Use of measurements to characterize firearms noise and comparison of methods for assessing the hazard to hearing and the selection of hearing protectors

Status:

completed 10/2018

Aims:

The German Social Accident Insurance Institution of the Federal Government and for the railway services (UVB) offers a firearms noise module in the form of an Excel program. This module can be used to perform a risk assessment and select hearing protectors in situations involving exposure of persons to firearms noise. The module is based upon the Pfander method. The program was developed by the WTD 91 Technical Center for Weapons and Ammunition of the Bundeswehr (German armed forces) in Meppen. It is used by the German federal and regional police forces and also by companies (such as firing range operators). The UVB would like it to be made available on the IFA website.

For assessment of workplace noise presenting a danger to hearing, reference must be made to the German Ordinance on noise and vibration protection (LärmVibrationsArbSchV). This regulation defines exposure action values and limits for the daily noise exposure level and the C-weighted peak sound pressure level. By contrast, the Pfander method employs the impulse duration (C-duration) and the unweighted (linear) peak sound pressure level as the values characterizing impulsive sound. Based upon these values, the energy content of a sound impulse can be estimated and thus the daily maximum permissible number of shots calculated. The efficacy of a hearing protector is described by the "impulsive sound attenuation value" (Knalldämmwert). This value is not identical to the M value to EN ISO 4869-2 otherwise used for the selection of hearing protectors for use with small arms (HML check).

Further variables are also used for the assessment of the harm to hearing caused by discrete events. These include the L_AImax (cf. VDI 2058 Part 2) and the L_AE. These also cannot be related mathematically to the results of the Pfander method. Before the firearms noise module can be made available on the IFA website, it must first be determined whether the methods employed by it are consistent with the LärmVibrationsArbSchV.

Activities/Methods:

In the first phase, two practical projects were conducted by students in which all relevant values for the characterization of impulses (including by means of the Pfander method) were determined simultaneously for a range of firearms. The measurement arrangement used for this purpose was set up at the IFA and then tested as part of a bachelor thesis in an indoor shooting range in Hangelar belonging to the German federal police force. Testing was performed on a number of common models of firearm serving as examples. For the purposes of comparison, the WTD 91 also conducted measurements in the same indoor firing range using the Pfander method.

In November 2017, the UVB organized a large-scale measurement campaign for new firearms and ammunition used by the German federal and regional security forces. The measurements were performed by the WTD 91 in accordance with the Pfander method, at firing ranges belonging to the federal police force in Ratzeburg. At a closed and an open firing range, the IFA recorded values for comparison with those of the WTD 91 for 14 firearm/ammunition combinations.

The values obtained were used to compare the different methods for assessing the risk to human hearing (Pfander and L_AE methods). For this purpose, the number of shots resulting in a mean noise exposure of 85 dB(A) over eight hours was calculated by each method. The difference in the consideration given to the sound attenuation of the hearing protector was also included in the analysis. A variant of the Pfander method was produced that exploits the correlations of the measured parameters for each shot and thereby reduces the uncertainty of the assessment.

Results:

For measurement and assessment by means of the Pfander method and the permissible number of shots derived from the results, the values obtained by the IFA and the WTD 91 were largely consistent for the majority of firearm/ammunition combinations studied. The scatter between the ten individual shots fired for each combination was generally lower in the values obtained by the WTD 91 than in those obtained by the IFA.

The Pfander method has a large inherent uncertainty of measurement. A critical variable is the impulse duration (C-duration), which is influenced heavily by reflection, particularly in indoor firing ranges. The IFA has produced a modified version of the Pfander method that leads to lower uncertainty in the number of shots (based upon the "sound value/S value" per measured shot). The lowest uncertainties overall were however produced by the L_AE method normally employed for occupational safety and health purposes.

A further difference between the Pfander and L_AE methods lies in the consideration given to the hearing protector. The Pfander method employs an impulsive sound attenuation value. This value is derived from the sound attenuation values at 500, 1,000 and 2,000 Hz. The L_AE method employs the "M value" (sound attenuation value for medium-frequency sounds). The differences between these values give rise to clear differences in the resulting permissible numbers of shots. With a confidence level of 50% (i.e. the declared sound attenuation value is attained or exceeded with only 50% of all users), the impulsive sound attenuation value fails to satisfy the standard for values otherwise used in occupational safety and health, which have a confidence level of 84%. The sound attenuation values used by the Pfander method are thus excessively high, and the method thereby yields higher permissible numbers of shots. If the same sound attenuation value (M value used in the L_AE method) is employed for calculation in both methods, the L_AE method permits higher numbers of shots.

The L_AE method satisfies the occupational safety and health criteria for assessment of noise harmful to hearing. This method is already used in a number of countries, particularly by the armed forces.

A rapid changeover from the Pfander method to the L_AE method is not possible, since L_AE values for the numerous firearm/ammunition combinations are not available in the firearms module of the UVB (apart from the measurements performed by the IFA in the course of this project).

In addition, no values for the C-weighted peak sound pressure level are available for the Pfander method. The LärmVibrationsArbSchV also requires consideration of this value. The available Z-weighted (linear) peak sound pressure levels permit an upper estimate, however.

In summary, it can be concluded that in its present form, the firearm noise module cannot be made available in the form of IFA software. A modification of the module's assessment method that could be implemented relatively easily would be replacement of the impulsive sound attenuation value with the M value. The UVB would however first have to determine whether the reduction in the number of shots resulting from this change (by a factor of approximately 2) is consistent with the exposures arising in practice. With this adjustment, the module would provide an upper limit to exposure. Should the raw data of the measurements (pressure/time characteristics) from 2017 and possibly also earlier measurements still be available at the WTD 91, it would in principle be possible to calculate the L_AE and L_Cpeak values retrospectively from them. A method for interpreting the raw data would however have to be produced for this purpose. In addition, it is not clear at this point whether the raw data of the WTD 91 still exist and if so to what extent they could be made available to the IFA.

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