Introduction: Measurements were made of the sound pressure levels on a military ship HMS Grimsby during firing of a Heavy Machine Gun (HMG) mounted on the starboard bridge wing. The measurement positions comprised three locations on the ship’s bridge (the wheelhouse) and one location on the starboard bridge wing. Equipment and Procedure: The three locations on the bridge were the starboard door, centre and port door. A total of 255 burst firings were measured during the survey comprising 850 rounds with each burst encompassing from 1 to 10 rounds. Analysis: The data have been assessed and interpreted in accordance with the Control of Noise at Work Regulations 2005. Results: The highest peak sound pressure levels measured on the bridge wing and on the bridge were 160.7 dB(C) (2170 Pa) and 122.7 dB(C) (27.3 Pa), respectively. The highest sound exposure levels measured on the bridge wing and on the bridge corresponding to one round being fired were 127.8 dB(A) and 88.9 dB(A), respectively. The ship’s structure provided about 40 dB attenuation in the transmitted noise. Discussion: The operator of the weapon would be required to wear some form of hearing protection. On the basis of the measured peak noise levels, there would be no requirement for bridge crew to wear any hearing protection during firing of a HMG. However, crew exposure to noise on the bridge is likely to exceed the upper exposure action value corresponding to 85 dB(A) after about 11,750 rounds. Conclusions: Measurements made on the bridge wings are likely to be affected by reflections from the ship’s structure.
Keywords: Hearing protection, peak noise, ship, sound exposure level, weapon
|How to cite this article:|
Paddan GS. Occupational noise exposure on a Royal Navy warship during weapon fire. Noise Health 2016;18:266-73
| Introduction|| |
There are many types of noise that occur on ships. Among other noise sources, these include noise from engines, ventilation and compressors. Depending on the type of vessel, noise exposure might also include activity from rotary wing aircraft. The acceptable levels of noise that occur on ships for health and comfort (rather than hearing protection) are covered by the International Maritime Organisation (IMO). With respect of occupational exposure to noise, on the other hand, the assessments of exposure could either be carried out in accordance with the Control of Noise at Work Regulations (CNAWR) in force in the UK, or for merchant ships the Merchant Shipping and Fishing Vessels (Control of Noise at Work) Regulations (MSFV(CNW)R). The CNAWR implement the European Commission’s Physical Agents Directive on Noise PA(N)D. One source of noise that would also be present on a military ship, and is not covered by the IMO, is that from weapon fire.
Noise exposure in most compartments on a ship would not usually be an issue for a vessel involved in normal navigation and travel. This would also be expected to be the case for a military vessel involved in most manoeuvres including training of crew and in peacekeeping operations. However, military ships would necessarily be involved in naval operations for some amount of the time. Crew exposure to noise within a military ship could be addressed using the exemptions stated in the CNAWR. The UK Regulations state that ‘… the Secretary of State for Defence may … exempt any person … in the interests of national security …’. However, policy is to comply with the CNAWR so far as possible.
Many training regimes that are conducted on a military ship include firing of weapons so that operators are able to maintain their trained capabilities. There are many locations on the ship that would be used for mounting guns on the ship; these include the bridge wings. For a weapon firing from the bridge wings, crewmembers exposed to noise include the operator of the weapon and the crew on the bridge (wheelhouse). The noise exposure of the operator on the bridge wings would be of concern as the peak levels are likely to exceed the values specified in the Control of Noise at Work Regulations CNAWR (for example, Willams). However, it is not known whether the exposure of the crew on the ship’s bridge would be of concern and what type of hearing protection should be worn, if any. That is, the attenuation of noise afforded by the ship’s structure is not known.
There are different types of gun that could be used on a military ship including Heavy Machine Gun (HMG), General Purpose Machine Gun, Minigun and 30-mm Automatic Small Calibre Gun. However, the mountings on the bridge wings would normally accommodate the HMG and the General Purpose Machine Gun.
This study is concerned with determining many parameters of noise on a ship as a result of firing a weapon, a HMG, mounted on the bridge wing of a military ship, HMS Grimsby. The aims of the study included: (i) determining the noise exposure of the weapon operator, (ii) noise exposure of crew on the ship’s bridge and (iii) the attenuation afforded by the ship’s structure in reducing noise transmission from the bridge wing to the bridge.
| Equipment and Procedure|| |
Noise measurements were made on a Royal Navy Sandown Class vessel HMS Grimsby (Pennant Number M108). The ship is a Mine Counter Measure Vessel (MCMV) involved in clearing mines. The ship was fitted with gun mounting positions on the bridge wings (port (left) and starboard (right)) on 01 deck. The bridge is a fully enclosed compartment.
There are two access points between the bridge compartment and the bridge wings located on the port and the starboard sides. Each point comprises an internal wooden door and an external metal watertight door. Both doors were closed during the noise measurements. Noise measurements were made during operations off the West coast of Scotland.
A HMG was fitted on to the bridge wings on the ship. The HMG, shown in [Figure 1], is a 12.7 mm calibre (0.50 inch) weapon type L111A1 and was operated in both single round mode (individual round fired per trigger pull) and burst round mode (up to 10 rounds fired per trigger pull). Each measurement included the firing of many rounds (either in single round mode or in burst round mode firing). A total of 255 burst firings comprising 850 rounds were fired during the survey. The weapon was fitted on the starboard side of the bridge wings during the measurements.
The weapon was used in different modes of operation including firing in all directions that the weapon was capable of covering. This involved the weapon pointing in minimum and maximum elevations (i.e. pitch angles) and covering greatest angles to the left and the right sides (i.e. yaw angles). There will be no distinction between the noise measured on the ship and the orientation of the gun during firing.
Noise measurements were made at four locations on the bridge (01 deck); these comprised the following: (i) outside on the starboard bridge wing; (ii) inside the bridge, by the starboard door (near side to weapon); (iii) inside the bridge, centre and (iv) inside the bridge, by the port door (far side to weapon). That is, one location was outside of the ship’s bridge whereas three locations were inside the ship’s bridge. The bridge was approximately 6.4 m wide and approximately 4.7 m deep. Therefore, the distance between the two bridge doors (starboard and port) was about 6.4 m. The port and starboard doors on the bridge were closed during firing of the weapon.
G.R.A.S. type 40BH high pressure microphones connected to G.R.A.S. type 26AC microphone preamplifiers were used to measure the noise at the different locations on the ship. The microphones were placed approximately 1.5 m above the deck of the ship and orientated at 90° to the direct line with the HMG (grazing incidence). All four microphones were fitted with foam windshields. The microphones were connected to two G.R.A.S. Power Modules type 12AA. The signals were then acquired into a digital computer-based data acquisition system using a data analysis terminal system (DATS) software package and Prosig 8004 hardware, wherein 24-bit rate time histories were acquired simultaneously at a sampling rate of 100,000 samples per second. The waveforms were low-pass filtered at 40,000 Hz via anti-aliasing filters. For each test, the measurement duration covered the complete burst firing with each burst ranging from 1 round to 10 rounds.
Calibration of the complete recording system was performed using two types of G.R.A.S. calibrator: a type 42AB Sound Calibrator which gave a sinusoidal calibration tone of 114 dB at a frequency of 1000 Hz, and a type 42AC Pistonphone which gave a sinusoidal calibration tone of 134 dB at a frequency of 250 Hz. The calibration procedure was performed both before and after the sound recordings, which showed the equipment to be stable over the measurement periods. The recorded data were analysed using the HVLab (v3.81) software package.
The human ear does not respond equally at all frequencies; therefore, the A-weighting is applied to the audible frequency range to represent the reduction in sensitivity to the low frequencies. This is illustrated in [Figure 2]. Also shown in [Figure 2] is the frequency response of the C-weighting filter, which is suitable for assessing peak sound pressure levels. Long-term damage to hearing from moderate to loud noise is related to the noise exposure in dB(A). The mechanism of instant damage to the ear for extremely loud noise is different, and is related to peak C-weighted levels.
|Figure 2: Frequency characteristics for the A-weighting (─────) and C-weighting (─ ─ ─ ─ ─) filters|
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The equivalent continuous sound pressure levels, Leq, were calculated for the measured time histories; these correspond to the energy equivalent in the steady sound pressure level over a specified period of time, to the actual fluctuating sound. (The Leq is also known as the time-averaged noise level.) It is described in the following equation [Equation 1], where tm is the specified period of time, P1 is the instantaneous sound pressure and P0 is the reference sound pressure of 20 μPa.
Analysis has also been done using A-weighted sound exposure levels, LAE. Sound exposure level is defined as: ‘The sound pressure level (in dB) which, if it lasted for 1 second, would produce the same energy as the actual noise event’. (Another definition of the term is given elsewhere as ‘For an event lasting less than 1 second, the sound energy is “smeared out” to fill the reference time; conversely, long duration sounds are “squashed” into 1 second’.) The use of sound exposure level is recommended by the health and safety executive (HSE) for measuring a single event or a known number of events. Unlike the equivalent continuous sound pressure levels Leq, the LAE does not reduce with an increasing quiet measurement period. LAE is a measure of noise dose per round, and the daily noise exposure can be calculated from the number of rounds and the LAE.
The time histories of noise measured at four locations were also used to calculate C-weighted peak sound pressure levels, LCpeak. Unweighted sound exposure levels, LE, were calculated for each one-third octave band between the frequencies of 20 Hz and 20,000 Hz for one burst firing to show, as an example, the frequencies present in this type of noise.
| Results|| |
Time histories of the sound pressure levels measured at the four locations (bridge wing, bridge starboard door, bridge centre and bridge port door) during firing of a HMG on the starboard bridge wing are shown in [Figure 3]. As would be expected, higher sound pressure levels are seen at the bridge wing at the operator of the weapon compared with the three locations on the bridge. Also, higher sound pressure levels are seen at the starboard door (closer to the weapon) compared with the other two locations. Note the different vertical scales on the time histories shown in [Figure 3]. The highest (unweighted) peak sound pressure level for this recording was 2750 Pa (162.8 dB) for the bridge wing and 24.8 Pa (121.9 dB) for the bridge (starboard door). A slight increase in the delay in the occurrence of the peak sound pressure is seen as the distance from the noise source increased. [Figure 3] shows data encompassing the complete event over a period of 0.2 s including any reflections and reverberation of the sound pressure wave within the bridge. The time histories for the bridge locations show the complex nature of the noise that is affected by the reflections within the bridge.
|Figure 3: Sound pressure levels measured on the bridge during firing of a HMG on the starboard bridge wing|
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Peak sound pressure
Peak sound pressure levels, LCpeak, for measurements at the four locations on the vessel are shown in [Figure 4]. These data show the variation in the measured peak sound pressure levels during firing. The large variation is related to the different orientation and elevation of the gun during firing. [Table 1] shows the spread of the peak sound pressure levels and the median of the logarithmic values (i.e. median pressure) for the measured data. The highest peak sound pressure level measured on the bridge wing was 160.7 dB(C) (2170 Pa), and the highest peak on the bridge (starboard door) was 122.7 dB(C) (27.3 Pa). It is seen from the median values that attenuation provided by the bridge structure is approximately 40 dB (compare the bridge wing with the starboard door). Lower peak values were seen at the centre of the bridge compared with the starboard and port doors.
|Figure 4: Peak sound pressure levels, LCpeak (dB(C)), measured on HMS Grimsby during the firing of a HMG on the starboard bridge wing|
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|Table 1: Range of peak sound pressure levels, LCpeak (dB(C)), measured on HMS Grimsby during the firing of a HMG on the starboard bridge wing|
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Sound exposure level
Sound exposure levels, LAE, corresponding to a single round fired over a 1-s period are shown in [Figure 5] for all measurements on the ship. The sound exposure levels show the total energy of the signal corresponding to a 1-s period. Logarithmic median sound exposure levels for the data shown in [Figure 5] are shown in [Table 2]. The highest sound exposure level on the bridge wing was 127.8 dB(A). The sound exposure levels were higher at the starboard door compared with the other two locations on the bridge. These data show that the highest sound exposure level measured on the bridge (starboard door) was 88.9 dB(A). On the basis of the median sound exposure levels, the attenuation of the sound exposure level provided by the ship’s structure was approximately 40 dB (compare the bridge wing with the starboard door). Coincidently, the attenuation in the sound exposure level (40 dB) is the same as the attenuation in the peak sound pressure level (40 dB).
|Figure 5: Sound exposure levels, LAE (dB(A)), measured on the bridge corresponding to single rounds fired from the HMG on board HMS Grimsby|
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|Table 2: Range of sound exposure levels, LAE (dB(A)), measured on HMS Grimsby during the firing of a HMG on the starboard bridge wing|
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All measured data comprised one burst firing of the HMG, and each burst comprised from 1 round to 10 rounds. As an example of the frequencies present in the time history, unweighted one-third octave band sound exposure levels, LE (dB), have been calculated for one burst firing containing 10 rounds; these are shown in [Figure 6]. The corresponding overall A-weighted sound exposure levels, LAE (dB(A)), for the four locations were:
- starboard bridge wing, outside 119.6 dB(A)
- bridge starboard door, inside 80.8 dB(A)
- bridge centre, inside 80.2 dB(A)
- bridge port door, inside 76.0 dB(A)
|Figure 6: One-third octave band sound exposure levels (LE) measured on HMS Grimsby for one burst firing of 10 rounds from the HMG|
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The data show higher sound exposure levels in each of the one-third octave frequency band for measurements on the bridge wing, that is, at the location of the weapon firer, compared with the three locations on the bridge. Marginally higher sound exposure levels occurred at the starboard door compared with the central location and the port door. These data can be used to determine the suitability of hearing protectors for this environment.
α- and β-durations
Other parameters that can be used in identifying the energy in impulse or impact type noise are terms called the α-duration (alpha duration) and the β-duration (beta duration). (These two terms are generally considered to be of a historical nature and have been replaced by the terms LAE and LCpeak. Also, these terms were mostly used in conjunction with noise generated by weapons used in the military sector and are not needed for the CNAWR.) The α-duration is defined as the time interval between the peak (either negative or positive) and the first time that the pressure reduces to atmospheric pressure before falling to a value below the atmospheric pressure. The β-duration is defined as the time interval between the peak (either negative or positive) and the last point on the waveform where the value of the pressure has reduced to 10% of the peak value with succeeding values remaining below 10% of the peak value. The β-duration could be used with signals comprising reflections and reverberations from surrounding walls and fixtures. These parameters have been defined by Coles et al.; the β-duration has been a part of the UK Ministry of Defence standard Defence Standard 00-27 (although the β-duration is not included in the latest version of Defence Standard 00-27). However, both parameters (α- and β-durations) can only be measured for single impulse or impact type events. The estimation of the β-duration would not be appropriate for ‘burst’ or ‘rapid’ fire events.
[Figure 7] shows a detailed image of the time history shown in [Figure 3] of sound pressure levels at the firer’s location on the bridge wing. The α-duration corresponding to this waveform is 0.57 ms. The β-duration for this waveform is measured to be 15.1 ms (the whole time history is not presented in [Figure 7]); this parameter is highly influenced by sound pressure levels reflecting of the ship’s structure. The sound pressure level waveform measured at the bridge starboard door shown in [Figure 3] is influenced by reflections and reverberations of the sound within the ship’s bridge. The β-duration corresponding to this waveform is calculated to be 108 ms; this parameter would have been affected by the fixtures and fittings in the bridge, thus resulting in a significantly longer duration compared with the β-duration measured of noise exposure of the operator on the bridge wing (i.e. outside of the bridge). Powell and Forrest state that ‘β-durations are generally longer with larger calibre weapons’: noises which tend to be dominated by low frequencies. In this case, where the β-duration could be considered to be long for measurements on the bridge, the noise is likely to be dominated by low frequencies; this is evident from the frequency data shown in [Figure 6].
|Figure 7: Sound pressure levels measured on the starboard bridge wing during firing of a HMG on that bridge wing|
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| Discussion|| |
Variation in the measured peak sound pressure levels, LCpeak, and the sound exposure levels, LAE, is shown in [Figure 5], respectively. This variation in the measured values is thought to correspond to the greatest variation that should be expected in these data since the orientation and elevation of the gun were not controlled. That is, the direction of fire varied and was limited only by the restrictions imposed by the gun mounting. Smaller variation would have been expected if the orientation and elevation of the gun had been controlled during the firings. Visually, the variation in the peak sound pressure levels and the sound exposure levels appears to be similar for all locations apart from measurements at the centre of the bridge. However, a quantitative measure of the variation would be required since the variation would be related to the absolute value of the parameter. A measure that could be used is the normalised variability that takes into account the absolute magnitude of the parameter:
Normalised variability in the peak sound pressure values and the sound exposure levels for the measured data are shown in [Table 3]. The data show, for example, that the normalised variability in the peak sound pressure level at the bridge wing was 1% of the median of the peak sound pressure levels. Greater variability occurred (3%) in peak sound pressure level at the starboard door compared with the other two locations on the bridge. The data also show lower normalised variability in the sound exposure level on the bridge wing compared with the other locations on the bridge. This measure of variability shows the merits of using both the median and the interquartile range in determining the variation in the measured data.
|Table 3: Normalised variability in the measured sound data during the firing of the HMG|
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The time history in [Figure 7] shows the measured sound pressure level on the bridge wing; this also shows the reflections occurring around the operator. The figure shows an initial peak value of 2750 Pa which decays followed by a second peak of approximately 1460 Pa. The time difference between the first and the second peak is measured as 0.33 ms. If it as assumed that the speed of sound is 330 ms−1, then this difference in the time between the two peaks corresponds to a distance of 11 cm. The microphone was mounted on the ship’s structure approximately 5 cm from the starboard door. In accordance to the time delay between the first and the second peak, the second peak is consistent with a reflection of the sound pressure wave. That is, the first peak corresponds to the direct sound transmitted from the gun to the microphone, and the second peak corresponds to the reflected sound. These data could be used to determine the effect of noise on the operator. Although the peak sound pressure level LCpeak (i.e. the first peak) is unlikely to be affected by the reflections, if the sound exposure level LAE were to be calculated, this would be influenced by the reflected sound from the local ship structure. This demonstrates the importance of ‘in-situ’ measurements compared with measurements made in unrepresentative conditions.
The assessment of exposure to noise would involve comparison with standards and regulations. For example, the CNAWR (2005) could be used. The CNAWR specify certain exposure action values and limit values; these are as follows:
- The lower exposure action values (LEAVs) are −
- a daily or weekly personal noise exposure of 80 dB (A-weighted); and
- a peak sound pressure of 135 dB (C-weighted)
- The upper exposure action values (UEAVs) are −
- a daily or weekly personal noise exposure of 85 dB (A-weighted); and
- a peak sound pressure of 137 dB (C-weighted)
- The exposure limit values (ELVs) are −
- a daily or weekly personal noise exposure of 87 dB (A-weighted); and
- a peak sound pressure of 140 dB (C-weighted).
[Table 1] shows that the highest peak sound pressure level on the bridge wing (160.7 dB(C)) exceeded the peak UEAV (and the ELV if no hearing protection is worn) whereas the highest peak on the bridge (122.7 dB(C)) was below the peak LEAV. Therefore, different types of assessment would be required depending on the location of measurement. The selection of hearing protection would involve both the peak sound pressure level and the daily personal noise exposure for people on the bridge wing, but only the daily personal noise exposure for persons on the bridge. For people on the outside on the bridge wing (LCpeak 160.7 dB(C)), hearing protection should be chosen to reduce the peak to below 137 dB(C). Hence, attenuation of more than 24 dB would be required (minimum of 21 dB for a reduction to 140 dB(C)) and hearing protection selected accordingly.
There would be a requirement to wear hearing protectors for the operator of the weapon (and other people in the vicinity) on the bridge wings. There are a number of Standards applicable for selection of hearing protectors. British Standard (BS) EN 352 covers selection and use of hearing protectors; this Standard applies to both earmuffs and earplugs. BS EN 352 states that the attenuation provided by hearing protectors can be measured using the procedure specified in BS EN 24869-1. BS EN 458 also provides recommendations for the selection, use, care and maintenance of the devices. With respect to assessing the suitability of hearing protection for the operator who would be exposed to impulse noise, the UK Health and Safety Executive recommends that the following guide be used to estimate the attenuation provided by personal hearing protection (PHP):
‘… the attenuation provided by a hearing protection device is predicted according to the modified sound attenuation values in [Table 4]. The effective peak sound pressure level at the ear is estimated by subtracting the modified sound attenuation value from the peak sound pressure level of the impulsive noise source’.
|Table 4: Sound attenuation values for different impulse or impact noises|
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The requirement for hearing protectors for people on the bridge would be determined with a continuous exposure assessment, that is, a different method of assessing and controlling exposure to noise would be used for people on the ship’s bridge. The sound exposure level, LAE, would be used to calculate the number of rounds that would be required to reach the LEAV and the UEAV specified in the CNAWR. If the highest sound exposure level were to be used in the assessment (88.9 dB(A) as shown in [Table 2]), then the numbers of rounds to reach the various values would be:
The numbers of rounds calculated assume that the crew on the bridge wear no hearing protection. To put these numbers of rounds into context, one round fired every 5 s would equate to 5760 rounds being fired over an 8-h period.
| Conclusion|| |
The highest peak sound pressure level measured at the operator’s location on the bridge wing was 160.7 dB(C) (2170 Pa) and the highest value measured on the bridge was 122.7 dB(C) (27.3 Pa). The ship’s bridge structure provided about 40 dB attenuation in peak noise. The corresponding sound exposure levels equating to single rounds being fired from the weapon were 127.8 dB(A) for the bridge wing and 88.9 dB(A) for the ship’s bridge.
On the basis of the noise measurements in this survey, it is unlikely that crew on the ship’s bridge would be exposed to noise exceeding the UEAV during firing of the HMG. The exposure of the operator of the HMG on the bridge wing would be expected to exceed the peak sound pressure ELV corresponding to 140 dB(C) unless some form of hearing protection is worn.
The measured sound exposure levels on the bridge show that firing of 3700 rounds would reach the exposure corresponding to the LEAV, and the corresponding numbers of rounds to reach the upper action value would be 11,750.
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Conflicts of interest
There are no conflicts of interest.
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Gurmail Singh Paddan
Institute of Naval Medicine, Crescent Road, Alverstoke, Gosport PO12 2DL
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3], [Table 4]