Impulse noise is particularly dangerous for hearing due to its fast-changing nature. A single exposure to noise produced from an artillery shot can lead to a temporary hearing loss, while repeated exposure to noise associated with gunfire can result in permanent noise-induced hearing loss [1
]. Auditory dysfunction after the blast results from the loss of outer hair cells and decreased spiral ganglion neurons and afferent nerve synapses [2
]. Studies also showed that exposure to impulse noise caused a loss about 30% of inner hair cell synaptic ribbons in rat cochleae and the loss of about 10% of outer hair cells [3
]. The occurrence of impulse noise is very often associated with the inability to apply engineering measures or administrative methods to reduce noise, which is especially true for people who are inside shooting ranges, where the sound source is in close proximity to the people. In contrast to sources of impulse noise in industry, there is no possibility to replace people with devices on a shooting range. The only known technical method of reducing the noise produced by firearms is the use of firearm suppressors [4
]. It has been found that the effectiveness of suppressors can significantly exceed the noise reduction of hearing protectors [5
]. However, this effectiveness, at a level of 20–28 dB, was also determined as not always sufficient [4
]. In one of the reported studies, it was found that, despite the fact that suppressors significantly lower the peak sound pressure level (SPL) produced during shots—e.g., from AR-15 rifles—in most cases, its value exceeds 140 dB [6
]. Due to the fact that the values of parameters of noise generated by shots most often exceed those that may be harmful to hearing, it becomes necessary to apply the latter possible solution, namely hearing protectors. The need to use hearing protectors is highlighted by the negative effects of the lack of such protection during the exposure of military personnel to noise from rifle shots [7
]. The average threshold shift of soldiers exposed to noise produced by rifle shots was over 21 dB [7
]. A recommendation for the use of hearing protectors was also formulated in light of conclusions from studies carried out among military personnel which suggested that even exposure to noise levels lower than the exposure limit value may lead to acoustic trauma [8
]. Recommendations were also made that firearm users should always wear hearing protectors when shooting or hunting, since the use of firearm suppressors may not be sufficiently effective [4
]. Re-educating people about the importance of proper hearing protection was also a result of research which found, among other things, that the risk of high-frequency hearing loss, e.g., in men involved in hunting, increased by 7% for every 5 years of participation in this type of activities [9
The need for communication in the presence of noise, in terms of the use of hearing protectors, is becoming increasingly common in many workplaces [10
]. For noise with an SPL that changes over time, it is advisable to use level-dependent hearing protectors. The level-dependent function is carried out by means of electronic circuits. In moments of relative silence, this function supports verbal communication and the possible hearing of other non-speech sounds. This does not require the removal of hearing protectors, which is beneficial on the one hand, and protects against randomly occurring sounds on the other. This functionality is particularly valuable at shooting ranges, because a person who wears hearing protectors at the shooting range is not exposed to the accidental acoustic impulse created by firing a firearm.
Regardless of the aforementioned functionality of level-dependent hearing protectors in terms of support in the perception of ambient sounds, these protectors should limit exposure to noise in such a way that the hearing of their users is safe. Therefore, it is necessary to properly select hearing protectors on the basis of the noise parameters present in the location of the user of the hearing protectors. The selection of hearing protectors can be carried out using computational methods. It is then necessary to determine both the noise parameters present in the workplace and information about the sound attenuation of the hearing protector in question. The selection of hearing protectors is not often discussed and, for example, is limited to presenting the basic principles of such a selection or discussing the main problems involved [10
]. It is also mentioned that one of the problems in the assessment of the effectiveness of using hearing protectors is when noise is impulsive [11
It is not difficult to obtain information about the sound attenuation of hearing protectors used in computational selection methods for continuous noise. This attenuation is one of the parameters measured during the conformity assessment process for the EU regulation [12
], and its values are included in the user instructions for the hearing protectors. Some difficulties are related to the assessment of impulse noise parameters. The selection of hearing protectors requires precise data on the noise parameters, and the information that the exposure limit values of these parameters have been exceeded is inadequate. Due to the high SPL values of impulse noise exceeding the upper limits of the measurement range of standard sound level meters, measurements of noise parameters must be carried out with the use of appropriate equipment. The experience of research personnel is also crucial to properly determine representative cycles of tasks when determining noise parameters, as required by the test methodology [13
The objective of this study was to answer the question of whether it is possible to properly protect the hearing of a shooting instructor at an indoor shooting range, using level-dependent hearing protectors. The study includes a large number (10) of level-dependent hearing protectors. In addition, the assessment of impulse noise reduction was carried out not in the presence of laboratory-produced acoustic impulses, as in the method described in ANSI/ASA S12.42-2010 (The American National Standards Institute, Inc./The Acoustical Society of America) [14
], but instead measurements of noise parameters were carried out in a real acoustic environment at an indoor shooting range. During the tests, a full program of shooting was carried out using various types of weapons and ammunition. The only known example of a study referring to the possibility of reducing the exposure to noise at an instructor’s location is a study which concerned the use of suppressors rather than hearing protectors [4
The research conducted indicated that the noise produced in individual training situations at the shooting range was different in terms of the content of acoustic energy components in individual frequency bands (Figure 3
). Taking this fact into account, and taking into consideration that the characteristics of the sound attenuation of hearing protectors as a function of frequency are uneven (Table 1
), it should be stated that the assessment of the reduction of the noise produced at a shooting range by individual hearing protectors requires appropriate calculations in each situation considered.
The measurements of the impulse noise parameters generated in the places where the shooting instructor was located indicated relatively high SPL values produced by the firearms, which is in line with the results of other studies on shots from firearms. Measurements conducted in this study showed that the instructor, when standing at a distance of 1.1 m behind the shooters, was exposed to acoustic impulses whose LCpeak
, depending on the variant of weapon/ammunition combination and the shooting scenario, assumed values ranging from 145.9 dB to 158.1 dB. There are, however, no data that could be directly compared with the data obtained in this study at the indoor shooting range for the instructor’s locations. Published results usually characterize noise close to the shooter’s ear (although not always), or there is no precise information about the measurement location and C-weighted peak SPL or unweighted peak SPL (Lpeak
) being considered; moreover, measurements are usually carried out on military fields. For example, in [1
], there is general information that the peak SPL produced by small-caliber rifles, shotguns and large-caliber handguns ranged from 132 dB up to more than 172 dB for high-powered firearms. In the study in which the measurements were carried out in field conditions, a few meters away from the shooter, the Lpeak
associated with C7 rifle shots was 148.3 dB, while at the distance of 30 cm from the shooter, it was 154.7 dB [23
]. For shots from a 9 mm caliber pistol, values of 148.4 dB and 155.6 dB were obtained, respectively [23
]. Subsequent data [24
] indicated that for Lpeak
determined on the basis of waveforms associated with a 5.56 mm caliber C7 rifle shot, recorded at a distance of 4 m from the muzzle at 90° and 6.4 m at 39°, in both cases, the value was approximately 157 dB. In one of the studies, the Lpeak
values associated with AR-15 rifle shots were 168 dB at the distance of 1.8 m, 150 dB at 4.3 m, and 132 dB at 25.7 m [25
]. Despite the mentioned differences between the aforementioned studies and this study regarding the measurement conditions, the common point is that large and comparable SPLs are measured in the places where the shooting instructor is located. The exposure of the shooting instructor to the noise produced at the shooting range, which, according to the previously mentioned criteria should be classified as dangerous for hearing, requires the use of appropriately selected hearing protectors. Another conclusion is that in order to correctly determine the values of the noise parameters to obtain the necessary data for the selection of hearing protectors, the measurements could not be carried out using standard sound level meters. The upper measuring range of such devices is usually limited to approximately 145 dB. In response to the problem of measuring high SPL values related to impulse noise, hardware solutions were developed [14
In one of the studies, where strategies for possible ways to reduce the negative impact of impulse noise on shooters’ ears were formulated, among others, the use of outdoor or acoustic-treated indoor shooting ranges was recommended [27
]. The results obtained on the shooting range included in the present study confirmed that when applying acoustic treatment, acoustic conditions similar to the free field conditions can be obtained. Although the LAeq
values shown in Figure 4
and Figure 5
do not fit perfectly in straight lines with the theoretical slope for free field conditions, the deviations of results from the theoretical lines are relatively small. This proves that the use of acoustic treatment on the shooting range effectively limited the impact of sound reflections. Therefore, it can be assumed that the effect of the distance of the shooting instructor from the sound source on the degree of hearing hazard is similar to the situation in free field conditions.
In the case of impulse noise, it is not possible to characterize the reduction of this kind of noise by a specific hearing protector using one value only. This is due to the fact that the parameters of acoustic impulses produced by different sources differ from one another [22
], and the reduction of the impulse noise strongly depends on the source of this noise [25
]. According to the methodology for hearing protector assessment referring to the LCpeak
parameter, in this paper, for each hearing protector, two attenuation values were distinguished: one for shots from pistols and a submachine gun, and the second for shots from smooth-bore shotguns (shotgun 1 and shotgun 2). The different values of noise parameters produced by each weapon/ammunition combination and the different values of the acoustic parameters of individual hearing protectors resulted in different noise values determined under the hearing protectors. For example, the highest
values (Figure 8
) under the HP1 hearing protector were 122.3 dB (pistols), 120.4 dB (submachine gun), 138.1 dB (shotgun 1) and 134.5 dB (shotgun 2). The range of these values is therefore 17.7 dB. Similarly, the range of the
values among the four weapon/ammunition combinations, depending on the hearing protector, ranges from 12.7 to 18.7 dB for all other hearing protectors. At the same time, it should be emphasized that with a particular hearing protector, the
parameter values are much less varied (from 1 dB to 3 dB—Figure 9
) between different weapon/ammunition combinations than they are in the case of the
parameter. Therefore, for impulse noise, the parameter reflecting the energy properties of noise (
) differentiates individual noise sources to a much lesser extent than the parameter referring to the instantaneous values characterizing impulses (
). It is therefore confirmed that for exposure to impulse noise, the assessment of hearing protectors is insufficient due to the value of the
parameter, as is the case for continuous noise. It is necessary to take into account the
parameter, which of course is primarily due to the fact that the LCpeak
value of noise is exceeded.
There are no published data on the reduction of impulse noise by individual hearing protectors, which could have been directly compared with the results presented in this paper. For example, data are available for the insertion loss of impulses generated during shots from an AR-15 rifle with a Lpeak
of 150 dB [25
]. These data were measured using an acoustical test fixture for earmuffs with similar properties to the hearing protector designated in this paper as HP2. The earmuff from the aforementioned study [25
] reduced the Lpeak
to 108.8 dB. In similar measurement conditions in the next study, data were obtained that indicated that another earmuff reduced Lpeak
to 118 dB [28
]. In this study, in the most similar situations to the two mentioned studies, the average LCpeak
value with the HP2 protector at distances from the shooter equal to 1.1 m and 2.8 m was 113.3 dB for submachine gun shots and 125.0 dB for shotgun 2 shots. Examples of published data can be supplemented with the result of 132 dB concerning the Lpeak
parameter measured using an acoustical test fixture with an earmuff, in the presence of an impulse produced during a shot from a 12.7 mm machine gun [29
]. The impulses produced by this source were characterized by a Lpeak
value of 152 dB. The studies in the referenced papers [25
] and this paper differ in the type of weapon, noise parameter analyzed, hearing protector model, distance from the shooter and measurement method used; however, independently obtained values of noise parameters potentially reaching the user of the hearing protector exceed 100 dB and are below the exposure limit values. It can be observed that in all the studies discussed, the reduction of impulse noise by hearing protectors was measured using a head and torso simulator; i.e., an acoustical test fixture. This is different from this particular study, in which a calculation method was employed in which sound attenuation data of hearing protectors was used, which are determined with the participation of subjects.
Since a hearing protector can only be considered suitable for protecting hearing against noise when its use obtains correspondingly reduced parameter
values at the same time. On the basis of the data in Table 2
, it can be stated that, according to the adopted criteria for the assessment of hearing protectors,
= 80 dB and
= 135 dB, an appropriate hearing protection for the shooting instructor will be possible when using hearing protectors 2, 4, 8 and 9. Two of the earmuffs (hearing protectors 2 and 4) and earplugs in two variants of eartips (hearing protectors 8 and 9) were found to be appropriate.
At the same time, it can be noted that by changing the criterion for
to less severe, i.e.,
= 140 dB, as used in many countries and defined in the Directive [18
], an adequate protection of hearing would also be guaranteed by hearing protectors 1 and 7.
The results obtained indicated that the parameter associated with the instantaneous value of the signal, i.e., LCpeak, is crucial in the process of assessing hearing protectors in terms of their ability to reduce the impulse noise produced on a shooting range. In both situations in which the parameter value was exceeded, the criterion value referring to the parameter was also exceeded. At the same time, in four consecutive situations of exceeding the criterion value of the parameter, these situations did not involve exceeding the parameter. It is therefore important that, for impulse noise, the selection of hearing protectors cannot be limited to taking into account the parameter, as in the case of continuous noise, where there is no problem in exceeding the exposure limit values of LCpeak.
At the same time, it should be noted that hearing protection requires not only properly selected hearing protectors, but that they must be used and set up correctly, and other sources of noise exposure, often unrelated to professional work, should be avoided. For example, practical instances of the improper use of earplugs were observed, and the reason behind this was a lack of training or that it was deliberately done to better hear messages spoken by the Range Safety Officer [23
]. In turn, in one of the papers investigating the hearing condition of 20 policemen after 10 years of service, a deterioration of hearing was found despite the use of double hearing protection [30
The results of the study indicate that impulse noise produced on the shooting range should be assessed as dangerous for hearing. The exposition of the shooting instructor to this noise requires the use of appropriately selected hearing protectors. The results of the analysis of noise parameters measured at different distances of the shooting instructor from the shooters confirmed that increasing this distance, if possible, is a good method for reducing the exposure of the instructor to noise. It also appeared that with a certain number of shooters who are to be trained, their division into subgroups consisting of three or six people at the same time does not significantly affect the shooting instructor’s exposure to noise.
The results of the assessment of level-dependent hearing protectors demonstrated that it is possible to adequately protect the hearing of a shooting instructor who is at an indoor shooting range when using protectors of this type. It is possible to choose hearing protectors, both earmuffs and earplugs, that will sufficiently reduce the impulse noise to which the instructor is exposed.
It appears that, taking into account the program of exercises at the shooting range at which shooting was carried out with pistols, a submachine gun and smooth-bore shotguns, the assessment of hearing protectors with a more restrictive criterion = 80 dB, = 135 dB) showed that only four out of the 10 level-dependent hearing protectors included in the analysis sufficiently reduced the noise. Assuming a less restrictive criterion regarding the C-weighted peak SPL, i.e., = 140 dB, six out of 10 hearing protectors would be appropriate. Therefore, regardless of which criterion for the assessment of earmuffs is used, only some of the hearing protectors will be a potentially suitable means of protecting the shooting instructor’s hearing, and the correct selection of hearing protectors is necessary. However, in the case of impulse noise, this selection must be based not only on the standardized selection methods related to the continuous noise but must also take into account the parameter associated with the instantaneous value of the signal level; i.e., the C-weighted peak SPL.