Assessment of hearing loss risk due to impact noise in industrial environments

Jorge P. Arenas 1

Institute of Acoustics, Univ. Austral of Chile PO Box 567, Valdivia 509000, Chile

Jorge Cárdenas 2

Institute of Acoustics, Univ. Austral of Chile PO Box 567, Valdivia 509000, Chile

Christian Robertson 3

Institute of Acoustics, Univ. Austral of Chile PO Box 567, Valdivia 509000, Chile

José L. Urnia 4

Asociación Chilena de Seguridad Ramon Carnicer 163, Providencia, Santiago, Chile

ABSTRACT Impact noises are often found in industrial environments, and they are predominant in mining, construction, factories, workshops, and shipyards. Impulse noises are more likely to cause noise- induced hearing loss than continuous noise of equal energy. Many countries have defined impulse noise exposure limits and criteria in occupational settings. They are based on the sound level measurements made using standard sound pressure level meters and dosemeters. However, it is not appropriate to use these instruments when dealing with such high peak levels and short duration times because of their metrological limitations. Studies on hearing damage produced by firearms, mainly on police and military personnel, have led to damage risk criteria in some standards. Although industrial noises can reach similar peak sound pressure levels, not many results have been published. In this work, several common sources of industrial impact noise were measured in-situ at the worker locations, using a specialized system equipped with high-dynamic-range microphones and a very high data acquisition rate. The signals were post-processed to obtain some metrics used for impulse noise exposure assessment. It is shown that many standard industrial processes reported a very high risk of impulsive noise to human hearing.

1 jparenas@uach.cl

2 jcardenas@uach.cl

3 croberts@soporteinteligente.cl

4 jurnia@achs.cl

a slaty. inter.noise 21-24 AUGUST SCOTTISH EVENT CAMPUS O ¥, ? GLASGOW

1. INTRODUCTION

The study of impulse noise and its potential e ff ects on hearing loss is a real challenge for researchers [1]. The term impulse noise is used generically here to include all forms of high intensity, short-duration sound, i.e., from the impacts present in industrial environments to the intense shock waves associated with military operations. The range of parameters defining an impulse is wide. Pulse durations can range from tens of microseconds, in the case of small firearms, to several hundred milliseconds for a sonic boom or reverberating industrial impact. Pulse intensities can range from 100 dB to over 185 dB peak sound pressure level ( L peak or Lpk). The energy of an impulse is usually widely distributed, but spectral concentrations of energy can cover several frequencies throughout the audible range. The number of occurrences of impulsive noise in industrial or military environments can range from one impulse or less per hour to several impulses per second with no fixed time interval between impulses. In addition to the physical parameters of the impulse, other environmental conditions can seriously a ff ect the outcome of exposure, i.e., whether the impulses occur in a free or reverberant field, the angle of incidence, the presence of other noise or vibration, etc. [2]. The waveforms or temporal characteristics of impulse noise can be very di ff erent. One extreme is industrial impact noise. Impact noise is reverberant ("ringing"), and its physical behavior is generally explained by the laws of acoustics. The physical specification of impulse noise is further complicated when impulses are mixed with continuous noise. The combination of impulse and continuous noise is very common in industrial environments. Crest factors of up to 50 dB are commonly found in these environments.

1.1. Assessing Hearing Loss Risk due to Impulse Noise The mechanism of hearing damage produced by impulsive noise is still under study. One di ffi culty in studying the e ff ects of impulsive noise on hearing is that it is unethical to conduct research that exposes unprotected subjects to high-energy impulsive noise to determine permanent changes in the hearing threshold. Because of this, most studies have relied on capturing temporal changes in the hearing threshold after noise exposure. It has been established that impulsive noises are more likely to cause noise-induced hearing loss than a continuous noise of equal energy [3]. The fundamental reason is that impulsive noises have a wide bandwidth and concentrate enormous sound energy in a small time interval. The protective mechanisms of the human ear are also ine ffi cient in activating the acoustic reflex [4]. In contrast to the damage to the sensory hair cells in the inner ear caused by continuous noise exposure, this corresponds to a more physical event in the case of high-level impulsive noise,. For this reason, it has been reported that even exposure above a peak of 140 dB (equivalent to an RMS sound pressure of 200 Pa) can cause permanent hearing loss and tinnitus, often when the impulse is unexpected [5]. Exposure to high-intensity impulses can also cause acoustic trauma and instantaneous mechanical damage to the inner ear. Impulse noise exposure is common in metalworking industries, construction, shipyards, mining, and police and military personnel. Price and Kalb [6] developed a mathematical model to assess hearing risk in the presence of high- level impulses. This model is the basis for what is now known as the Auditory Hazard Assessment Algorithm for Humans (AHAAH). It is described in detail in the work of Price [7]. In 1990, the ISO 1999 standard was published to consider impulsive noise of moderate value for application in industry. It was agreed to use the A-weighted equivalent level (LeqA) to assess impulsive noise levels up to 145 dB at the ear [8]. It is established that exposure to a peak sound level higher than 140 dB is not allowed. A similar criterion was adopted by the American Standards Institute (ANSI), in its ANSI S3.44 standard [9], the National Institute of Occupational Safety and Health of the U.S.A. [10] and by the European Community in its directive 86 / 188 [11]. One of the ISO 1999 standard features is that it integrates the hazard of exposure to impulsive noise with exposure to continuous noise. However, it has been established that this is not appropriate for levels exceeding

140 dB peak sound pressure level. The three damage risk criteria contained in various versions of the standard MIL-STD-1474 di ff er significantly in their risk assessment. For example, the MIL-1474-E recommendation [12], employs ARU and the criterion of equal equivalent energy averaged over 100 ms intervals (LIAeq100ms), while the MIL-1474-D recommendation [13], relied on the A- and B-duration measurement, and provided a limiting category to impulsive noise. Among the criteria for impulsive noise, mention can also be made of ANSI S12.7 [14], which uses the sound exposure level (SEL), expressed as LxE, where x is the frequency weighting. Although less objective than the above, the use of the average peak level graph (measured as Lzpeak), expressed in pascals as a function of time in seconds, has also been recommended as a method of hearing damage assessment. Methods for estimating risk criteria for impulsive noise exposure damage remains a very active area of research. Despite the scientific evidence of the hearing damage caused by impulsive noise and the importance of the metrics needed to assess the risk, validated and accepted results for impulsive noise had lagged far behind their steady or fluctuating continuous noise counterparts. One reason for this dichotomy is metrological limitations. There is general agreement that the best method for measuring an isolated impulse is to record the sound pressure history over time. However, because of the extreme limits of impulsive noise parameters, care must be taken when performing a measurement. For example, rising times in impulsive noise can be less than a few microseconds, and intensities can be as high as 185 dB in sound pressure level. Impulsive noise peaks above 140 dB are challenging to measure because many instruments have been designed initially to measure continuous noise and intermittent sound levels in the industrial environment. Therefore, they are not capable of accurately capturing such intense sound energy. For this reason, to adequately measure impulsive noise and assess hearing risk, unique instrumentation is required to acquire a signal at a high sampling rate by employing high-dynamic- range microphones. One problem with that is that these types of equipment are often costly because of the instrumentation’s demanding technical requirements. Although industrial noises can reach similar peak levels than firearms, not many results have been published. In this work, several common sources of industrial impact noise were measured at four industrial sites at the worker locations, using a specialized system equipped with high-dynamic-range microphones and a very high data acquisition rate. Note that this work does not report the total impact noise to which workers may be subject during a working day, but the impact noise from typical sources encountered in industrial sites where impact noise is commonly prevalent.

2. EXPERIMENTAL MEASUREMENTS

The measurements of impact noise were conducted at four industrial sites that included:

– Site 1: a large mechanical factory that manufactures hoppers for large mining trucks, – Site 2: a factory of large metal parts for the mining industry, – Site 3: a workshop for vulcanization and maintenance of tires and vehicle accessories, and – Site 4: a shipyard that produces azimuth tugs, ferries, cruise boats, supply vessels, and well boats.

Most of the noise sources at these sites corresponded to impact noise produced by shocks, riveting, cutting, and hammering on large metal plates. A few sources at Site 3 involved impulsive noise from rapid gas releases at the workshop. The B&K 7963 impulsive noise evaluation system consisting of a 3-channel LAN-XI data acquisition module (type 3052), and a LAN-XI battery module (type 2831), was used for field

measurements. The data acquisition module was connected to a military-grade notebook (DELL Latitude 5424) via a shielded F-UTP / STP network cable. The transducers used to measure noise were three B&K (type 4944-A), 1 / 4-in pre-polarized pressure field microphones (with a CCLD preamplifier), with a nominal sensitivity of 0.9 mV / Pa and e ff ective frequency response over a range of 16 Hz to 70 kHz ( ± dB). The microphones were connected to each of the three inputs of the data acquisition module using the cables supplied with the system. A 20 cm diameter expanded polystyrene sphere simulating a human head to provide a hypothetical receiver with a microphone on each side corresponding to the left and right ear positions. The microphones were installed using the adapters supplied with the equipment attached with Velcro to the sphere. The sphere was then mounted on a tripod to regulate the receiver’s height, which was set at 1.5 m above the ground. The sphere was placed in a position representative of the operator most exposed to the noise source. The third microphone, denoted as muzzle in the system, was located close to the noise source and mounted on a tripod using the accessories supplied with the equipment. A noise dosimeter (B&K type 4448) was also attached to the sphere, and the recorded data was post-processed with its software. Before each set of measurements, the system was calibrated using a sound level calibrator, class 1 (B&K type 4231), with an adapter for 1 / 4-in microphones. The calibration procedure is automated by the system. For later postprocessing and analysis, 198 usable measurements were recorded and saved as audio files (WAV). Impulsive noise metrics were obtained directly from the B&K 7963 impulsive noise evaluation system. Figure 1 shows a photograph of one of the measurements.

Figure 1: Photograph of an in-situ measurement using the impulsive noise evaluation system.

3. RESULTS

This section presents the field measurements carried out at the four industrial sites. At the four sites, highly variable values of di ff erent activities that involve impulsive noise were measured. The maximum peak level value measured was 149.3 dB at site 3, which corresponded to the operation of a Cheetah tool that generated impulsive noise due to the rapid release of pressurized gas. The values obtained at each site were very heterogeneous, so their median values are shown in Table 1. We can observe that the highest peak level values were measured at sites 2 and 3, although

site 3 reported SEL values approximately 5 dB lower than site 2. The lowest impulsive noise values were measured at site 1.

Table 1: Median value of some of the impulse noise metrics measured at each industrial site.

Site L pk , dB A-duration, ms B-duration, ms SEL, dB SPL, dB

1 126.5 0.1842 357.089 105.8 110.7

2 138.6 0.3429 424.087 118.5 122.4

3 138.6 0.0655 208.279 113.8 121.0

4 132.5 0.1978 58.8712 107.5 119.7

Figure 2 shows the results of a histogram of the peak level and sound exposure level (SEL) values as calculated by the system. We can see that a significant number of peak level values are in the vicinity of 140 dB (maximum allowed noise level in some legislations [15]), while most of the SEL values are between 105 to 120 dB. In fact, it is noted that about 15% of the measurements carried out in the four industrial sites reported peak levels greater than 140 dB. These values represent inadequate levels for workers and require the mandatory use of hearing protection equipment.

70

60

SEL Lpeak

50

40

30

20

10

0

40 60 80 100 120 140 dB (20 Pa)

Figure 2: Histogram of the peak sound pressure level and Sound Exposure Level (SEL) obtained from all the measurements.

The relationship between the measured peak level and SEL values at all sites can be seen in Figure 3. The results show high linearity, particularly for peak level values below 80 dB. In this case, we could indicate that the SEL values could be approximated from the peak levels by the equation in the figure. Figure 4 shows a plot between the metrics A-duration and B-duration values, calculated from the time histories of the recorded signals. We can observe that very few values of the A-duration are similar to those of the B-duration, which is associated with the fact that the impulsive noises were measured in rather reverberant environments, which is typical of these industrial sites. In fact, we can observe a big cluster of B-duration results concentrated around 450 ms.

150

L pk = 0.97*SEL + 26 dB

130

110

L peak , dB

90

70

50

20 30 40 50 60 70 80 90 100 110 120 130 140 SEL, dB

Figure 3: Plot showing the relationship between the peak sound pressure levels and the Sound Exposure Levels (SEL) measured at the four sites.

1

0.8

A-duration, ms

0.6

0.4

0.2

0

0 100 200 300 400 500 B-duration, ms

Figure 4: Plot showing the relationship between the values of A-duration and B-duration metrics calculated for each measurement.

4. CONCLUSIONS

This work showed that the noise levels produced by impact noise at industrial sites could reach excessively high values. Some of the measured noises exhibited peak levels equivalent to those measured with firearms. It is noted that about 15% of the measurements carried out in the four industrial sites reported peak levels greater than 140 dB. Several national legislations do not allow levels higher than these values [15]. This fact means that noise control actions should be undertaken at these industrial sites to comply with the legislation. Another critical issue is the workers’ proper use of hearing protection

devices. Given the high peak values measured in the industry and the consequent potential hearing hazards, adequate monitoring and supervision of hearing protection use should be a constant concern of employers. Since this is still a work in progress, it is planned to apply the AHAAH model [6,7] to assess the risk of exposure to impulsive noise using the results measured at industrial sites and carry out additional measurements to build a more extensive database of impulsive industrial noise.

ACKNOWLEDGEMENTS

This work was selected in the Call for Research and Innovation Projects in Prevention of Occupational Accidents and Diseases (2020) of the Superintendence of Social Security (Chile). It was financed by the Asociación Chilena de Seguridad with resources from the Social Security of Law No. 16,744 on Occupational Accidents and Occupational Diseases.

REFERENCES

[1] Arenas JP. Impulse Noise: A real threat for workers and a challenge for acousticians. Int. J. Acoust. Vib. 26:272-273, 2021. [2] Henderson D, Hamernik RP. Auditory hazards of impulse and impact noise. In Handbook of Noise and Vibration Control (Ed. MJ Crocker), John Wiley and Sons, New York, 2007. [3] Starck J, Toppila E, Pyykko I. Impulse noise and risk criteria. Noise and Health 5:63-73, 2003. [4] Berger EH, Royster LH, Royster JD, Driscoll DP, Layne M. The Noise Manual . 5th ed. Fairfax, VA: Am. Ind. Hygiene Assoc., 2000. [5] Mrena R, Savolainen S, Kuokkanen JT, and Ylikoski J. Characteristics of tinnitus induced by acute acoustic trauma: A long-term follow-up. Audiol. NeuroOtol. 7:122-130, 2002. [6] Price GR, Kalb JT. Insights into hazard from intense impulses from a mathematical model of the ear. J. Acoust. Soc. Am . 90:219-227, 1991. [7] Price GR. Validation of the auditory hazard assessment algorithm for the human with impulse noise data. J. Acoust. Soc. Am . 122:2786–2802, 2007. [8] ISO 1999. Acoustics - Determination of occupational noise exposure and estimation of noise- induced hearing impairment. Geneva: International Organization for Standardization, 1990. [9] ANSI S3.44. Determination of occupational noise exposure and estimation of noise-induced hearing impairment, American National Standard ANSI S3.44-1996. Acoustical Society of America, Melville, New York, 1996. [10] NIOSH. Criteria for a recommended standard—Occupational noise exposure (revised criteria 1998). Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health (NIOSH) Pub. No. 98–126, 1998. [11] European Council Directive 86 / 188 / EEC. Protection of workers from risks related to exposure to noise at work. Brussels, Belgium, 1986. [12] MIL-STD-1474E. Design Criteria Standard, Noise Limits. U.S. Department of Defense, Washington, D.C., 2015. [13] MIL-STD-1474D. Design Criteria Standard, Noise Limits. U.S. Department of Defense, Washington, D.C., 1997. [14] ANSI S12.7. Methods for measurement of impulse noise, American National Standard ANSI S12.7-1986. Acoustical Society of America, Melville, New York, 1986. [15] Arenas JP, Suter AH. Comparison of occupational noise legislation in the Americas: An overview and analysis. Noise and Health 16:306-319, 2014.