An exchange rate of 3 dB is used in occupational noise exposure criteria by some European countries. This exchange rate is equivalent to basing noise exposure on L eq . Environmental Protection Agency (EPA) identified levels. Using a 4 kHz threshold shift criterion, protective noise levels are substantially lower than the OSHA criteria. EPA (1974, 1978) in its “Levels” document identified the equivalent sound level of intermittent noise: L eq24 dBAϭ 70 as the “(at ear) exposure level that would produce no more than 5 dB noise-induced hearing damage over a 40 year period”. This value is based on a predicted hearing loss smaller than 5 dB at 4 kHz for 96% of the people exposed to 73 dBA noise for 8 hr/da ϫ 250 da/yr ϫ 40 yr. With the following corrections, the 73 dBA level is adjusted to L eq24 ϭ 70 dBA the protective noise level: Ϫ1.6 dBA to account for 365 da/yr exposure, Ϫ4.8 dBA to correct for 24 hr/day averaging, ϩ5 dBA assuming intermittent exposure and Ϫ1.6 dBA for a margin of safety. NON-AUDITORY EFFECTS OF NOISE The relationship between long-term exposure to industrial noise and the probability of noise-induced hearing loss is TABLE 5 Background noise correction DIF COR DIF COR 0.2 Ϫ13.5 — — 0.4 Ϫ10.6 — — 0.6 Ϫ8.9 — — 0.8 Ϫ7.7 6.5 Ϫ1.1 1.0 Ϫ6.9 7.0 Ϫ1.0 1.2 Ϫ6.2 7.5 Ϫ0.9 1.4 Ϫ5.6 8.0 Ϫ0.7 1.6 Ϫ5.1 8.5 Ϫ0.7 1.8 Ϫ4.7 9.0 Ϫ0.6 2.0 Ϫ4.3 9.5 Ϫ0.5 2.2 Ϫ4.0 10.0 Ϫ0.5 2.4 Ϫ3.7 10.5 Ϫ0.4 2.6 Ϫ3.5 11.0 Ϫ0.4 2.8 Ϫ3.2 11.5 Ϫ0.3 3.0 Ϫ3.0 12.0 Ϫ0.3 3.2 Ϫ2.8 12.5 Ϫ0.3 3.4 Ϫ2.7 13.0 Ϫ0.2 3.6 Ϫ2.5 13.5 Ϫ0.2 3.8 Ϫ2.3 14.0 Ϫ0.2 4.0 Ϫ2.2 14.5 Ϫ0.2 4.2 Ϫ2.1 15.0 Ϫ 0.1 4.4 Ϫ2.0 15.5 Ϫ0.1 4.6 Ϫ1.8 16.0 Ϫ0.1 4.8 Ϫ1.7 16.5 Ϫ0.1 5.0 Ϫ1.7 17.0 Ϫ0.1 5.2 Ϫ1.6 17.5 Ϫ0.1 5.4 Ϫ1.5 18.0 Ϫ0.1 5.6 Ϫ1.4 18.5 Ϫ0.1 5.8 Ϫ1.3 19.0 Ϫ0.1 6.0 Ϫ1.3 19.5 Ϫ0.0 DIF ϭ total noise level–background noise level. Sound level due to source ϭ total noise level ϩ COR. TABLE 6 Allowable exposure times Time T hr/da Sound level L dBA 32* 80 16 85 890 495 2 100 1 105 1/2 110 1/4 or less 115 * The 32 hr exposure time is used in evaluating noise dose when sound levels vary. C014_003_r03.indd 775C014_003_r03.indd 775 11/18/2005 10:46:10 AM11/18/2005 10:46:10 AM © 2006 by Taylor & Francis Group, LLC 776 NOISE well-documented. And, we can estimate the effect of intru- sive noise on speech intelligibility and masking of warn- ing signals. Equivalent sound levels and day-night sound levels based on hearing protection, activity interference, and Bureau identified noise as the top complaint about neighbor- hoods, and the major reason for wanting to move. In a typi- cal city, about 70% of citizen complaints relate to noise. The most common complaints are aircraft noise, highway noise, machinery and equipment, and amplified music. Noise tolerance varies widely among individuals. It is difficult to relate noise levels to psychological and non- auditory physiological problems. But there is anecdotal evi- dence that violent behavior can be triggered by noise. In a New York case, one man cut off another man’s hand in a dispute over noise. In another noise-related incident, a New Jersey man operated his motorcycle engine inside his apart- ment, leading a neighbor to shoot him. Chronic noise exposure has been related to children’s health and cognitive performance. In a study of British schools, Stansfield and Haines (2000) compared reading skills of students at four schools with 16-hour equivalent sound levels less than 57 dBA and four schools with levels greater than 66 dBA. After adjustment for socioeconomic factors, lower average reading scores were found at the nois- ier schools. The difference was equivalent to six months of learning over four years. A study by Zimmer et al. (2001) examined aircraft noise exposure and student proficiency test results at three grade levels. Communities with comparable socioeconomic status were selected for the study. Noise-impacted communities with a day-night sound level greater than 60 dBA and com- munities with a level of less than 45 dBA were compared. If proficiency test results are extrapolated to educational attain- ment and salary level, one could predict a 3% salary level dis- advantage for students from the impacted communities. COMMUNITY NOISE Contributors to community noise include aircraft, highway vehicles, off-road vehicles, powered garden equipment, construction activities, commercial and industrial activities, public address systems and loud radios and television sets. The major effects of community noise include sleep interfer- ence, speech interference, and annoyance. Highway noise. Noise levels due to highway vehicles may be estimated from the Federal Highway Administration (FHWA) model summarized by the sound level vs. speed relationships in Table 7. These values make it possible to pre- dict the impact of a proposed highway or highway improve- ment on a community. The contribution that a given class of vehicles makes to hourly equivalent sound level is given by LL DVSAAAA A S eqH o o B D F G lgϭϩ ϩϩϩϩ ϩϪ 10 25 ր [] (10.1) where D o ϭ 15 m, V ϭ volume (vehicles/hr), S ϭ speed (km/hr). A B, D, F, G and S are adjustments for barriers, distance, finite highway segments, grade and shielding due to buildings, respectively. Each term is applied to a given class of vehicles and traffic lane. For acoustically absorptive sites, the distance adjustment is ADD Do ϭ15lg ⁄ [] (10.2) where D ϭ distance from the traffic lane (m). Hourly equiva- lent sound level at any location is predicted by combining the contributions from all vehicle classes and traffic lanes. The result is L i eqH COMBINED N ( ) ⁄ ∑ ϭ ϭ 10 10 10 1 lg . LHieq (10.3) Design noise levels for highways. Design noise levels specified by the Federal Highway Administration (1976) are traffic on proposed highways are compared with the design levels. These data aid in selecting a highway design and routing alternative including the “no-build” alternative. Aircraft noise. Noise contour maps are available for most major airports. These enable one to make rough predic- tions of the impact of aircraft noise on nearby communities. Federal Aviation Administration publications (1985a and b) outline aircraft noise certification procedures and aircraft noise compatibility planning. Many of the existing airport noise contour maps are based on the descriptor Noise expo- sure forecast (NEF). An approximate conversion from NEF to L DN is given by L DN NEF 35ഠ ϩ (10.4) where L DN ϭ day-night sound level (Ϯ about 3 dBA). Community noise criteria. There are thousands of dif- ferent community noise ordinances, with a wide range of permitted sound levels. Their effectiveness depends largely on the degree of enforcement in a particular community. The Environmental Protection Agency has identified the noise TABLE 7 Energy mean emission levels for vehicles Vehicle class Sound level L 0 dBA Speed S km/hr Autos 31.8 lg S Ϫ 2.4 у50 Autos 62 Ͻ50 Med. trucks 33.9 lg S ϩ 16.4 у50 Med. trucks 74 Ͻ50 Heavy trucks 24.6 lg S ϩ 38.5 у50 Heavy trucks 87 Ͻ50 Sound levels at 15 meters. Source: Barry and Reagan (1978). C014_003_r03.indd 776C014_003_r03.indd 776 11/18/2005 10:46:11 AM11/18/2005 10:46:11 AM © 2006 by Taylor & Francis Group, LLC annoyance are given in Table 9. The United States Census summarized in Table 8. Noise predictions based on projected NOISE 777 levels in Table 9 as protective of public health and welfare. All are based on an average 24 hour day. Control of community noise. Environmental noise problems are particularly difficult to solve due to problems of shared responsibility and jurisdiction. In many cases, Federal laws preempt community regulations. Highway noise and aircraft noise, often the most significant contribu- tors to community noise levels, are largely exempt from local control. In spite of the difficulties encountered, however, the importance of protecting the quality of life makes environ- mental noise control efforts worthwhile. Depending on the circumstances, some of the following courses of action may be considered. a) Review the applicable noise ordinance. Compare it with a model noise ordinance. Check to see if specific limits are set in terms dBA. Determine whether or not sound level meters are available and whether or not the ordinance is actually enforced. b) Meet with representatives of the local governing body or environmental commission. Make them aware of noise related problems in the community. c) Initiate a campaign for public awareness with regard to the environment including the noise environment. Make use of the local papers. d) Consider a ban or limitation on all-terrain- vehicles (ATV’s). Determine whether muffler requirements are actually enforced. e) Encourage planning and zoning boards to require an environmental impact statement (EIS), including a noise report, before major projects are approved. f) Support noise labeling for lawn mowers and other power equipment. g) Attend and participate in hearings involving plans for airports, heliports, and highways. Consider noise impact when evaluating the cost/benefit ratio for proposed facilities. h) Evaluate the feasibility of noise barriers on exist- ing and proposed highways in sensitive areas. i) Support legislation to reduce truck noise emission limits. j) Support legislation enabling airport curfews. REFERENCES Barry, T.M. and Reagan, J.A. FHW A highway traffic noise prediction model FHWA-RD-77-108, 1978. Environmental Protection Agency, Information on levels of environmental noise requisite to protect public health and welfare with an adequate margin of safety, EPA 550/9-74-004, 1974. Environmental Protection Agency, Model community noise control ordinance, EPA 550/9-76-003, 1975. Environmental Protection Agency, Protective noise levels, EPA 559/979-100, 1978. Federal Aviation Administration, Noise standards: aircraft type and air- worthiness certification, FAR part 36, 1985(a). Federation Aviation Administration, Airport noise compatibility planning, FAR part 150, 1985(b). Federal Highway Administration, Procedures for abatement of highway traffic noise and construction noise, FHPM 7-7-3, 1976. Federal Register, Code of federal regulations, 29, parts 1900 to 1910, 1985. Miller, J.D., “Effects of noise on people,” J. Acoust Soc. Am. 56, no. 3, pp. 729–764, 1974. Noise control act of 1972, PL 92-574, HR 11021, Oct. 27, 1972. Occupational Safety and Health Administration, “Occupational noise exposure hearing conservation amendment” Federal Register, 46 (11), 4078–4181 and 46 (162), 42622–42639, 1981. Occupational Safety and Health Administration, “Guidelines for noise enforce- ment”, OSHA Instruction, CPL2-2.35,29 CFR1910.95(6) (1), 1983. Peterson, A.P.G., Handbook of noise measurement, GenRad, Concord, MA, 9th ed., 1980. Stansfield, S. and M. Haines, “Chronic aircraft noise exposure and chil- dren’s cognitive performance and health: the Heathrow studies”, FICA symposium, San Diego CA, 2000. Wilson, C., Noise Control, Krieger, Malabar FL, 1994. Zimmer, I.B., R. Dresnack, and C. Wilson, Modeling the impact of aircraft noise on student proficiency”, NOISE-CON Portland ME, 2001 . The following Internet resources may contain current information of interest: www.faa.gov Federal Aviation Administration www.icao.int International Civil Aviation Administration www.fhwa.dot.gov. Federal Highway Administration environment/noise www.osha.gov Occ upational Safety and Health Administration TABLE 8 Design noise levels Sound level L eqH dBA Measurement location Land use category 57 Exterior Tracts of land in which serenity and quiet are of extraordinary significance. 67 Exterior Residences, schools, churches, libraries, hospitals, etc. 72 Exterior Commercial and other activities. 52 Interior Residences, schools, churches, libraries, hospitals, etc. TABLE 9 Protective noise levels Effect Level (dBA) Area Hearing protection L eq24 р 70 All areas. See Section 9. Outdoor activity L DN р 55 Outdoors in residential areas. Interference and annoyance L eq24 р 55 Outdoor areas where people spent limited amounts of time. Indoor activity L DN р 45 Indoor residential areas. Interference and annoyance L eq24 р 45 Other indoor areas with human activities such as schools, etc. Source: EPA (1974, 1979). C014_003_r03.indd 777C014_003_r03.indd 777 11/18/2005 10:46:11 AM11/18/2005 10:46:11 AM © 2006 by Taylor & Francis Group, LLC 778 NOISE www.cdc.gov/niosh Nat ional Institute for Occupational Safety and Health europe.osha.eu.int Eur opean Agency for Safety and Health at Work europa.eu.int. European Union en/record/green www.epa.gov Environmental Protection Agency www.who.int/ World Health Organization environmental_ information/noise CHARLES E. WILSON New Jersey Institute of Technology C014_003_r03.indd 778C014_003_r03.indd 778 11/18/2005 10:46:11 AM11/18/2005 10:46:11 AM © 2006 by Taylor & Francis Group, LLC . intermittent exposure and Ϫ1.6 dBA for a margin of safety. NON-AUDITORY EFFECTS OF NOISE The relationship between long-term exposure to industrial noise and the probability of noise- induced hearing. protectors). Communities often resort to ordinances that limit noise levels and restrict hours of operation of noise- producing equipment and activities. Community noise con- trol methods also include. Estimates of the cost of noise to society range from 0.2% to 2% of gross domestic product. Noise control involves reduction of noise at the source, control of noise transmission paths, and protection