Impulse noise measurement in view of noise hazard assessment and use of hearing protectors

Abstract

Experience shows the occurrence of situations when the measurements of impulse noise parameters are made with measurement equipment unsuitable for such conditions. The results of using such equipment were compared with the results of using equipment with a sufficiently large upper limit of the measurement range. The analysis was carried out on the example of noise generated during shots from a Mossberg smooth-bore shotgun and AKM rifle, as well as produced in the forge. The use of the unsuitable equipment allowed to indicate the exceeding of the exposure limit value of the peak value of the signal (L_Cpeak), but this is not always possible when determining the energy properties of the signal (L_EX,8h). While the inadequate properties of the measurement equipment will generally not prevent the conclusion that noise in a particular workplace is hazardous to hearing, the results of measurements cannot be used to select hearing protectors.

KEYWORDS:

• impulse noise
• noise measurements
• exposure to noise
• earmuffs
• hearing protector selection

1. Introduction

In the case of exposure to noise, it is important to determine its parameters in order to carry out an assessment of noise hazards. In many cases, it is also necessary to obtain the data necessary for the selection of hearing protectors. The problem of obtaining reliable results may occur when measuring the impulse noise parameters. The value of impulse noise parameters often exceeds the dynamic range upper limit of commonly used standard sound level meters.

An employer is obliged to perform the assessment of noise hazards. The results of such an assessment may indicate the need for that employer to take certain measures to reduce exposure to noise. The assessment is carried out on the basis of specific criteria, i.e., limit values for noise parameters. These issues are described both in European legislation, i.e., Directive 2003/10/EC [1], and in Polish national regulations [2]. While the principles of hearing conservation recommend noise reduction at the source as the first step, engineering and administrative controls are the next steps when the former is not possible [3,4]. In the case of impulse noise produced during the use of firearms, firearm suppressors [5] may be such engineering noise control measures. The effectiveness of noise reduction in this way is not always sufficient [6]. On the other hand, the application of technical means limiting exposure to impulse noise in industry, according to our earlier observations, is not always possible [7]. In the absence of their sufficient effectiveness or applicability, it is essential to use hearing protectors. These can be both passive hearing protectors and hearing protectors equipped with electronic systems to support the perception of relevant ambient sounds. Reviews of the various types of hearing protectors have been published by others [8,9]. Hearing protectors should be properly selected [1,10] by assessing their effectiveness in reducing noise. The proper assessment of the effectiveness of noise reduction by hearing protectors requires specific selection methods. Selection methods for hearing protectors are, in turn, based on knowledge of the specific parameters characterizing noise in places where hearing protectors are to be used [11].

There is practical evidence that measurements of noise parameters, under conditions of high sound pressure level values, are performed with measurement equipment which is not suitable for such conditions, i.e., standard sound level meters. When the noise signal saturates the upper limit of a sound level meter, an overload is indicated and the displayed sound pressure level is incorrect. However, in the case of noise exposure assessment, despite the incorrect result, information is obtained that the noise exposure limit value is exceeded, as required by Directive 2003/10/EC [1]. The situation is worse when there is a need to select hearing protectors. The use of measurement data obtained in an inadequate manner for obvious reasons must result in incorrectly determined values for the parameters of noise present under hearing protectors. As a result, this may lead to erroneous assessment of hearing protectors as to whether they can sufficiently reduce exposure to noise.

The measurement of high-level impulse noise such as gunfire can only be accurately conducted with the use of specialized equipment [12,13]. The purpose of this article is to discuss how incorrect measurement of impulse noise parameters with the use of sound level meters affects the noise hazard assessment and the selection of hearing protectors.

2. Noise parameters and their measurements

2.1. Parameters applied for the assessment of noise hazard

The assessment of noise hazard is carried out based on the comparison of the values of selected parameters with the relevant criterion values, i.e., exposure limit values. Directive 2003/10/EC [1] contains exposure limit values relating to two noise parameters: C-weighted peak sound pressure level (L_Cpeak) and A-weighted noise exposure level normalized to an 8-h working day, otherwise known as the daily noise exposure level (L_EX,8h). The maximum values for these parameters have been set as 140 and 87 dB, respectively. According to the Polish regulations [2], the exposure limit values are L_Cpeak = 135 dB and L_EX,8h = 85 dB. In addition, the Polish regulations specify a criterion value for the A-weighted maximum sound pressure level: L_Amax = 115 dB. However, in the case of exposure to impulse noise, the assessment using this parameter practically duplicates the assessment made with L_Cpeak. Due to the essential duality of the assessment of the impulse noise instantaneous values using the L_Amax and L_Cpeak parameters, the L_Cpeak parameter will be taken into account in further analysis. For the parameter L_EX,8h, its value is based on the exposure time and the A-weighted equivalent sound pressure level (L_Aeq).

The L_Amax parameter – although, as mentioned, used to assess exposure to noise in Poland – is not used in the process of selecting hearing protectors. For the instantaneous value of the signal, the impulse noise selection methodology, i.e., ‘Method for assessing the sound attenuation of a hearing protector for impulsive noise’ included in the informative Annex B of the relevant Standard No. EN 458:2016 [11], includes the use of the L_Cpeak parameter. Due to the fact that the article deals with two aspects related to the measurement of noise parameters, i.e., the assessment of exposure to impulse noise and the selection of hearing protectors, the conducted analysis was based on the two mentioned parameters (L_EX,8h, L_Cpeak), which are taken into account in the noise exposure assessment in Directive 2003/10/EC [1] and at the same time are considered in the selection of hearing protectors for impulse noise [11]. The analysis was supplemented with other parameters used in the selection of hearing protectors. Details in both of these areas, i.e., assessment of noise hazard and selection of hearing protectors, will be described later in the article.

2.2. Problems with the measurement of impulse noise parameters when assessing noise hazard

Standard No. ISO 9612:2009 [14] for the determination of occupational noise exposure includes the requirement that the sound level meters used shall meet the requirements for IEC 61672-1 [15], class 1 or class 2 instrumentation. In accordance with the provisions of Directive 2003/10/EC [1], the measurement equipment used for the assessment of noise hazard should make it possible to measure the noise parameters taken into account in the assessment and determine whether, among others, the exposure limit values for these parameters have been exceeded. When measuring the parameters of impulse noise, it may happen that the sound pressure level characterizing this noise will exceed the dynamic range upper limit of the sound level meter. Table 1 presents examples of the limits of the possibilities of measuring noise parameters by type of approved sound level meters. These data, which are included in the sound level meter user manuals, include L_Cpeak, which, as a parameter related to the instantaneous value of the signal, is particularly applicable to characterize the properties of impulse noise. The upper limit of the measurement range is also determined for L_Aeq, which is also used in the assessment of noise hazard.

Table 1. Examples of class 1 sound level meters (according to IEC 61672-1 [15]) with type approval often used in Poland and their upper limits of measuring ranges.

Manufacturer and model of sound level meter with a given microphone	Upper limit of the LCpeak measurement range (dB)	Upper limit of the LA/LAeq measurement range (dB)
Brüel & Kjær 2250 with 4189 microphone	142.7	139.7
Norsonic Nor 140 with Nor 1225 microphone	140.0	137.0
Sonopan DSA 50 with WK-21 microphone	138.0	135.0
Svantek SVAN 979 with GRAS 40AE microphone	140.0	137.0

Note: Information based on product data/instruction manuals. L_A = A-weighted sound pressure level; L_Aeq = A-weighted equivalent sound pressure level; L_Cpeak = C-weighted peak sound pressure level.

Noise with high peak sound pressure levels can occur in both industrial and recreational settings. The peak sound pressure level produced by high-powered firearms can even exceed 172 dB [16]. An example of an industrial noise source that generates noise exceeding the exposure limit value for L_Cpeak is a forging hammer. The parameters of noise present at work in the forge were included by the team of authors in one of their previous studies [7], where the possibility of reducing this noise by using hearing protectors was analyzed. In another study, the team of authors described issues related to the selection of hearing protectors for shooting instructors [17]. An inherent element of these two works were the measurements of noise parameters in the places where people use the selected hearing protectors. The next study presents the results of the measurements of noise parameters occurring during military field exercises [18]. Table 2 presents some sample results of noise parameter measurements taken from the three aforementioned works. The results shown in this table were obtained using measurement equipment whose dynamic range upper limit does not limit the ability to characterize the noise produced by the sources considered and is 174 or 184 dB. A GRAS 67SB transducer (GRAS Sound & Vibration A/S, Denmark) or Brüel & Kjær 4941 microphone (Brüel & Kjær, Denmark) was used. A signal from the transducer/microphone was supplied to a Brüel & Kjær 3052-A-030 input module or Brüel & Kjær PULSE 3560 C measurement unit. The waveforms of this signal were recorded, which enabled further analysis with the use of Brüel & Kjær Pulse Reflex or Brüel & Kjær LabShop software, during which noise parameters were determined.

Table 2. Exemplary values of parameters of impulse noise generated in the forge and during firing of firearms.

Impulse noise source	LCpeak (dB)
Forgers’ workplace	148.9
PM-98 Glauberyt submachine gun at a distance of 1.1 m	151.4
Glock 17/Walther P99 pistol at a distance of 1.1 m	153.3
Mossberg 590 smooth-bore shotgun at a distance of 1.1 m	158.1
ZU-23-2K anti-aircraft gun (at crew position)	158.3
Anti-tank guided missile (at operator position)	160.7

Note: Values based on other studies of the authors’ team [7,17,18]. L_Cpeak = C-weighted peak sound pressure level.

In Table 2, impulse noise is characterized by L_Cpeak, which, as already mentioned, is a parameter used to assess noise hazard. Data available in the literature most often refer to another noise parameter, i.e., (unweighted) peak sound pressure level, and other types of weapons. Moreover, they were obtained at other distances from the noise source. Despite all these differences, the available data are comparable or even exceed the values presented in Table 2. For example, in the case of C7 rifle shots, at a distance of 30 cm from the shooter, L_peak was 154.7 dB [19]. In the case of AR-15 rifle shots, at a distance of 4 m it was even 168 dB [20]. The L_Cpeak values presented in Table 2 exceed the upper limit of the L_Cpeak measurement range presented in Table 1 for each of the four sound level meters. Thus, measurement of the parameters of noise generated by the sources presented in Table 2 with a standard sound level meter will result in a message about the measurement range distortion and the displayed indication will be erroneous. This will not give a reliable value of L_Cpeak. Information on exceeding the measuring range of the instrument, for obvious reasons relating to the correct use of the measurement equipment, makes it necessary to reject the results obtained from the noise parameters measured in this case. Using each of the four sound level meters presented in Table 1, it is only possible to determine that the L_Cpeak limit value has been exceeded. This means that the aforementioned requirement in Directive 2003/10/EC [1], which states that the measurement equipment must be capable of detecting the fact that the permissible noise parameters have been exceeded, is met. It should be kept in mind that such measurement data, i.e., erroneous measurement results, will make it impossible to determine correctly the indicators used to show how far the limit value for a certain noise parameter is exceeded.

2.3. Noise parameters for the selection of hearing protectors

The problem of exposure to noise in the workplace is often associated with the need for hearing protectors. As mentioned in Section 1, hearing protectors should be properly selected [1,10]. Depending on the calculation method of selection of hearing protectors [11], it is necessary to know the specific noise parameters to use. When selecting hearing protectors to be used in the presence of impulse noise, it is necessary to characterize the instantaneous values of acoustic impulses using the L_Cpeak parameter. The selection must also involve determining L_Aeq under a hearing protector. The use of the ‘octave band method’ for this purpose will require measurement of the sound pressure level in octave bands (L_p). When using the ‘HML method’ for selecting hearing protectors, it is necessary to measure L_Aeq as well as the C-weighted equivalent sound pressure level (L_Ceq). The name of the HML method is related to the fact that the calculation uses the H, M, and L values representing the attenuation of hearing protectors: H = high-frequency attenuation value, M = medium-frequency attenuation value, L = low-frequency attenuation value [21]. In the case of the ‘SNR method’, it is necessary to obtain a result of the L_Ceq measurement. The name of the SNR method is related to use of the SNR value in the calculations, which expresses the attenuation of the hearing protector with only one number: single number rating [21]. As the octave band method is the most accurate of these methods, it is recommended to use it first [11]. In this situation, it is necessary to use measurement equipment with an option of analysis in octave bands.

As already mentioned, in the case of a noise hazard assessment one can accept the lack of correct noise values in certain situations when the technical capabilities of the sound level meter are not sufficient. On the other hand, with regard to the data required for the selection of hearing protectors, information on the mere fact of exceeding the noise limit values is no longer sufficient. The use of calculation methods for the selection of hearing protectors requires the availability of correctly determined noise parameter values [11]. Thus, measurement equipment with an insufficient dynamic range upper limit will not provide the data necessary for the correct selection of hearing protectors.

3. Methods

3.1. Method for illustrating the consequences of the use of standard sound level meters for impulse noise

As already mentioned in Section 1, experience shows that measurements of noise parameters, under conditions of high sound pressure level values, are carried out with standard sound level meters, with too low a value of the dynamic range upper limit. The further part of this study describes the effects of using measurement equipment not adapted to measure the parameters of characterized noise.

For this purpose, a simulation was conducted to determine the values of noise parameters by means of a sound level meter, using previously recorded noise waveform. The diagram illustrating the procedure adopted to show the effects of using sound level meters in the case of impulse noise is shown in Figure 1.

Figure 1. Procedure adopted to show the effects of using sound level meters in the case of impulse noise parameter measurements. Δ = difference; L_Aeq = A-weighted equivalent sound pressure level; L_Cpeak = C-weighted peak sound pressure level.

The noise waveforms occurring when firing the Mossberg smooth-bore shotgun and the AKM rifle and during metal processing with a forging hammer were recorded using measurement equipment whose dynamic range upper limit does not limit the possibility of noise characterization, i.e., with a GRAS 67SB transducer connected to a Brüel & Kjær 3052-A-030 input module. Original and modified noise waveforms were analyzed in two situations e.g., when the dynamic range upper limit was 139.7 and 135.0 dB. These were the highest and lowest values of this limit among the data presented in Table 1. The noise waveforms recorded in real conditions have been modified to reflect the effect of a signal trimmed due to the limited measuring range of the sound level meter. Examples of noise waveforms for the recorded signal and signals after modification are shown in Figure 2.

Figure 2. Original and trimmed waveforms of shots from the Mossberg smooth-bore shotgun with different dynamic range upper limit values of the measurement equipment.

The recorded waveforms of the signal (i.e., the original) and the waveforms after modification were used to determine the values of noise parameters, in accordance with the definitions of these parameters and the rules contained in Standard No. ISO 9612:2009 [14]. The original noise waveform was used as a basis to determine reference data. The noise parameters were determined using program procedures designed to be run in MATLAB R2019a version 9.6.0.1174912. The proper performance of the prepared procedures has been demonstrated by their validation with calibrated measurement equipment based on the Brüel & Kjær 3052-A-030 input module and Brüel & Kjær LabShop. The determined values of the noise parameters, in the case of original and modified waveforms, were used to calculate the differences between them in order to show what the consequences of incorrect use of the measurement equipment may be. The obtained values of noise parameters were also used to show the influence of incorrect measurement results on the selection and evaluation of hearing protectors. The parameters determined included L_Cpeak, L_Aeq, L_EX,8h and L_Ceq. Additionally, the results of the determination of L_p were presented.

All signal waveforms included in the analysis were representative in accordance with the principles of noise exposure assessment [14] and corresponded to the principles of selection of hearing protectors [11]. For each of the sources, an exemplary cycle of activities was used, representative for a specific place of residence of the person who performs these activities. The results obtained included an exemplary working day. All of the signals included in the analysis corresponded in terms of their duration to situations that took place in real conditions. At the same time, all activities that should be analyzed in the case of individual places of residence of a person were taken into account. Details on the selection of representative signals for analysis are provided in the cited paper on selection of hearing protectors for use in a shooting range [17]. For example, one full representative cycle of the task includes not only shooting, but also giving instructions, loading weapons and other preparatory activities, unloading weapons, checking their condition, etc. For example, in the case of the Mossberg shotgun, three-shot exercises were included, each of which consisted of several shots and the activities accompanying this shooting. In the case of the AKM rifle, the analysis included exercises consisting of 50 shots.

3.2. Method for illustrating the consequences of the incorrect selection of hearing protectors

In the case of exposure to impulse noise, two parameters of that noise are considered when selecting hearing protectors [11]. It shall be checked whether the use of a hearing protector will cause the C-weighted peak sound pressure level under the hearing protector ($L_{C\text{peak}}^{\mathrm{\prime }}$) to be sufficiently lowered, i.e., below the exposure limit value, which, as mentioned earlier, is 135 dB. At the same time, appropriate conditions must be met for the value of the A-weighted equivalent sound pressure level of noise under a hearing protector ($L_{\text{Aeq}}^{\mathrm{\prime }}$). It is necessary to ensure that the value of 80 dB is not exceeded for this parameter, as demonstrated by the analysis of the data in Standard No. ISO 1999:1990 [22] concerning the expected hearing permanent threshold shift due to noise exposure. In addition, due to the perception of sound signals, hearing protectors should be selected in such a way that the $L_{\text{Aeq}}^{\mathrm{\prime }}$ value is not less than 65 dB, as defined in Standard No. EN 458:2016 [11]. This condition is associated with the effect of overprotection, which can lead to a failure to hear, e.g., a warning signal and, consequently, even an accident. The overprotection phenomenon may be dangerous wherever the reception of sounds that can be treated as useful is important for safety reasons. In the case of using firearms, including, e.g., exercises at the shooting range, it is important to receive commands. In addition to warnings, in the case of the shooting instructor’s work, verbal communication between the instructor and the trained person is also important. Then, too much sound attenuation can affect the difficulty in communication between people.

Once the $L_{\text{Aeq}}^{\mathrm{\prime }}$ value has been determined, the hearing protectors can be assessed for proper hearing protection for their users. This assessment consists of classifying the result of the $L_{\text{Aeq}}^{\mathrm{\prime }}$ calculation into one of three categories: sufficient protection, insufficient protection or overprotection. The protection is sufficient if $L_{\text{Aeq}}^{\mathrm{\prime }}$ is within the range from 65 to 80 dB. Insufficient protection means that $L_{\text{Aeq}}^{\mathrm{\prime }}$ is higher than 80 dB. Overprotection is observed if $L_{\text{Aeq}}^{\mathrm{\prime }}$ is below 65 dB. It is important to note that in the final evaluation of hearing protectors that provide proper hearing protection due to the $L_{\text{Aeq}}^{\mathrm{\prime }}$ value, it is also necessary to consider whether they sufficiently limit the $L_{C\text{peak}}^{\mathrm{\prime }}$ value.

The limited value of the dynamic range upper limit (data presented in Table 1) in the case of measurement of impulse noise parameters with a standard sound level meter will result in the indication of the measured L_Cpeak value reaching 143 dB at most. Using such results to calculate the value of this parameter under hearing protectors ($L_{C\text{peak}}^{\mathrm{\prime }}$) for each hearing protector will lead to a conclusion of sufficient noise reduction. Obviously, in all cases where the actual L_Cpeak value will differ significantly from the indicated value, because the dynamic range upper limit of the sound level meter is exceeded, this conclusion may be false. The further part of the study will show how the potential failure to ignore the information about exceeding the sound level meter’s measuring range can affect the selection of hearing protectors carried out due to the $L_{\text{Aeq}}^{\mathrm{\prime }}$ parameter, i.e., using L_p, a parameter not associated with peak signal level.

The selection covered 15 different hearing protectors with different sound attenuation values represented by SNR values between 21 and 37 dB in the user manuals of these protectors. The SNR has been defined in Standard No. ISO 4869-2:2018 [21] on estimation of the effective A-weighted sound pressure level when hearing protectors are worn. Determination of the $L_{\text{Aeq}}^{\mathrm{\prime }}$ value was conducted using the octave band method. The data on L_p determined in the case of a correctly recorded (original) noise waveform and the data determined assuming that they come from sound level meters with insufficiently high dynamic range upper limit were used.

4. Results

4.1. Consequences of the use of standard sound level meters for impulse noise

The results of determining the values of the L_Cpeak, L_Aeq, L_EX,8h and L_Ceq parameters in the case of noise generated during shots from a Mossberg smooth-bore shotgun and an AKM rifle and during metal processing with a forging hammer are presented in Tables 3–5, respectively. Each table gives the results of the analysis of the original noise waveform, i.e., without exceeding the dynamic range upper limit of 174.0 dB. The results of the analysis carried out in the case of modified noise waveforms with the assumption of limiting the dynamic range upper limit to 139.7 and 135.0 dB have also been included.

Table 3. Results of the analysis of the original and modified noise waveforms associated with shots from the Mossberg smooth-bore shotgun at a distance of 1.1 m from the shooter (shooting trainer’s position).

Waveform	LCpeak (dB)	ΔLCpeak (dB)	LAeq (dB)	LEX,8h (dB)	ΔLAeq (ΔLEX,8h) (dB)	LCeq (dB)	ΔLCeq (dB)
Original, DRUL = 174.0 dB	158.3	–	103.8	90.2	–	105.6	–
Trimmed, DRUL = 139.7 dB	142.2	16.1	100.2	86.6	3.6	102.0	3.6
Trimmed, DRUL = 135.0 dB	137.6	20.7	98.3	84.7	5.5	100.1	5.5

Note: Δ = difference; DRUL = dynamic range upper limit; L_Aeq = A-weighted equivalent sound pressure level; L_Ceq = C-weighted equivalent sound pressure level; L_Cpeak = C-weighted peak sound pressure level; L_EX,8h = A-weighted noise exposure level normalized to an 8-h working day (daily noise exposure level).

Table 4. Results of the analysis of the original and modified noise waveforms associated with shots from the AKM rifle at a distance of 1.1 m from the shooter (shooting trainer’s position).

Waveform	LCpeak (dB)	ΔLCpeak (dB)	LAeq (dB)	LEX,8h (dB)	ΔLAeq (ΔLEX,8h) (dB)	LCeq (dB)	ΔLCeq (dB)
Original, DRUL = 174.0 dB	148.9	–	105.4	85.6	–	107.2	–
Trimmed, DRUL = 139.7 dB	140.4	8.5	102.9	83.1	2.5	105.0	2.2
Trimmed, DRUL = 135.0 dB	135.9	13	100.9	81.1	4.5	103.0	4.2

Note: Δ = difference; DRUL = dynamic range upper limit; L_Aeq = A-weighted equivalent sound pressure level; L_Ceq = C-weighted equivalent sound pressure level; L_Cpeak = C-weighted peak sound pressure level; L_EX,8h = A-weighted noise exposure level normalized to an 8-h working day (daily noise exposure level).

Table 5. Results of the analysis of the original and modified noise waveforms associated with the noise generated during metal processing in the forge.

Waveform	LCpeak (dB)	ΔLCpeak (dB)	LAeq (dB)	LEX,8h (dB)	ΔLAeq ΔLEX,8h (dB)	LCeq (dB)	ΔLCeq (dB)
Original, DRUL = 174.0 dB	148.3	–	109.5	107.5	–	114.1	–
Trimmed, DRUL = 139.7 dB	141.8	6.5	108.2	106.2	1.3	112.3	1.8
Trimmed, DRUL = 135.0 dB	137.6	10.7	106.6	104.6	2.9	110.4	3.7

Note: Δ = difference; DRUL = dynamic range upper limit; L_Aeq = A-weighted equivalent sound pressure level; L_Ceq = C-weighted equivalent sound pressure level; L_Cpeak = C-weighted peak sound pressure level; L_EX,8h = A-weighted noise exposure level normalized to an 8-h working day (daily noise exposure level).

Tables 3–5 also present the values of the differences between the original and modified noise parameters. For the L_Aeq and L_EX,8h parameters, values of these differences will be the same, as the L_EX,8h parameter is determined on the basis of the L_Aeq parameter, after taking into account the exposure time, which is related to 8 h [14].

When determining the value of the L_EX,8h, two other activities with a lower sound pressure level were taken into account, in addition to the activities related to the operation of forging hammer when acoustic impulses are generated (L_Aeq = 109.5 dB). The L_Aeq values measured during two other activities are 84.3 and 45.0 dB and are correct when using a sound level meter. In practice, the impact of these two other activities, apart from the direct operation of the hammer, slightly affects the value of L_EX,8h and this impact does not exceed 0.8 dB.

The rating of noise hazard in the shooting trainer’s workplace indicated not only that the permissible value concerning the signal’s instantaneous values, i.e., the L_Cpeak parameter, was exceeded, but also that the permissible value of the L_EX,8h parameter, reflecting the signal’s energy properties, was too high. For each of the noise parameters, the difference between the value of this parameter determined with a sound level meter with an insufficient value of the dynamic range upper limit and the result obtained with measurement equipment of the appropriate measuring range increases as the dynamic range upper limit of the sound level meter decreases.

The effects of using a sound level meter with the dynamic range upper limit not adjusted to the parameters of impulse noise for L_p are shown in Figure 3. The measurement results relating to noise generated during shots from the Mossberg smooth-bore shotgun are included. Results for the two other noise sources being considered are presented in Figures 4 and 5. The values of the differences in indications determined from the original noise waveform and trimmed waveform (when the dynamic range upper limit is 139.7 or 135.0 dB) are greatest for L_p measured during shots from the Mossberg smooth-bore shotgun, while the smallest are for the noise generated in the forge.

Figure 3. Sound pressure levels in octave bands for the original and trimmed waveforms of shots from the Mossberg smooth-bore shotgun, with different dynamic range upper limit values of the measurement equipment.

Figure 4. Sound pressure levels in octave bands for the original and trimmed waveforms of shots from the AKM rifle, with different dynamic range upper limit values of the measurement equipment.

Figure 5. Sound pressure levels in octave bands for the original and trimmed waveforms of noise generated in the forge, with different dynamic range upper limit values of the measurement equipment.

4.2. Consequences of the use of standard sound level meters for impulse noise

Figures 6–8 present the results of the determination of $L_{\text{Aeq}}^{\mathrm{\prime }}$. The figures show the lines that distinguish the three evaluation areas for hearing protectors, i.e., insufficient protection, sufficient protection and overprotection.

Figure 6. Results of $L_{\text{Aeq}}^{\mathrm{\prime }}$ determination for hearing protectors used during shots from the Mossberg smooth-bore shotgun. DRUL = dynamic range upper limit; $L_{\text{Aeq}}^{\mathrm{\prime }}$ = A-weighted equivalent sound pressure level of noise under a hearing protector; SNR = single number rating.

Figure 7. Results of $L_{\text{Aeq}}^{\mathrm{\prime }}$ determination for hearing protectors used during shots from the AKM rifle. DRUL = dynamic range upper limit; $L_{\text{Aeq}}^{\mathrm{\prime }}$ = A-weighted equivalent sound pressure level of noise under a hearing protector; SNR = single number rating.

Figure 8. Results of $L_{\text{Aeq}}^{\mathrm{\prime }}$ determination for hearing protectors used in the presence of noise generated during metal processing in the forge. DRUL = dynamic range upper limit; $L_{\text{Aeq}}^{\mathrm{\prime }}$ = A-weighted equivalent sound pressure level of noise under a hearing protector; SNR = single number rating.

The data presented in Figures 7 and 8 indicate that among the earmuffs, there are both those which, due to their L_Aeq, provide sufficient protection against noise generated at the workplace in the forge and those whose assessment indicates insufficient protection. In the case of noise generated during shots from the Mossberg smooth-bore shotgun (Figure 6), apart from these two categories, some earmuffs lead to overprotection.

5. Discussion

Commercially available sound level meters and dosimeters are convenient for conducting noise assessments, but recent studies have raised awareness of their limitations [23]. The use of measurement equipment not suitable for the parameters of the measured signal must naturally result in incorrect indications. The results of noise parameter determination obtained in our work on the basis of the waveforms, which was recorded correctly, and the modified waveforms according to the measurement range limits considered, were used to assess the noise hazard and to select hearing protectors. In this way it is shown what the consequences can be of using the results of measurements carried out using inappropriate equipment.

The results presented in Tables 3–5 show that the L_Cpeak value indicated by sound level meters with an insufficient dynamic range upper limit exceeds the exposure limit value related to this parameter (135 dB), similar to the result obtained in the case of a correctly performed measurement. The assessment of the noise hazard, i.e., finding that the exposure limit value has been exceeded, despite incorrect L_Cpeak values, is therefore concurrent in the case of correct and incorrect measurements. It should be noted, however, that when assessing the noise generated during shots from the Mossberg smooth-bore shotgun (Table 3), carried out using the L_EX,8h parameter, a sound level meter with a dynamic range upper limit of 135.0 dB resulted in indications which led to the result opposite to the reference measurement. For this meter, a value of L_EX,8h amounting to 84.7 dB has been determined, which indicates that the criterion value of 85 dB is not exceeded. The situation of obtaining an incorrect result of noise hazard assessment due to the parameter L_EX,8h also occurred in the case of shots from the AKM rifle (Table 4). Concluding on the basis of the results measured by any of the two sound level meters with an insufficiently high value of the dynamic range upper limit indicates that the L_EX,8h value (even 3.9 dB below the criterion) is not exceeded, when in fact it is exceeded. The examples shown therefore indicate that the provision in Directive 2003/10/EC [1] that the measurement equipment must enable to determine whether the noise limit values are being exceeded may not be complied with when measuring impulse noise. Despite this, in the end, the assessment of noise in the workplace (not including the problem of selection of hearing protectors) will generally indicate that the noise is dangerous, since, as is known, the assessment also includes a component on the peak value of the signal. In the assessment of noise in the workplace, it is sufficient that there is an exceedance for one of the noise parameters for the noise to be considered hazardous.

It follows from the aforementioned that the measurements of impulse noise parameters should be carried out with the use of equipment that enables the correct indication of relatively high values of the sound pressure level. Therefore, in the case of noise sources considered in this article, the dynamic range upper limit of the equipment used should reach 165 dB. As mentioned in Section 1, the issue of equipment for measuring impulse noise parameters is rarely undertaken in published works. The first example of including this subject is Standard No. ANSI/ASA S12.42-2010 [13] describing methods for the measurement of insertion loss of hearing protectors in continuous or impulsive noise. This standard includes, inter alia, indications for the implementation of a transducer for measuring impulse noise parameters. It is required that this transducer (a free-field pressure probe/microphone) should be capable of measuring impulse levels of 180-dB peak sound pressure level. The need for a sufficiently large dynamic range upper limit of measurement equipment for the characterization of impulse noise was also highlighted in the work on the development of a noise dosimeter [12]. This work presents two developed noise dosimeters appropriate for the military. The dynamic range upper limit of these noise dosimeters is 165 and 180 dB.

It is natural that the greater the value of the peak sound pressure level characterizing impulse noise, the greater are the differences between the values of noise parameters determined using sound level meters with insufficiently high dynamic range upper limit in relation to the measurement data obtained correctly. Data presented in Tables 3–5 allow to obtain information about the scale of potential deviations of the results produced with the use of sound level meters in inappropriate impulse noise measurements from the results that can be described as correct. The indication of L_Cpeak, if measured with an unsuitable sound level meter, may deviate from the value characterizing the impulses produced when firing a firearm by 13 dB (ΔL_Cpeak in the case of the AKM rifle) or by even more than 20 dB (Mossberg smooth-bore shotgun). For the noise that is characteristic of industrial conditions, i.e., produced in a forge, the deviation of the indication of the L_Cpeak may reach almost 11 dB. The largest of the numerical values of the analyzed indication differences in the case of parameters related to the energy properties of the signal, i.e., ΔL_Aeq (ΔL_EX,8h) and ΔL_Ceq, are 5.5, 4.5 and 3.7 dB, respectively, in the case of noise from the Mossberg smooth-bore shotgun, from AKM rifle shots and in the forge. The largest deviations of the values obtained by the sound level meter from the reference results, when measuring L_p (Figures 3–5), are 4.3 dB (ΔL_p1 – the difference in indications determined from the original noise waveform [dynamic range upper limit: 174.0 dB] and the trimmed waveform when the dynamic range upper limit is 139.7 dB) and 6.5 dB (ΔL_p2 – the difference in indications determined from the original noise waveform [dynamic range upper limit: 174.0 dB] and the trimmed waveform when the dynamic range upper limit is 135.0 dB). Therefore, the referred values of deviations (ΔL_Cpeak, ΔL_Aeq [ΔL_EX,8h], ΔL_Ceq, ΔL_p1, ΔL_p2) confirm that it is sometimes incorrect to make an assumption among people carrying out measurements that the impact of exceeding the dynamic range upper limit of the measurement equipment during impulse noise measurements is negligible.

Although L_Cpeak in the case of noise generated during shots with the AKM rifle exceeds the value of this parameter characterizing noise generated in the forge only by 0.6 dB, the discrepancy of the L_EX,8h determination in the case of noise in the forge (compared to the parameter determined from the recorded noise waveform) is lower (2.9 dB) than that of the AKM rifle (4.5 dB). The noise in the forge is not only composed of the impulses produced by the forging hammer. Impulses are emitted against the background of the continuous noise generated during the operation of many machines in the forge hall. The energy contained in the peaks of impulses generated in the forge which are not taken into account in the signal analysis by sound level meters with a limited measuring range shall represent a smaller proportion of the total signal energy than that of the impulses associated with gunshots. Hence, the impact on the final value of L_EX,8h is smaller than for the noise generated by a firearm.

An integral part of the selection of hearing protectors is the need to know the given parameters of noise that is present in the place where their user is located. The data shown in Figures 6–8 show that the transfer of erroneous noise measurement results to the $L_{\text{Aeq}}^{\mathrm{\prime }}$ value determined during the selection process under the hearing protector leads to a different erroneous assessment for some earmuffs. For example, in the case of an earmuff with SNR = 25 dB (Figure 6), the data determined based on the waveform recorded correctly lead to the ‘insufficient protection’ assessment. In turn, data reflecting the use of sound level meters with an insufficiently high dynamic range upper limit result in ‘sufficient protection’ assessment. A situation of this kind can be particularly dangerous for the potential user of a hearing protector. If an incorrect noise measurement is ignored and erroneous measurement data are used to select a hearing protector, the results would indicate correct hearing protection. In fact, when the data obtained from a correctly performed measurement are taken into account, the selection indicates insufficient hearing protection. This means that such hearing protectors should not be used to protect hearing in the presence of the noise under consideration. There may also be an incompatible result of the assessment of an earmuff of a slightly different nature. Selection of hearing protectors is also associated with the need to take into account instances of overprotection [24]. For example, in the case of an earmuff with SNR = 37 dB, a selection result based on data obtained using sound level meters indicates overprotection. At the same time, a selection based on properly obtained measurement data indicates sufficient protection. In this case, therefore, it is not reasonable to remove the earmuff from the set of hearing protectors as a result of using incorrect measurement data. Unlike in the previous case, an incorrect assessment will not harm the potential user of hearing protectors.

When hearing protectors are selected for shots from the Mossberg smooth-bore shotgun, the use of measurement data obtained using sound level meters with an insufficient dynamic range upper limit results in an erroneous evaluation of the earmuffs for 6 out of 15 earmuffs examined. At the same time, in three out of the six situations mentioned, incorrect assessment was performed for both sound level meters under consideration. In three of the remaining six erroneous cases, a different rating based on the data obtained correctly occurred only in the case of a sound level meter with a lower dynamic range upper limit (i.e., 135.0 dB). It should also be noted that, in general, a particularly unfavorable assessment (sufficient protection instead of insufficient protection) was given for 3 out of 15 earmuffs. When selecting hearing protectors for the noise generated when firing from the AKM rifle (Figure 7), an incorrect assessment was given for 3 out of 15 earmuffs (with SNR of 24, 25 and 26 dB). In contrast, the evaluation of the results of the selection of hearing protectors for the noise present in the forge was different for 4 out of 15 hearing protectors (with SNR of 30, 31, 32 and 33 dB).

In general, it can therefore be concluded that the selection of hearing protectors with regard to the $L_{\text{Aeq}}^{\mathrm{\prime }}$, parameter, using measurement data obtained with a sound level meter with an insufficient dynamic range upper limit, may result in an incorrect assessment in about 20–40% of the earmuffs. It should be added that the full selection of hearing protectors in the case of impulse noise exposure requires additional analysis due to the value of the parameter $L_{C\text{peak}}^{\mathrm{\prime }}$.

In summary of the discussed measurement problems in the case of noise with sound pressure level values exceeding the dynamic range upper limit of the sound level meter, it can be said that the occupational health and safety services, the employer and employees, during the assessment of noise parameters in the workplace, although they will not receive a precise measurement result, will obtain information about the occurrence of exceeded values of permissible noise parameters. At the same time, the article shows (Tables 3 and 4) that an insufficient high dynamic range upper limit can, unfortunately, in some cases result in a failure to demonstrate an exceedance of the noise parameter value related to the energy properties of the signal (L_EX,8h). This poses a potential problem when evaluating noise in a workplace where impulse noise is present. Moreover, this problem is potentially very significant because practically all sound level meters that have type approval, i.e., can be used to evaluate noise parameters in the workplace, have a high dynamic range upper limit close to about 140 dB. Fortunately, however, this evaluation also includes peak signal values. In every situation where a noise assessment due to the L_EX,8h parameter failed, the exceedance was shown by the L_Cpeak parameter. Thus, in the end, although standard sound level meters are characterized by a high dynamic range upper limit of about 140 dB and will not always allow to obtain numerical data characterizing noise, their use can be conditionally accepted in practice. This acceptance is possible when there is sufficient information about the exceedance of any of the assessed noise parameters. The situation is different in the case of the selection of hearing protectors. The selection of a hearing protector that properly protects hearing requires the determination of the value of noise parameters. At the same time, the proper selection of a hearing protector means, on the one hand, verifying that it sufficiently reduces noise in terms of hearing protection, and on the other means avoiding overprotection due to the provision of safe working conditions. This is primarily about being able to hear certain important sounds, such as auditory danger signals. In a situation where data are needed to carry out the selection of hearing protectors, noise parameters should be measured using measuring equipment suitable for testing noise at high sound pressure levels. Manufacturers of measurement equipment offer measurement microphones suitable for this purpose, which can be part of specialized measurement systems or can be used in place of standard microphones with which sound level meters are equipped.

6. Conclusions

The study presents the effects of the use of sound level meters to measure the parameters of impulse noise in a situation where the dynamic range upper limit of equipment was not large enough in relation to the parameters of characterized noise. The analysis was carried out on the example of noise generated during shots from the Mossberg smooth-bore shotgun and AKM rifle, as well as produced in the forge. The use of the sound level meter to assess the hazard of impulse noise, despite erroneous measurement results, allowed to indicate the exceeding of the exposure limit value of the parameter related to the instantaneous signal properties (L_Cpeak). The situation was different in the case of the parameter reflecting the energy properties of the signal, i.e., L_EX,8h. The use of a sound level meter in the case of impulse noise led, in part of the cases, to the result that the exposure limit value of this parameter was not exceeded, where it was actually exceeded. Consequently, the results of the article indicate that the provision of Directive 2003/10/EC [1], stating that the measurement equipment used to assess the noise hazard must make it possible to determine the fact that the limit values for noise parameters have been exceeded, may not be met when the properties of impulse noise are characterized. Fortunately, noise assessment in the workplace takes into account both the parameter reflecting the energy properties of the signal (L_EX,8h) and the parameter related to the instantaneous signal properties (L_Cpeak). This helps to prevent lack of recognition of the presence of a noise hazard for hearing.

While the inadequate properties of the measuring equipment will generally not prevent the general conclusion that noise in a particular workplace is hazardous to hearing, the results of measurements obtained with this equipment cannot be used to select hearing protectors. The selection of earmuffs to be used in the presence of impulse noise, due to the parameter reflecting the signal energy properties ($L_{\text{Aeq}}^{\mathrm{\prime }}$), using measurement data obtained with a sound level meter with an insufficient dynamic range upper limit, may result in an incorrect assessment in about 20–40% of the earmuffs.

The conclusions of the analyses carried out therefore show that the use of a standard sound level meter for impulse noise assessment in a workplace with relatively high sound pressure levels is only conditionally acceptable. However, obtaining data for the selection of hearing protectors requires the use of equipment with a sufficiently large upper limit of the measuring range. Improper measurements not only can lead to an incorrect results, but also can result in inadequate hearing protection for employees forced to use hearing protectors.

Acknowledgements

This paper has been based on the results of a research task carried out within the scope of the fifth stage of the National Programme ‘Improvement of Safety and Working Conditions’ partly supported in 2020 — within the scope of state services — by the Ministry of Family, Labour and Social Policy. The Central Institute for Labour Protection – National Research Institute is the Programme's main co*ordinator.

Disclosure statement

No potential conflict of interest was reported by the authors.