Age-WEIGHTED SOUND LEVELS

Transcription

1 Age-WEIGHTED SOUND LEVELS By Michael A. Staiano 1923 Stanley Avenue Rockville, Maryland Report No. M 06723D 17 August 2007 Published in Noise Control Engineering Journal Volume 55, Number 5, September October 2007 In many areas where established noise goals, in terms of A-weighted sound levels, have been achieved particularly in the vicinity of airports, the public remains dissatisfied. A-weighting accounts for the non-uniform frequency sensitivity of human hearing but is based upon data collected from yr olds. However, the mean age for the overall U.S. adult population is now almost 50 yr. For this reason, the importance of ageing on the suitability of A-weighted sound levels for assessing annoyance response was examined. Age-corrections were applied to the A-weighting curve and the resultant overall Age-weighted sound levels compared to the corresponding A-weighted sound levels for a variety of spectra. The analyses found that the change in hearing frequency response due to hearing loss typical of aging does not greatly alter the overall frequency-weighted sound levels of common environmental noises. Thus, the A-weighted sound level remains suitable for quantifying the noise exposures for the general population. However, the perception of certain natural sounds is likely to be attenuated with ageing, and its absence may serve to heighten the adverse response to manmade sounds. Overall reductions in hearing sensitivity do appear to be important and may alter the adverse response to noise for many individuals older than 50 yr. Ageing may yet explain some variation in observed annoyance response of individuals, but in terms of age-dependent shifts in overall hearing sensitivity rather than changing frequency response. 1 INTRODUCTION The human ear responds to sound over a range of about 20 Hz to 20 khz a span wider than that of a piano keyboard. However, the ear does not sense all frequencies equally well. The human auditory mechanism is less sensitive to extremely high and low frequencies. Therefore, the perceived magnitude of audible sounds, i.e., loudness, will be greatest for middle-frequency sounds of the same sound pressure level. Sound levels that are weighted to account for the non-uniform frequency sensitivity of human hearing are known as A-weighted sound levels and are given in units of "A-weighted decibels" (dba). A-weighted sound level is the metric used almost universally for assessing the effects noise upon humans. For evaluating the annoyance-evoking potential of environmental noise and the compatibility of land uses with respect to noise exposures, the nighttime-weighted, 24-hr energy-average of the overall A-weighted sound level, day-night average sound level (L dn ), commonly is used. Noise often is defined as unwanted sound. Annoyance is a feeling of irritation that an individual or group feels as a result of being subjected to noise. Annoyance depends primarily upon the intensity of the sound, its frequency, and its duration. Other factors also influence annoyance response, including noise source type. 1 In public opinion surveys polling community annoyance to noise, the respondents were asked to rate the noise environment on a scale of annoyance ranging from "none" to "extreme," or similar descriptors. The verbal descriptions were then -1-

2 M 06723D August 2007 related to measurements of noise in the canvassed area to allow the definition of a dose-response relationship. These surveys have been performed in a variety of environments primarily those with aircraft or highway traffic or railtransportation noise exposures. The results of a number of social surveys on noise annoyance were synthesized such that those responses that could be described as highly annoyed were focused upon to produce a dose-response function, often referred to as the Schultz Curve. 2 The curve implies that the response to aircraft, highway, and rail noise sources is the same. It has been applied to non-transportation noise sources and has become widely accepted for estimating annoyance response although it leaves unaccounted much of the variance in the observations upon which it is based. 3 This characterization has been reexamined by a number of investigators with additional data producing substantially similar results. The representation, shown as Figure 1, was recommended by the Federal Interagency Committee on Noise (FICON) for the prediction of the effects of general transportation noise on people. 4 Based in part upon the results of the social surveys, a day-night average sound level of 65 dba has become commonly established as the limit for acceptable exposures to residential land uses. However, in many areas where 65 dba[l dn ] has been achieved particularly in the vicinity of airports, the public remains dissatisfied. While this should not be surprising considering that the survey results indicate that about one in eight exposed persons will be highly annoyed at 65 dba[l dn ], a number of investigators have questioned the adequacy of the day-night average sound level as a predictor of human response. This brief article examines a fundamental aspect of the noise descriptor and of A-weighting generally. 2 DERIVATION of A-WEIGHTING A-weighted sound levels are used for the prediction of a variety of noise effects (including annoyance, hearing-loss risk, speech interference, and sleep disturbance), although the weighting is derived from judgments on sounds in controlled laboratory circumstances unrepresentative of ordinary listening environments. A-weighting and the companion B- and C-weightings were based upon research conducted in the 1930s. In this research, test participants were: presented a pure tone at some frequency, then presented a reference pure tone at 1 khz, then directed to judge the magnitude of the first signal with respect to that of the reference tone. 5 The test tone frequencies and magnitudes, and magnitudes of the reference tone were altered (over a sound pressure level range of db) and the process repeated. The test was performed on a number of volunteers yielding the results shown (in updated form 6 ) in Figure 2. The equal-loudness contours shown in Figure 2 were based upon data collected from men and women aged yr. The equal-loudness curves provided a basis for designing sound-measurement instrumentation that displayed single-number ratings appropriate to human response of complex noise spectra. A-, B-, and C-weightings were conceived to permit simple sound pressure level measurements at various amplitudes. The weightings correspond to the loudness of 1-kHz tones with sound pressure levels of 40 db, 70 db, and 100 db, respectively, as also shown in Figure 2. 7 B-weighting found little application and is rarely implemented in modern instrumentation. C- weighting is useful primarily for transient sound or sound containing significant low-frequency content. The A- and C-weighting filter responses typically are represented as in Figure 3. The equal-loudness contours are only valid for young persons listening with both ears to frontally incident, individual tones in a free field without background noise. Nonetheless, A-weighting has been applied to a variety of sounds (including broadband and random noise) over a wide range of magnitudes because it is simple, practical, and nothing better has been found. Other frequency weightings have been proposed with more complex frequency-response curves that particularly emphasize high frequencies. (Frequencies above 1 khz are weighted by as much as +11 db.) D-weighting was offered as a means of estimating the noisiness or the perceived noise level of an exposure. 8 E-weighting was proposed as a means for measuring the loudness or the perceived level of noise. 9 Both of these schemes were based upon laboratory experiments with mostly or entirely young adults usually students. Ultimately, these weightings failed to gain wide acceptance since they were sufficiently like the simpler A-weighting that the exposure ratings they produced were not appreciably different. More recently, G-weighting has been specified for measuring infrasound. 10 The response of this weighting peaks at 20 Hz with rapidly diminishing response above and below the peak. G-weighting has attracted interest as a means for quantifying low-frequency, wind turbine noise.

3 M 06723D August POPULATION DISTRIBUTION by AGE The United States population contains a range of ages well beyond the yr group. With the Baby Boom generation now reaching its 60s, much of the U.S. population is substantially older. The age and gender distribution of a population within a given nation is commonly represented by means of a population pyramid. This triangular distribution plots the portion of the population along the horizontal axis versus age group along the vertical axis, where male population is shown to the left of the axis and female to the right. Thus, the shape of the chart illustrates population development over time. High infant mortality and high risk of early death produce a triangular shape with a wide base. Low infant mortality and greater longevity yield a more rectangular shape since more of the population lives to old age. The 2006 population pyramid for the United States is given in Figure If the fundamental noise metrics are based upon the hearing of young adults and much of the U.S. population is substantially older, are the errors in predicting annoyance response a result of an un-representativeness of the A-weighted frequency response? Per Figure 4, the median ages for men and women are 33 and 35 yr, respectively. Combining men and women, the mean age for the overall population is 37 yr. However, considering the concern in noise assessment is adverse community response, the real issue with respect to annoyance is the adult population. The mean age for the overall U.S. population 20 years of age is 47 yr. Respondent age in annoyance-response surveys either has not been addressed or not found significant. 12 However, Miedema and Vos closely examined the effect of age on annoyance and concluded that age is significant having the equivalent of up to 5 dba exposure difference where the greatest annoyance is found between ages yr and where the relatively young and relatively old are less annoyed. 13 Ageing may affect annoyance response by changes in: hearing sensitivity, attitudes, or life style (e.g., retired vs. working). The possible consequence of hearing sensitivity changes with ageing is explored here. 4 HEARING LOSS The influence of aging is not reflected in the loudness contours and consequently, A-weighting. In the general population, hearing performance may be reduced by a number of conditions. Conductive hearing loss is an abnormal functioning of the outer or middle ear to receive or transmit sounds. It may be genetic or traumatic in origin. Sensorineural hearing loss is a malfunctioning of sensory receptors, auditory nerve, or higher processing. Sensory hearing loss may be: Presbycusis caused by the aging process; Noise-Induced Hearing Loss o Occupational hearing loss caused by work-related noise, or o Sociacusis caused by noise in every-day life; and Nosoacusis due to other causes (disease, trauma, or ototoxic drugs). In occupational, industrial hygiene contexts, quantification of non-occupational hearing losses is of interest to enable discrimination of factors contributing to observed hearing levels in periodic audiometric testing. Hearing levels (i.e., the magnitude by which an audiometric test tone at a given frequency must be increased above a reference sound pressure level to be heard) for 60-yr old workers in an industrialized society but not exposed to workplace noise exhibit substantial loss of hearing particularly at the higher frequencies and especially for men. 14 These data exclude workers who have been exposed to military or farm noise; excessive gunfire, or noisy hobbies; are users of ototoxic drugs, or have a history of other ear problems. Consequently, they may tend to represent the minimum hearing loss for much of the population. Occupational-noise-induced hearing loss exhibits a largely similar pattern with frequency although hearing levels may be generally greater. The effect of hearing loss may be offset by the use of hearing aids to the extent that they are worn by those with hearing impairment.

4 M 06723D August Age-WEIGHTING To evaluate the potential influence of ageing on A-weighted sound levels, age correction curves developed to aid industrial audiometric evaluations were utilized. The age correction curves are averages of several studies of non-industrial-noise exposed populations comparing the median hearing of a particular age group to that of young individuals in the same study. 14 Such data have been adopted in an international standard for the estimation of hearing threshold deviations, * for example as shown in Figure 5, 15 and include not only the effects of aging but also a moderate amount of sociacusis and nosoacusis. (In this investigation, hearing threshold deviations and subsequent weightings were calculated at third-octave-band center frequencies. Where a center frequency did not correspond to a frequency at which a hearing threshold coefficient is reported, the deviation value was interpolated from the coefficient values of the nearest tabulated frequencies corresponding to third-octave-band frequencies.) As can be seen in Figure 5 for 60-yr olds, men and women have substantially similar ageing characteristics up to about 2 khz. Above 4 khz, men show greater hearing loss up to 19 db by age 70 yr. Women have tended to exhibit better hearing than men even in industries where men and women work alongside each other. Whether this is due to some hearing-loss resistant characteristic of the female hearing mechanism or to employment or lifestyle factors is unknown. 14 (If not the result of a female hearing loss resistance, changes in employment profiles and life style in recent years may have reduced women s advantage vis-à-vis men.) In any event, male-female average age corrections were computed for simplicity to evaluate the importance of ageing for the overall population, as also shown in Figure 5. Other than the loss of sensitivity and altered frequency response, the growth of perceived loudness with increasing sound level was assumed unchanged from normal hearing. Weighting Normalization. The audiometric age-corrections were algebraically subtracted from the A- weighting for each third-octave band yielding the family of age-adjusted weighting curves given in Figure 6. (The curves in Figure 6 are based upon hearing threshold deviations which have been extrapolated beyond their reported, 125 8k-Hz values to 16 10k Hz.) The resultant weighting curves reflect both the increasing loss of high-frequency hearing response with advancing age as well as the overall loss of sensitivity with ageing. However, the sound weighting concept as implemented by the A-, B-, and C-weightings only addresses relative frequency response. Consequently, the age-adjusted A-weightings were then normalized to 0 db at their maximum band values. The resultant family of Age-weightings is given in Figure 7 for 40-, 50-, 60-, and 70-yr olds. Not surprisingly, the Ageweighting curves differ from A-weighting primarily in the high frequencies. With normalization, the Age-weighting curves in Figure 7 do not differentiate any effects due to overall increases in hearing level although Figure 6 shows low-frequency deviations for 70-yr olds of more than 10 db. This is reasonable provided loudness judgments by individuals with hearing losses across the hearing range are made relative to some familiar, known source, perhaps conversational speech. If the known and unknown sounds are both attenuated equally by the lost hearing sensitivity, then only the frequency content may be important. The consequences of weighting normalization were examined. A-weighting, and un-normalized and normalized 40- through 70-Age-weightings were applied to pink-noise (0-dB/octave-slope) spectra and the band levels summed. The resultant overall sound levels are provided in Table 1. Table 1 then compares the various weighted overall sound levels directly to the A-weighted overall levels. To the nearest whole decibel, normalization reduces the effect of age by 2, 4, 7, and 11 db at ages 40, 50, 60, and 70 yr, respectively. Although the overall loss of hearing sensitivity for the elderly is large, the following analyses consider normalized Age-weighting consistent with the conventional definition of frequency weightings. Simple Spectra. The effect of Age-weighting on the overall sound levels depends upon the shape of the frequency spectrum. The importance of altered hearing response initially was investigated by assuming various simple sound pressure level spectra (all with 60-dBA overall A-weighted sound level), shown in Figure 8: o o o o +3-dB/octave slope beyond a Hz plateau, 0-dB/octave slope (pink noise), 3-dB/octave slope beyond a Hz plateau, and 6-dB/octave slope beyond a Hz plateau. * Hearing threshold deviation is the expected hearing threshold level of an individual relative to the median threshold of a population of 18-yr olds.

5 M 06723D August 2007 The 40- through 70-Age-weightings, and A- and C-weightings were applied to the spectra and the band levels summed. The resultant overall sound levels are provided in Table 2. While all four spectra have 60-dBA overall magnitude, their magnitudes range db for a 40-yr old (per the age correction curves) and db for a 70-yr old (where the Age-weighted overall magnitude of the strongly low-frequency spectrum is the result of the severe de-emphasis of the high frequencies and maximum band normalization to 0-dB weighting). Table 2 then compares the various weighted overall sound levels directly to the A-weighted overall levels. The 6-dB/octave and 3-dB/octave spectra are, by design, low-frequency dominated with L C L A values of 17 and 10 db, respectively. Considering the effects of age a person with hearing sensitivity per the age-correction for a 40-yr old is estimated to hear the various spectra up to 4-dB differently than A-weighted sound levels indicate. For 50-yr olds, the difference is up to 6 db, and up to 8 db and 9 db different for 60- and 70-yr olds, respectively. Given that the mean age of the U.S. adult population is almost 50 yr, these preliminary results suggest that some sounds may be quantified incorrectly for assessing annoyance response or other similar effects. Real Spectra. While the above comparison indicates Age-weighting may be noteworthy, the findings are speculative since the spectrum shapes may not represent real noise sources. To address this, sound levels from actual 25-Hz 10-kHz measurements of various environmental noise sources were examined. These sources included: o o o o Traffic noise from highways and urban streets, Rail vehicle noise (including an idling locomotive, a light-rail vehicle pass by, and wheel squeal from a railcar in a tight curve), A low-flying jet fighter flyby, and Ventilation equipment (including a rooftop chiller and a small cooling tower). The source spectra are shown in Figure 9. (Note that unlike the simple spectra, the overall A-weighted sound levels are not normalized to 60 dba.) As previously, the Age-weightings, and A- and C-weightings were applied and the band levels summed. Table 3 compares the various weighted overall sound levels directly to the A-weighted overall levels. Some of the spectra have considerable low-frequency content with L C L A values ranging 3 14 db. Despite the greatly differing character of the sounds, their Age-weighted magnitudes did not differ greatly from their overall A-weighted values i.e., by no more than 2 db through age 70. Aircraft Spectra. Aircraft noise is somewhat under-represented in the real spectra examined above, but such exposures are probably the most likely to produce questions regarding noise assessment criteria. To address this, aircraft spectral data incorporated into the U.S. Federal Aviation Administration Integrated Noise Model (INM) computer-based noise prediction method were considered. INM contains an aircraft noise emissions reference database consisting of spectral classes for various aircraft categories in departure and approach operations. Departure sound level spectra were selected for very typical aircraft spectral classes : two-engine, high-bypass ratio turbofan aircraft in , and 757 and A300 aircraft categories. 16 The 1000-ft reference spectra were shaped for propagation to 2000-, 4000-, 8000-, and ft distances considering the frequency-dependent atmospheric absorption of sound over distance with atmospheric conditions producing minimal attenuation, 59 F temperature and 50% relative humidity (ignoring geometric divergence). The spectra in Figure 10 were obtained. As before, the Age-weightings, and A- and C-weightings were applied and the band levels summed yielding 50-Hz 10-kHz overall sound levels and the magnitudes referenced to the A-weighted overall levels, as given in Table 4. The spectra have low-frequency content, L C L A, ranging 4 9 db. The Age-weighted levels did not differ greatly from their overall A-weighted values, i.e., by no more than 2 db for any age group. Again, A- weighted sound level appears to be an adequate descriptor. Natural Sounds. The spectral content of most manmade sounds as experienced in environmental noise exposures is predominantly in the low and middle frequencies. Consequently, Age-weighted sound levels differ relatively little from A-weighted sound levels. However, certain animal sounds are predominantly high-frequency. The sources may include birds, insects, and amphibians. Most of the sound energy produced by common songbirds The source spectra were derived from available file data of actual measurements of the identified noise sources but are not presented as necessarily generally representative of that source.

6 M 06723D August 2007 and some tree frogs is at frequencies 2 khz. Examples of three insect signatures are given in Figure 11, where the high-frequency ( 2 khz) insect sounds were measured in suburban areas with typical low-frequency, residual background noise without prominent noise sources. The overall Age- and C-weighted sound level magnitudes relative to the A-weighted overall levels are presented in Table 5. The spectra have considerable low-frequency content, with L C L A, 9 14 db. The Age-weighted levels differ from their overall A-weighted values by 2 5 db with the low frequencies. If natural sounds are more acceptable (or at least more readily tolerated) than manmade sounds, then the presence of intruding unnatural noise may be more noticeable and objectionable to older persons with typical hearing loss patterns possibly heightening their annoyance response. Because of the high frequency of the natural sounds, the changed hearing response becomes prominent relatively early (between yr age) then remains relatively stable possibly explaining the observation of Miedema and Vos that annoyance peaks between yr (before overall sensitivity changes become dominant). However, since masking has limited downward spread with frequency, the lost benefit due to reduced perception of these high-frequency natural sounds is uncertain. 6 CONCLUSIONS The change in hearing frequency response due to hearing loss typical of aging (i.e., ignoring workplacenoise-induced loses) does not greatly alter the overall frequency-weighted sound levels of common environmental noises. Thus, for the usual progression of hearing ability, older individuals are likely to hear manmade sounds much as younger persons do if overall weighted sound levels are generally representative of hearing ability. Consequently, the A-weighted sound level remains a suitable metric for quantifying the overall noise magnitudes for the general population. The perception of high-frequency natural sounds (e.g., crickets) is likely to become attenuated by middle age, and its absence may serve to heighten the adverse response to manmade sounds. Overall reductions in hearing sensitivity, which are inappropriate to incorporate into frequency weightings, do appear to be important and may alter the adverse response to environmental noise for many individuals older than 50 yr. Ageing of the human hearing mechanism may yet explain some variation in observed annoyance response of individuals but addressed as age-dependent shifts in overall hearing sensitivity and not in terms of frequency response. Tabulation of respondent history of high-level noise exposure in occupational and recreational settings, ear problems, and regular hearing aid usage is recommended in annoyance-response surveys. From this information, approximate age-dependent hearing sensitivity adjustments can be estimated based upon: the ISO-7029 hearing threshold deviations, suitable generic or actual measured noise exposure spectra, and the survey respondent s age and gender. Annoyance response could then be related to the respondent-sensitivity-adjusted day-night average sound levels. 7 REFERENCES Miedema, H.M.E., and C.G.M. Oudshoon, Annoyance from Transportation Noise: Relationships with Exposure Metrics DNL and DENEL and Their Confidence Intervals, Environmental Health and Perspectives, Vo. 109, No. 4, April Schultz, T.J., "Synthesis of Social Surveys on Noise Annoyance," Journal Acoustical Society of America, Vol. 64, No.2, August Fidell, S. and P. Schomer, "Uncertainties in Measuring Aircraft Noise and Predicting Community Response to It, "Noise Control Engineering Journal, Vol. 55(1), January February Finegold, L.S., C.S. Harris, and H.E. von Gierke, "Community Annoyance and Sleep Disturbance: Updated Criteria for Assessing the Impacts of General Transportation Noise on People," Noise Control Engineering Journal, Vol. 42(1), January February Scharf, B., Loudness, Ch. 91, Handbook of Acoustics, (M.J. Crocker, Ed.), Wiley-Interscience, ISO 226, Acoustics Normal Equal-Loudness-Level Contours, International Organization for Standardization, 2003.

7 M 06723D August Earshen, J.J., Sound Measurement: Instrumentation and Noise Descriptors, Ch. 3, The Noise Manual Fifth Edition, (E.H. Berger, et al., Eds.), AIHA Press, Kryter, K.D., and K.S. Pearsons, Some Effects of Spectral Content and Duration on Perceived Noise Level, Journal of the Acoustical Society of America, Volume 35, No. 6, Pg. 866, June Stevens, S.S., Perceived Level of Noise by Mark VII and Decibels (E), Journal of the Acoustical Society of America, Volume 51, No. 2, Pt. 2, Pg. 575, February ISO 7196, Acoustics Frequency-Weighting Characteristics for Infrasound Measurements, International Organization for Standardization, United States Census Bureau, International Data Base. Fields, J.M., Effect of Personal and Situational Variables on Noise Annoyance in Residential Areas, Journal of the Acoustical Society of America, Volume 93, No. 5, Pg. 2753, May Miedema, H.M.E., and H. Vos, Demographic and Attitudinal Factors that Modify Annoyance from Transportation Noise, Journal of the Acoustical Society of America, Volume 105, No.6, Pg. 3336, June Ward, W.D, J.D. Royster, and L.H. Royster, Anatomy and Physiology of the Human Ear: Normal and Damaged Hearing, Ch. 4, The Noise Manual Fifth Edition, (E.H. Berger, et al., Eds.), AIHA Press, ISO 7029, Acoustics Statistical Distribution of Hearing as a Function of Age, International Organization for Standardization, Bishop, D.E., Beckman, J.M., Bucka, M.P., Revision of Civil Aircraft Noise Data for the Integrated Noise Model (INM), BBN Laboratories Incorporated Report 6039, September 1986.

8 M 06723D August 2007 TABLE 1. UN-NORMALIZED and NORMALIZED OVERALL LEVELS for pink noise spectrum (0-dB/octave slope) DESCRIPTOR OVERALL 16 10k Hz SOUND LEVEL (db) (db re L A ) un-normalized normalized un-normalized normalized L A L Age L Age L Age L Age TABLE 2. OVERALL LEVELS of SIMPLE SPECTRA for spectrum slope (db/octave) DESCRIPTOR* OVERALL 16 10k Hz SOUND LEVEL for Spectrum Slope (db/octave) (db) (db re L A ) L p L C L A L Age L Age L Age L Age * L p = un-weighted overall sound pressure level

9 M 06723D August 2007 TABLE 3. LEVELS of SAMPLE REAL SPECTRA re OVERALL A-WEIGHTED SOUND LEVEL based upon data from actual measurements of the identified noise sources but not presented as necessarily generally representative of that source DESCRIPTOR Urban Traffic OVERALL 25 10k Hz SOUND LEVEL (db re L A ) for Source Type Highway Idling Light-Rail Wheel Military Rooftop Traffic Loco- Vehicle Squeal Aircraft Chiller motive Small Cooling Tower L p L C L Age L Age L Age L Age TABLE 4. LEVELS of AIRCRAFT SPECTRA re OVERALL A-WEIGHTED SOUND LEVEL based upon mean 737 and 757 INM spectral class departures propagated to distance (ft) for 59 F temperature and 50 % relative humidity, ignoring divergence DESCRIPTOR OVERALL 50 10k Hz SOUND LEVEL (db re L A ) for Propagation Distance (ft) L p L C L Age L Age L Age L Age

10 M 06723D August 2007 TABLE 5. LEVELS of SPECTRA with NATURAL SOUNDS re OVERALL A-WEIGHTED SOUND LEVEL per insect sounds (crickets, cicadas, and katydids) measured in suburban areas with typical low-frequency, residual background noise without prominent noise sources DESCRIPTOR OVERALL 25 10k Hz SOUND LEVEL (db re L A ) for Spectra Containing Crickets Cicadas Katydids L p L C L Age L Age L Age L Age

11 M 06723D August 2007 FIGURE 1. ANNOYANCE RESPONSE to NOISE FIGURE 2. EQUAL-LOUDNESS CONTOURS per ISO 226:2003 and WEIGHTING CURVES

12 M 06723D August 2007 FIGURE 3. COMMON WEIGHTING CURVES FIGURE 4. POPULATION PYRAMID

13 M 06723D August 2007 FIGURE 5. EXPECTED HEARING THRESHOLD DEVIATIONS for otologically normal 60-yr olds relative to 18-yr olds, estimated at third-octave-band center frequencies from ISO 7029:2000 FIGURE 6. UN-NORMALIZED AGE-ADJUSTED A-WEIGHTING CURVES derived from A-weighted response with male-female-average hearing threshold deviations incorporated

14 M 06723D August 2007 FIGURE 7. NORMALIZED AGE-WEIGHTING CURVES derived from A-weighted response with hearing threshold deviations incorporated and with maximum band normalized to 0 db FIGURE 8. SIMPLE SPECTRA

15 M 06723D August 2007 FIGURE 9. REAL SPECTRA from actual measurements of the identified noise sources but not presented as necessarily generally representative of that source FIGURE 10. SAMPLE AIRCRAFT SPECTRA estimated from commercial jet aircraft departure spectra per reference noise emissions

16 M 06723D August 2007 FIGURE 11. SPECTRA with NATURAL SOUNDS consisting of insect noise 2 khz with suburban residual, background noise