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| Year : 2009
| Volume
: 11 | Issue : 45 | Page
: 223-230 |
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| Estimates of the auditory risk from outdoor impulse noise I: Firecrackers |
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Gregory A Flamme, Kevin Liebe, Adam Wong
Department of Speech Pathology and Audiology, Western Michigan University, Michigan, USA
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| Date of Web Publication | 2-Oct-2009 |
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Firecrackers are common impulse noise exposures in the United States. In this study, impulses produced outdoors by consumer firecrackers were recorded, described, and analyzed with respect to the amount of the auditory risk they pose to the unprotected listener under various listening conditions. Risk estimates were obtained using three contemporary damage risk criteria (DRC), including a waveform parameter-based approach (peak SPL and B duration), an energy-based criterion (A-weighted sound exposure level and equivalent continuous level), and a physiological model (the AHAAH model developed by Price and Kalb). Results from these DRC were converted into numbers of maximum permissible unprotected exposures to facilitate comparison. Acoustic characteristics of firecracker impulses varied with the distance, but only subtle differences were observed across firecrackers. Typical peak levels ranged between 171 dB SPL at 0.5 m and 142 dB SPL at 8 m. Estimates of the auditory risk did not differ significantly across firecrackers, but varied with the distance. Vast differences in maximum permissible exposures were observed, and the directions of the differences varied with the level of the impulse. Typical estimates of maximum permissible exposures ranged between 0 and 2 at 0.5 m and between 31 and 227,000 at 8 m. Unprotected exposures to firecracker impulses should be limited or avoided entirely if the firecrackers are ignited in batches within 8 m of the listener. Differences across DRC are inconsequential at 0.5 m, but have substantial implications at distances of 1 m and more. Keywords: Explosions, hearing loss, impulse noise, noise-induced, noise/adverse effects, risk factors
How to cite this article:
Flamme GA, Liebe K, Wong A. Estimates of the auditory risk from outdoor impulse noise I: Firecrackers. Noise Health 2009;11:223-30 |
| Introduction | |  |
This is the first in a series of two papers on auditory risk from impulse noise. In the current paper, we review contemporary damage risk criteria (DRC) for impulse noise and select three exemplars for use in this project. We also describe the acoustic characteristics of firecracker impulses and associated estimates of the auditory risk. The companion paper [1] describes acoustic characteristics and risk estimates from a small sample of firearms and examines the relationships among DRC using data from both firecrackers and fireworks.
Nonoccupational exposure to impulse noises produced by firecrackers is common. In 2007, approximately 260 million pounds of fireworks were imported into the United States. Fireworks were associated with over 6000 injuries (predominantly to children) treated in hospital emergency departments in 2007, with firecrackers being the second most common source of injuries treated in hospital emergency departments. [2]
Professionals involved in hearing healthcare are in a position to counsel patients exposed to noise from firecrackers and firearms. Such guidance can take the form of recommendations to allow unprotected exposures, to limit unprotected exposures to only a few impulses, to use hearing protectors, or to avoid any exposure to the impulse. These recommendations should be informed by the known risks associated with the exposure and concerns for the patient's overall safety and welfare. If a recommendation is too lax, the patient could unknowingly endanger his or her hearing. On the other hand, a patient could elect to discard an enjoyable recreational activity without reason if an unjustifiably strict recommendation is made.
Firecrackers produce impulsive noise through explosive release of chemical energy via decomposition of solid materials into gasses that require much greater volume. This combustion produces locally high temperatures and a high-intensity supersonic shock wave that emanates from the source [3] and can be visualized via special optical techniques. [4]
| Acoustic Parameters of Impulses | | |
Firecrackers can produce sound levels in excess of 160 dB SPL and there is evidence that permanent hearing loss can result from even limited exposure to these sounds. [5],[6] Estimates of peak sound levels produced by firearms are in the range of 143.5-174.7 dB SPL at the location of the shooter or bystanders in the immediate area. [7],[8]
The degree of the auditory risk from an impulse depends on the acoustic characteristics of that impulse. A complete characterization of an impulse requires more information than just the peak level. Impulses can be described with respect to the amount of energy present in the impulse, the duration of the impulse, and the kurtosis of the time waveform. The amount of energy in the impulse can be represented either as instantaneous peak amplitudes or integrated energy across the duration of the impulse, e.g., sound exposure level (SEL) and equivalent continuous level (Leq ).
Four major duration parameters have been used as source data for assessing the auditory risk. The A duration represents the duration of the pressure wave and is defined as the time interval between the onset of the impulse and the moment the time waveform first passes through the ambient pressure. [9] The A duration can be considered a characteristic of the energy of the explosive charge and will generally follow a Friedlander wavefunction. [3] The B duration represents the duration of the pressure envelope and is defined as the total amount of time within which the absolute value of the envelope of the impulse waveform resides within 20 dB of the instantaneous peak, including reflections and negative waveform amplitudes. The B duration can be considered a characteristic of the environment in which combustion takes place, including for example the firearm and any reflective surfaces in the vicinity. [9] The C duration [10] is similar to the B duration, but it includes only the time interval within which the waveform resides within 10 dB of the instantaneous peak, including reflections and negative waveform amplitudes. The D duration [11] also represents the time interval in which the energy resides within 10 dB of the instantaneous peak, and in this sense it is similar to the C duration. The defining aspect of the D duration is that calculations based on this duration use the Hilbert transform to represent both potential and kinetic energy in the waveform, which was intended to result in more stable estimates of duration.
Kurtosis represents the peakedness of the time waveform. Studies with chinchilla models revealed that kurtosis is directly related to the potential for auditory damage from an impulse, at least up to values of 50. [12]
| Damage Risk Criteria | | |
The acoustic characteristics of impulses are used to derive estimates of the auditory risk via damage risk criteria (DRC). Contemporary DRC can be separated into three types, depending on whether risk estimates are based on the parameters of the impulse waveform, the total energy contained within the impulse, or whether the impulse waveform is evaluated via an electroacoustic model of the auditory system.
| Waveform Parameter-Based DRC | | |
The seminal human research relating impulse waveform parameters to thr auditory risk was conducted prior to the 1970s, using instrumentation and analytic approaches that were quite advanced for the time, but which have been eclipsed by contemporary signal acquisition and analysis methods. Owing in part to the impracticality of spectral analyses of such signals, acoustic analyses of the impulses were limited to the time waveform. [9] As a result of these measurement considerations, DRC developed at that time were based on parameters of the impulse waveform (e.g., peak amplitude, duration). In the United States, the predominant approach for assessing the potential for harm associated with an impulse noise is described in Military Standard 1474D. [13] It assumes that for any exposure with a peak level over 140 dB SPL, hearing protectors shall be used. The standard was derived from a set of recommendations from the National Academies of Sciences Committee on Hearing, Bioacoustics, and Biomechanics (CHABA) that were published in 1968, [14] and was heavily influenced by a joint study conducted by military personnel in the United States and UK. [9] The works of Coles et al. and CHABA pertained to unprotected exposures to impulses and were extended to people wearing hearing protectors by assuming that a single hearing protector attenuated peak levels by 29 dB.
Maximum exposure limits [9] were reported for exposures to 100 impulses from a sound source located at grazing incidence. For A or B durations of 1 ms, the maximum permissible peak level was approximately 164 dB SPL, and the permissible peak level increased by 2 dB for each halving of duration until the duration reached approximately 200 μs. For durations greater than 1 ms, separate criteria were applied for A and B durations. The A duration was considered relevant for impulses emulating a Friedlander wavefunction, and the B duration was relevant for all other types of impulses. The B-duration criterion maintained a constant slope (-2 dB per doubling of duration), reaching a permissible peak level of 145 dB SPL for durations of approximately 700 ms. This criterion was developed to protect 75% of ears, with recognition that the most susceptible 25% of ears might not be protected. The maximum exposure limits per CHABA [14] followed a very similar function, but all values were shifted downward by 5 dB to account for sound sources located at normal (90) incidence to the head and another 5 dB to protect all but the most susceptible 5% of ears (i.e., 95th percentile). Both of these correction factors were recommended by Coles et al. [9] At the low end, the maximum permissible peak level terminated at 138 dB SPL (at a 200-ms B duration) as a way of accounting for the expected protective effect of the middle ear muscle reflex, which was hypothesized to have a meaningful effect on the middle ear transfer function for impulses with B durations greater than about 200 ms. [14] For numbers of exposures other than 100, the maximum exposure level was to be reduced by 5 dB for a 10-fold increase in the number of exposures.
| Energy-based DRC | | |
DRC for impulse noise have also been based on the quantity of energy present in the impulse. We considered the Smoorenburg-NATO recommendations [15] emblematic of the current level of advancement in this kind of DRC. An energy-based DRC assumes that auditory damage increases in proportion to the energy reaching the cochlea. The principal measure of this energy is represented by the A-weighted sound exposure level (dBA SEL), which represents the A-weighed energy in the impulse distributed over a reference duration of 1 s. For short-duration impulses (e.g., rifle noise) at normal incidence to the listener, the Smoorenburg-NATO [15] DRC recommended a limit of 116 dBA SEL for 1-50 impulses. When distributed over an 8-hour period that is otherwise silent, this total amount of sound energy corresponds to 71.4 dBA Leq . For listeners exposed to more than 50 impulses, the maximum exposure level for any one impulse would be reduced so that the 8-hour Leq would not exceed 80 dBA.
| Ear Model-based DRC | | |
Price and Kalb [16] developed an electroacoustic model of the ear called the Auditory Hazard Assessment Algorithm for Humans (AHAAH). This computational model of the ear uses components that represent the function of each anatomical structure of the ear. The model was originally based on the auditory physiology of the cat, [16] and the human version was developed by scaling model parameters to match the physical characteristics of the human ear. The model treats the outer and inner ear as essentially linear elements. Two nonlinear elements are included in the model of the middle ear. One of these elements represents the annular ligament surrounding the stapes, which is presumed to limit stapes' displacement to about 20 μm. [17],[18] For sound pressures greater than about 2 kPa, it appears that this nonlinearity reduces the middle ear transfer function, based on intracochlear pressure measurements in guinea pigs [19]
The other nonlinear element is intended to account for the expected effect of middle ear muscle contraction. The Price-Kalb model provides separate risk estimates for both warned and unwarned listeners. Middle ear muscles are presumed to contract prior to the onset of the impulse under the warned condition, and muscle contractions are presumed to be elicited by the sound under the unwarned condition. For short-duration impulses, such as firecrackers and gun blasts in outdoor environments, the contraction cannot be expected to occur during the pendency of the impulse. The likelihood that a randomly selected untrained human listener's middle ear muscles will contract prior to the arrival of blast noise from a gun has never been tested directly, and the effect of any such contraction on auditory risk from small arms is not entirely clear because middle ear muscle contractions can enhance the admittance of high-frequency signals into the cochlea. [20] However, it has been shown that human middle ear muscle contractions are not regularly triggered by classical conditioning. [21]
The Price-Kalb model estimates the basilar membrane's temporal displacement function, and damage at each location along the basilar membrane is assumed to accumulate as a result of upward basilar membrane displacement, in microns squared, summed across all upward displacements. Hazard estimates are determined by the location on the basilar membrane with the greatest sum, and the sum at this location is scaled into auditory risk units (ARUs). Summed displacements at other basilar membrane locations are discarded from ARU estimates. For listeners with occasional exposure to impulse noise, an exposure to 500 ARUs is considered unsafe (i.e., can be expected to produce more than 25 dB of threshold shift); a 200 ARU exposure is considered safe (i.e., would not be expected to produce any threshold shift), and a temporary threshold shift between 0 and 25 dB would be expected for exposures between 200 and 500 ARUs.
| Study Rationale | | |
Firecracker impulses are known risk factors for hearing impairment, [5],[6] but acoustic characteristics and risk estimates have not been obtained using contemporary measurement and analysis techniques. There could be substantial differences in the auditory risk across firecracker types, and there could be distance limits beyond which unprotected listening might pose minimal risk. Measurements of acoustic characteristics and estimates of risk are necessary before these kinds of practical recommendations can be developed to help firecracker users protect their hearing.
The current study was designed to inform a measurement method and obtain preliminary data on a small set of firecrackers. The overall study objectives were to (1) report the acoustic characteristics of impulses from these sources and assess the influence of methodological factors (e.g., microphone location, ammunition) on subsequent results; (2) describe the degree of risk to the auditory system posed by these impulse noise exposures according to three current types of DRC; and (3) describe the relationships and differences among the recommended maximum number of exposures developed using those DRC. Results from firearms and analyses of combined firecracker and firearm data are presented in the companion paper. [1]
| Method | | |
Firecrackers
All firecrackers in this study conformed to UN0336 and DOT 1.4G consumer firework classifications, which mean that they contained no more than 50 g of explosive material. Two of the firecracker types were 1.5 inches long by approximately 0.25 inches in diameter. These were the Black Cat (BC) model 101 Super Charged Flashlight Cracker, and the Country Cracker (CC). The CC product label suggested that it would be atypically loud because titanium was blended with the explosive material. We also included a firecracker (labeled M70) that was 1.75 inches long by approximately 0.5 inches in diameter.
Test conditions
Impulses from BC and CC firecrackers were recorded at 0.5, 1, 2, 4, and 8m distances from a fixed firing position on the pavement. The M70 impulses were recorded at only the 2- and 8-m distances. To obtain impulse interval data, recordings of BC and CC firecracker impulses were obtained for individual firecrackers and for groups of 16 firecrackers sharing a central fuse.
Environments
Firecracker impulses were recorded in an empty paved parking lot on the Western Michigan University campus. Firecrackers were ignited while resting on the pavement at a 13m distance from the nearest vertical reflective surface.
Instrumentation and data processing
Recordings were made using a GRAS type 40BD Ό-inch microphone oriented at grazing incidence. Signals were routed to a Tucker Davis RP2.1 real time processor through a GRAS type 26AC preamplifier and a GRAS type 12AA power supply. The real time processor was configured to perform analog-to-digital conversion at a 195,312.5 Hz sample rate and 24-bit depth. Data were first processed in Matlab (Mathworks, Inc., Natick, MA, USA) and then SPSS (SPSS Inc., Chicago, IL, USA) for descriptive and inferential statistics. Matlab software routines were prepared by NIOSH [22] and modified by the lead author for this study.
Procedure
Recording
Recordings of at least 10 impulses were made in each measurement condition. For firecracker impulse recordings, the measurement microphone was placed at the desired distance from the firing location. For individual impulse recordings, firecrackers were placed on the pavement at the firing location and ignited. For batch recordings, the group of firecrackers was placed at the firing location and the shared fuse was ignited. Firecrackers were ignited singly for the purpose of assessing acoustic characteristics and obtaining risk estimates. In addition, batches of 16 BC and CC firecrackers were ignited by lighting a single shared fuse so that the time interval separating successive impulses could be assessed.
Data processing and analysis
After data transfer to the analysis computer, impulse baseline adjustments were made by subtracting the mean value during a silent period in the waveform from all points on the recording. Each impulse was then analyzed independently using the Matlab software routines described above. Risk estimates were calculated in terms of maximum permissible exposures (MPEs) via each DRC for a listening condition in which the listener was directly facing the sound source (i.e., grazing incidence to the ear). MPEs via the Price-Kalb DRC were calculated using a maximum of 500 ARUss under unwarned listening conditions (i.e., no anticipatory middle ear response). We elected to use the unwarned condition based on the results of Bates et al. [21] who found that anticipatory middle ear reflexes cannot be conditioned in the majority of human listeners.
Descriptive statistics, analyses of variance (ANOVA), and correlations were obtained using SPSS v.14. Firecracker ANOVAs were conducted via a random effects model (impulse source Χ microphone location), with Bonferroni's correction for multiple comparisons in post hoc analyses. Linear (Pa) rather than logarithmically transformed (SPL) quantities were used in analyses of sound levels, and estimates of MPEs were logarithmically transformed to normalize variance.
| Results | | |
Acoustic characteristics
Typical firecracker impulse waveforms and acoustic characteristics of impulses from firecrackers are represented in [Figure 1] and [Table 1], respectively. Firecracker impulses generally emulate a Friedlander waveform. [3] Typical unweighted peak levels for all firecrackers were greater than 140 dB SPL through 8-m distances. The A-weighted sound exposure levels (SEL A ) of the BC and CC firecrackers ranged between 101 and 126 dB. Peak levels were not significantly different across firecrackers(F2,5 = 1.92; p = .239), but decreased significantly as a function of distance (F4,5 = 195; p < .005), as expected. There was also a modest firecracker Χ distance interaction (F5,119 = 4.24; p = .001), which was related to a greater reduction in peak level for the CC firecracker than the BC firecracker between the 0.5- and 1-m distance. A similar pattern of results was observed for A-weighted peak levels, 8-hour Leq , and A-weighted 8-hour Leq .
The A durations of the firecracker impulses were as short as 160 μs and as long as 1.9 ms. The B durations were related to distance and ranged between 0.5 and 3 ms. C and D durations tended to increase with the distance, ranging between 90 and 220 μs. A durations did not differ significantly across firecrackers (F2,5 = 1.06; p = .409) or distance (F4,5 = 2.52; p = .162) and no significant interactions were observed. B durations differed significantly across firecrackers (F2,5 = 15.1; p = .006) and distance (F4,5 = 12.2; p = . 007). Post hoc pairwise comparisons revealed that the BC and CC firecrackers had longer B durations than the M70 firecrackers and that B durations measured at 0.5 m were significantly shorter than at 1, 2, and 8 m. Analyses of C durations identified significant differences across firecrackers (F2,5 = 64.2; p < .005) and distances (F4,5 = 250; p < .005). M70 firecrackers had longer C durations than the BC or CC firecrackers, and C durations at nearly all distances were significantly different from each other. The exceptions were two pairs of distances, one at 0.5 and 1 m, and the other at 2 and 4 m. A significant main effect on D duration was only observed across distance (F4,5 = 10.4; p = .011). Post hoc pairwise comparisons revealed that D durations were significantly different from each other across distance, but two distance pairs (0.5 and 1, 2 and 4 m) were not significantly different and that D durations at 8 m were not significantly different from those at 2 or 4 m.
Analyses of the kurtosis of firecracker impulses indicated differences in kurtosis across firecrackers (F2,5 = 10.6; p = .014) and distance (F4,5 = 202; p < .005). Impulses from M70 firecrackers had lower kurtosis than the BC or CC firecrackers, and kurtosis values were significantly different from each other at all distance combinations except the combination of 2 and 4 m.
The BC and CC firecrackers also featured similar energy spectra. At 2 m, the spectral peak for these impulses was between 1 and 2 kHz, where one-third octave band RMS levels exceeded 95 dB SPL [Figure 2]. Impulses from the M70 contained more energy below 1.25 kHz and less energy above that frequency. The high-frequency content of the BC impulses decreased with increasing distance from the firing point [Figure 2]b.
The mean interval between impulses for firecrackers ignited in batches was 140 ms, but these intervals varied widely, with 95% of successive impulses occurring within a 51 to 644ms interval.
In summary, the firecrackers included in this study produced peak levels greater than 140 dB SPL through 8m distances. Pressure envelope durations were relatively short, ranging between 0.5 and 3 ms, and firecrackers produced greater kurtosis than was observed with firearms, [1] ranging between 250 and 790. The smaller diameter firecrackers (BC and CC) produced maximal energy between 1 and 2 kHz, while the larger firecracker (M70) produced the greatest amount of energy in a lower frequency range.
Risk estimates
Great differences in the numbers of permissible exposures to firecracker impulses were observed across the three approaches to estimating the auditory risk used in this study [Table 2], particularly at longer distances. At all but the 1 m distance, the Coles-CHABA DRC was most liberal, followed by the Smoorenburg-NATO and Price-Kalb criteria. However, at 1 m the Smoorenburg-NATO criteria permitted the greatest numbers of exposures. For example, the MPEs for BC firecrackers evaluated using the Coles-CHABA DRC were practically unlimited at distances of 4 m, while the estimated MPEs via the Smoorenburg-NATO and Price-Kalb DRCs were 107 and 18, respectively.
A significant main effect for distance (F4,5 = 121; P < .005) and interaction between distance and firecracker (F5,119 = 3.25; P = .009) were observed via the Coles-CHABA DRC, but no main effect of firecrackers (F2,5 = 2.26; P = .198) was observed. Post hoc testing identified the expected increase in the MPE with the distance.
Exposure to a single firecracker impulse at 0.5 m was judged unsafe via the Smoorenburg-NATO DRC. But a range of 31-50 exposures were allowed at distances of 1 and 2 m for the BC and CC firecrackers and at 2-m distances for the M70 firecrackers. Estimated MPEs via the Smoorenburg-NATO DRC are discontinuous for high-level impulses [15] , allowing either 0 or 50 impulses, depending on whether the A-weighted SEL exceeds 120 dB for impulses produced directly in front of the listener. An average of 31 was observed because some impulses exceeded this threshold value while other impulses did not. We consider this result as functionally equivalent to an MPE of zero because it is impossible to know in advance whether a given firecracker will produce an impulse above or below this cutoff value. An evaluation of differences in MPEs across firecrackers was not conducted at 0.5, 1, 2, and 4m distances because MPEs from at least one firecracker were in the categorical range of the Smoorenburg-NATO DRC. No significant difference was found in MPEs across firecracker types at the 8-m distance (F2,23 = 0.02; p = .983).
Maximum permissible exposures via the Price-Kalb DRC varied by firecracker (F2,5 = 8.97; p = .021) and distance (F4,5 = 382; p < .005), and no significant interaction between firecracker and distance was observed (F5,119 = 1.93; p = .094). Post hoc testing revealed that the MPEs to M70 impulses were lower than for either the BC or CC firecrackers, and that the numbers of permissible exposures increased with the distance.
| Discussion | | |
Unprotected exposures to impulses from fireworks are common in the United States, but few descriptions of the acoustic characteristics of impulses from consumer firecrackers or estimates of auditory risks from these sounds can be found in the contemporary archival literature. The current study examined recordings of impulses from firecrackers with respect to their acoustic characteristics and the risk they pose to the unprotected auditory system.
Firecrackers ignited nearby produce an auditory risk that is comparable to the auditory risk of gunfire to the shooter, [1] especially when one considers the manufacturer's recommendations that firecrackers be fired in groups of 16 or more. In the United States, 1½-inch firecrackers are often sold in packages containing large quantities (e.g., 250, 500, 1000), and two of the three risk estimation methods used in this study suggest that this many unprotected exposures to firecracker impulses would be inadvisable even at distances of 8 m. At present, there are no requirements to warn firecracker users of the auditory risk from these sounds, and the results of the current study suggest that such warnings are warranted. If one assumes that the person lighting a firecracker fuse is capable of moving 2 m from the firing point before the first firecracker ignites, fewer than five unprotected exposures would be allowed by the most conservative DRC. This number should be reduced to 2 or less at distances closer to the firecracker firing point, which would occur when the firecracker fuse was ignited not at the tip but somewhere in the middle.
There was little difference between the maximum permissible exposures for the two firecracker sizes included in this study or the two firecracker brands within the 1-inch size evaluated in this study. When the auditory risk was evaluated via the Price-Kalb model, more unprotected exposures were permissible via the larger firecracker than the smaller ones [Table 2], presumably due to the difference in the spectral content of the impulse from the larger firecracker.
Although we observed the expected decrease in the sound level with the distance, the BC and CC firecrackers differed with respect to the distance-related decrease in the sound level for recordings made near the firing position. Also, the amount of high-frequency energy was inversely related to the distance from the firing point [Figure 2] and kurtosis decreased with the distance. Estimates of the firecracker peak level presented here are somewhat higher than reported in prior studies, and this difference may be due to instrumentation and analysis differences across studies. Smoorenburg [6] reported a range of peak levels between 145 and 160 dB SPL, with a modal value of 151 dB SPL, measured at 2 m using a -inch microphone and a precision sound level meter. One can expect such measurements to underestimate the instantaneous peak level because the peak durations were considerably shorter than the rise time of the sound level meter.
The DRC of Smoorenburg [15] might underestimate permanent threshold shift. The Smoorenburg DRC depends on only the A-weighted energy in the impulse. However, kurtosis is a factor beyond A-weighted energy that predicts permanent threshold shift (PTS). [23] As kurtosis grows from a value of 3, which is the kurtosis of white noise, to a kurtosis of approximately 50, an increase in PTS has been observed in chinchilla models, after controlling total energy in the signal and temporal variation in amplitudes. [24] The minimum kurtosis among the impulses in the current study was well above this range [Table 1] and [Table 2]. A joint exposure criterion based on sound energy and kurtosis has been hypothesized as necessary and sufficient, [12] but this approach has not been evaluated with respect to impulses like those produced by firecrackers and firearms.
Biases and limitations
The current study provides these data for a limited set of firecrackers. We made the impulse recordings in an open area. The results presented here cannot be expected to generalize to conditions where the impulse source is located near large reflective surfaces or any kind of enclosure. Analyses of impulses recorded in such environments are required to determine the reduction in the maximum numbers of permissible exposures in such environments, but it is reasonable to expect that among the stimuli included in this study, only distant firecracker impulses could potentially have one or more permissible unprotected exposure via the most conservative approach to risk estimation.
Finally, MPEs reported in this study could underestimate the auditory risk for some listeners. Ward [25] showed that TTS varies with gender, possibly due to gender-related differences in outer and middle ears. However, Ward identified no gender differences for trains of comparatively low-level impulsive stimuli (136-156 peak SPL, unstated duration and interpulse interval) delivered via supra-aural earphones.
| Conclusion | | |
All three firecrackers evaluated in the current study produced impulses that put unprotected listeners at increased risk of hearing impairment. Listeners 8 m from the firing location should limit exposures to one batch of 16 firecrackers or less, using the most conservative DRC. A distance limit corresponding to the minimal auditory risk was not identified in this study, and can be expected to be 16 m or more from the firing location. At closer distances, no unprotected exposure would be permitted to firecrackers ignited in a batch sharing a single fuse. Great disagreements among DRC were observed at distances greater than 0.5 m. Current evidence concerning the validity of these DRC cannot identify which, if any, damage risk criterion is most accurate for these types of impulses. We recommend evaluating impulses via a variety of approaches and following the most conservative criterion until such information is available.
| Acknowledgements | | |
The authors thank William Murphy, Chucri Kardous, and Edward Zechmann (all from CDC/NIOSH, Taft Laboratories, Cincinnati, OH) for providing the Matlab software routines used to analyze the impulse data.
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Correspondence Address:
Gregory A Flamme
Western Michigan University, 1903 W. Michigan Ave., Kalamazoo, MI 490 08
USA
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/1463-1741.56216
[Figure 2]
[Table 1], [Table 2], [Figure 1] |
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