Ultrasonic Exposure Safety Recommendations for High-Frequency Hand-Held Diver Sonars
Prepared by: Ronald Kessel Craig Hamm Maritime Way Scientific Ltd. (MWS) 1420 Youville Drive Ottawa, Ontario K1C 7B3
PWGSC Contract Number: W7707-4501478116 Technical Authority: Vincent Myers, Defence Scientist
The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of the Department of National Defence of Canada.
Contract Report DRDC-RDDC-2017-C089 March 2017
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© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2017 © Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2017
Ultrasonic Exposure Safety Recommendations for High-Frequency
Hand-Held Diver Sonars
Maritime Way Scientific Full Report Prepared for: Defence R&D Canada – Atlantic and the Canadian Fleet Diving Unit (FDU) Halifax, Nova Scotia, Canada Contract Project Authorities: Vincent Myers, DRDC - Atlantic PWGSC Contract Number: W7707- 4501478116
Prepared by: Ronald Kessel, Chief Technical Officer Craig Hamm, Sonar Analyst Maritime Way Scientific Ltd. (MWS), 1420 Youville Drive, Ontario, Ottawa, Canada K1C 7B3
MWS Project Number: 13-027.9
Date: 23-Mar-2017
Maritime Way Scientific Ltd 1420 Youville Drive, Unit 5A Ottawa ON K1C 7B3 T: 613-841-0505 • E: [email protected]
Title & Project Identification Page
Company Maritime Way Scientific Ltd. name and 1420 Youville Drive, Unit 5A address Ottawa, Ontario • K1C 7B3 Phone 613.841.0505 • Fax 613.590.7231 13.027.9 MWS Project Number
Mr. Martin Taillefer Contact Phone 613-841-0505 Persons [email protected] Dr. Ronald Kessel Phone 613-841-0529 [email protected] Ultrasonic Exposure Safety Recommendations for High-Frequency Hand-Held RFP Title: Diver Sonars Imaging sonars provide visual-style real-time imaging of an underwater scene. They Abstract typically operate in the ultrasonic regime, at frequencies on the order of 300 kHz up to 1.5 MHz, depending on the imaging technology used. There are no known instances of harm or adverse sensation caused to divers by imaging sonars. From medical diagnostic ultrasound, however, it is known that exposure to ultrasonic energy can be harmful. The risks of harm that are of concern in diagnostic ultrasound have never been examined for imaging sonars. Here the risks of diver exposure to a diver hand-held imaging sonar are assessed here in light of the metrics used in diagnostic ultrasound, especially the risks of thermal and mechanical effects. The risks posed by one commercially-available imaging sonar in particular, the Teledyne Blueview P450-45 series sonar, is examined. Its ultrasonic field was characterized by direct measurement under anechoic conditions. Then the two main safety indices used in diagnostic ultrasound—the thermal and mechanical indices, MI and TI—were conservatively applied (erring squarely on the side of caution) in order to assess the risk of harm to divers who may be exposed to the ultrasonic beam of the sonar during dive operations. The results and implications for safe standoff distance and exposure time are reported. The recommendations for safe use are not expected to interfere significantly with relatively free use of the sonar. The methodology used here could be applied to other imaging sonar model-types and technologies.
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Executive Summary
To achieve high image-quality resolution, imaging sonars operate in the ultrasonic regime, at frequencies on the order of 300 kHz up to 1.5 MHz, depending on the imaging technology used. There are no known instances of harm or adverse sensation caused by imaging sonars. From medical diagnostic ultrasound, however, it is known that exposure to ultrasonic energy can be harmful. The risks of harm that are of concern in diagnostic ultrasound have never been examined for imaging sonars. Here the risks posed by a diver hand-held imaging sonar are assessed in light of the metrics used in diagnostic ultrasound, especially the thermal and mechanical indices. One imaging sonar in particular is assessed, the Teledyne Blueview P450-45 series imaging sonar, but the methodology can be applied to other sonar makes, models and technologies. This particular sonar features in a version of the diver hand- held sonar package made by Shark Marine Technologies deployed for operational use by the Canadian Fleet Diving Unit. Its ultrasonic field was characterized by direct measurement under anechoic conditions, and the two main safety indices used in diagnostic ultrasound—the thermal and mechanical indices, MI and TI—were conservatively applied to assess the risk of harm to divers who may be exposed to the ultrasonic beam of the sonar during dive operations. This report reviews the exposure characteristics of ultrasonic fields and their connection to metrics commonly used by sonar engineers, the indices of diagnostic ultrasound, the experimental setup and results, and the implications for safe standoff and exposure time. It was confirmed that the strongest exposures are concentrated along the center of the sonar beam (sonar axis), with higher exposures in the near field (closer than 1 m to the sonar) than in the far. More particularly it was found that: 1. at all distances from the sonar, the ultrasonic effects fall within the safety recommendations for mechanical effects (mechanical index ܯܫ ൏ ͳǤͻ), and for thermal effects under continuous exposure of 1.6 minutes or less (thermal index ܶܫ ൏ ͳǤͷ); 2. at distances greater than 1 m from the sonar, the thermal effects for exposure times up to 10 minutes fall within the safety recommendations (thermal index ܶܫ ൏ ͳǤͷ); and 3. in the event of bubble formation in the diver body during decompression, at distances greater than 0.60 m from the sonar, the ultrasonic mechanical effects fall within the safety recommendations (ܯܫ ൏ ͲǤͷ) (no change in thermal effects due to bubble formation) The main results are summarized in a single figure, Figure 13 on page 30 of this report. It is believed that the safety recommendations do not significantly interfere with sonar use. If these constraints are considered to be too restrictive, then the conservative assumptions made in their derivation could be revisited and relaxed somewhat. The additional complexity is not warranted if the present recommendations are not too restrictive.
Martin L. Taillefer, CD, M.Sc., B.Sc. President & Managing Director Maritime Way Scientific Ltd
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Table of Contents
1 Introduction ...... 7 1.1 The Teledyne Blueview P450-45 Sonar ...... 7 2 Metrics for Ultrasonic Exposure Risk ...... 9 2.1 RMS Pressure and Sound Pressure Level ...... 10 2.2 Peak Rarefaction (Under) Pressure ...... 11 2.3 Ultrasonic Intensity ...... 11 2.4 Derating of Intensities and Indices ...... 13 2.5 Effective Beam Area, ࢋࢌࢌ, and Effective Cross-Section Beam Dimensions ...... 14 2.6 The Ultrasonic Thermal Index ...... 14 2.7 The Ultrasonic Mechanical Index ...... 16 3 Safety Recommendations from Diagnostic Ultrasound ...... 16 4 Ultrasonic Field of the P450-45 ...... 18 4.1 Experimental Setup ...... 18 4.2 P450-45 Ultrasonic Pulses ...... 20 4.3 P450-45 Far-Field Beam Pattern ...... 21 4.4 P450-45 Beam at Close Range (0.85 m) ...... 22 4.5 P450-45 Field on Axis (sonar face to 10 m)...... 24 4.6 P450-45 Pulse-Average Acoustic Power ࢃࡼ and Average Intensity at the Sonar Face ...... 25 4.7 P450-45 Exposure Model ...... 26 5 P450-45 Ultrasonic Exposure Summary ...... 27 6 Conclusions and Recommendations for Use ...... 30 7 Acknowledgements ...... 33 8 References ...... 33
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List of Figures
Figure 1 The Teledyne BlueView P450-45 sonar head [8], the model used by Shark Marine, but operated removed from the Shark Marine unit for better control and operation of the sonar during testing. Two mounting positions were used for anechoic ultrasonic field measurements: horizontal mounting (left) for measurements beyond 2 m distance from the sonar face, and vertical mounting (right) for measurements less than 2 m from the sonar face. The sonar beam direction is indicated by the arrow...... 8 Figure 2 Schematic of the blazed-array transducers behind the sonar face. Each blazed-array is an echelon of staves whose geometry and excitation are phased to direct frequencies toward different angles in the far field in a manner similar to diffraction gratings or prisms...... 8 Figure 3 Schematic plan view of the ultrasonic field produced by the diver hand-held sonar under evaluation. The field is the superposition of two blazed-array transducers which together provide a horizontal field of coverage of 45°. Each transducer array spreads the ultrasonic energy into its spectrum, as indicated by the “rainbow” colours here. The far field begins roughly 2.25 m away from the sonar face. The strongest fields occur in the near field, near the face of the sonar...... 9 Figure 4 Example ultrasonic field as a function of time observed from the P450-45 ultrasonic imaging sonar. The acoustic pressure is shown above, and the acoustic intensity below. The inset illustrates the ultrasonic (high-frequency) content of the field. The field metrics used for intensity in diagnostic ultrasound are indicated by subscripts ࢀࡼ for temporal peak, ࡼ for pulse average, ࢀ temporal average, and (not shown here) ࡿࡼ spatial peak and ࡿ spatial average (see Table (1) below)...... 10 Figure 5 Schematic of near-field measurements (left) in Cartesian coordinates less than 2 m distance from the sonar, with the sonar looking downward in the calibration barge well; and far- field measurements (right) in cylindrical coordinates greater than 2 m distance, with the sonar looking horizontally at 4 m depth in the calibration barge well...... 19 Figure 6 The spectrogram and time series for a single pulse observed at 4 m along the axis of the sonar. The units are relative, in dB intensity for the spectrogram (-40 dB to 0 dB) in the upper figure, and in volts proportional to the acoustic pressure in the time series in the lower figure...... 21 Figure 7 The horizontal beam pattern for the P450-45 at a distance of 4 m from sonar. The sonar is at 4 m depth. The lighter curves are the beam pattern along 4 m circular arcs displaced vertically in 2 cm steps off the axis of the axis of the sonar...... 22 Figure 8 The vertical beam pattern of P450-45 at a distance of 4 m from the sonar. The sonar is at 4 m depth...... 22 Figure 9 Time series of acoustic pressure at a distance of 0.85 m from the sonar face (near-field set up in Fig. (5)). “Nadir” is on the central axis of the sonar where the acoustic intensity is highest. “Off Axis” are from points 2 to 4 cm off the axis of the sonar...... 23
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Figure 10 The pulse-average acoustic intensity ࡵࡼ in mW/cm2 across the sonar beam, 0.85 m from the face of the sonar (near-field set up in Fig. (5)). Left is ࡵࡼ across a grid of evenly spaced points across the beam. Right is ࡵࡼ along the horizontal line through the peak...... 24 Figure 11 The mechanical index ࡹࡵ computed from direct measurement of the peak rarefaction pressure at points across the beam of the sonar, 0.85 m from the face of the sonar. The highest value is ࡹࡵ ൌ Ǥ ૡ ...... 24 Figure 12 Measurements of the sound pressure level (SPL) along the axis of the P450-45 imaging sonar as a function of distance from the sonar face...... 25 Figure 13 The safe exposure time and distance, conservatively estimated, for a diver on the axis of the P450-45 sonar. Ultrasonic induced heating of the exposed diver is certain to be less than 1.5 °C for exposure times equal to or less than the ࢀࡵ ൌ Ǥ curve in this figure. ࡹࡵ ൏ Ǥ ૢ at all distances from the sonar as generally recommended in diagnostic ultrasound. ࡹࡵ ൏ Ǥ at distances greater than 60 cm from the sonar as recommended when gas bodies are present. The upper bound on the maximum at distances less than 60 cm is ࡹࡵ ൏ Ǥ ૠ...... 30
List of Tables
Table 1 List of ultrasonic intensity metrics adapted from [14]. Each has a role in characterizing the strength of the ultrasonic field...... 12 ι۱ for existing kinds of ultrasonic sonars to which divers may be̴ۯ۾۾܁Table 2 Example values of ۷ exposed (taken from [27])...... 16 Table 3 Selection of Diagnostic Ultrasound Safety Recommendations ...... 17 Table 4 The set of Canadian Ministry of Health Diagnostic Ultrasound recommendations [20] for ࢀࡵǡ ࡹࡵ and ࡵࡿࡼࢀ adopted here for the ultrasonic exposure of divers, plus additional recommendations for ࡹࡵ for tissues with gas bodies (bubbles) [13] and for ࡵࡿࡼࡼ from the USA FDA [22]...... 18 Table 5 Equipment used to characterize the ultrasonic field of the P450-45 imaging sonar...... 18 Table 6 Measurement Uncertainties ...... 19 Table 7 Summary of ultrasonic field characterization for the P450-45 imaging sonar...... 27 Table 8 Summary of thermal and mechanical indices for the P450-45 imaging sonar...... 29
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1 Introduction Imaging sonars provide visual-style real-time imaging of an underwater scene. They operate in the ultrasonic regime, at frequencies on the order of 300 kHz up to 1.5 MHz, depending on the imaging technology used. The high frequencies are required to achieve relatively high acoustic resolution of a scene (typically less than 1 ⁰ beam resolution) in a compact, light-weight low power sonar transducer. They are among highest frequencies used by sonars. There are no known cases of harm or sensation of effect caused to divers by imaging sonars. From medical diagnostic ultrasound, however, it is known that exposure to ultrasonic energy can be harmful. The risks of harm that are of concern in diagnostic ultrasound have never been examined for imaging sonars. Existing recommendations regarding the safety of divers exposed to sonars focus on the effects related to hearing and underwater blasts (references [1] to [5]). None consider the heating or mechanical effects that are of concern in diagnostic ultrasound. The purpose here is to assess and make recommendations about the ultrasonic exposure safety for the Teledyne Blueview P450-45 series imaging sonar (DEN) in Fig. (1), which features in a version of the diver hand-held sonar package made by Shark Marine Technologies (not shown in Fig. 1), which has been deployed for operational use by the Canadian Fleet Diving Unit. The method used for that sonar could be applied to other classes of imaging sonars. The approach followed here builds on that in a preliminary study [6], venturing now into measurement and more detailed analysis. The approach will therefore be to introduce the sonar (Section (1.1)), review the metrics of ultrasonic exposure risk (Section (2)), review and select the safety recommendations from diagnostic ultrasound (Section (3)), characterize the imaging sonar (Section (4)), apply the exposure metrics (Section (5)), and draw conclusions and recommendations (Section (6)).
1.1 The Teledyne Blueview P450-45 Sonar The P450-45 sonar operates in the 300 kHz to 600 kHz frequency band. Two similar blazed (echelon) arrays are collocated in the sonar head as shown in Fig. (2)—one array providing coverage for the right and left horizontal fields of view, roughly 22.5 ⁰ to either side of the sonar axis, as shown in Fig. (3). The blazed arrays are protected and hidden from view by the dome of the sonar face. The active acoustic surface of one single blazed array is approximately 15 cm long by 1.5 cm across. The array includes approximately 40 staves, each on the order of 4 mm X 1.5 cm, organized in a 45⁰ echelon (staircase) that is rotated (roughly 10 ⁰) horizontally about its center. Each array behaves acoustically in many respects like a rectangular array projected across the face of the array, and the double array like a rectangular array of twice as many staves. The patented blazed-array technology [9] splits the broad frequency spectrum into angular beams of different frequency as shown in Fig. (3). In the P450-45 sonar, overlapping 500 kHz beams from both arrays are directed along the axis of the sonar, and 300 kHz beams are directed toward the outer edge of sonar coverage. There is some overlap in the central region between the two beam sets in roughly ± 5⁰ horizontal field of view. In the overlapping central zone along the sonar axis, the 450-500 kHz beams from one array overlap the 500-600 kHz beams of the other. As with sonar arrays generally, the behavior of the ultrasonic field changes between the:
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1. Near Field—close to the sonar face, where the ultrasonic field is strongest, but where the field has not yet separated by frequencies into distinct beams, and where scene imaging is therefore not possible; and the 2. Far Field—starting on the order of 2 to 3 m away from the P450-45 sonar face, where the field has separated by frequencies into distinct beams, and where scene imaging is possible. The ultrasonic exposure in both the near and far field were measured and characterized in this work. The sonar and ultrasonic field metrics used to characterize them are described in the next section.
Figure 1 The Teledyne BlueView P450-45 sonar head [8], the model used by Shark Marine, but operated removed from the Shark Marine unit for better control and operation of the sonar during testing. Two mounting positions were used for anechoic ultrasonic field measurements: horizontal mounting (left) for measurements beyond 2 m distance from the sonar face, and vertical mounting (right) for measurements less than 2 m from the sonar face. The sonar beam direction is indicated by the arrow.
Figure 2 Schematic of the blazed-array transducers behind the sonar face. Each blazed-array is an echelon of staves whose geometry and excitation are phased to direct frequencies toward different angles in the far field in a manner similar to diffraction gratings or prisms.
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Figure 3 Schematic plan view of the ultrasonic field produced by the diver hand-held sonar under evaluation. The field is the superposition of two blazed-array transducers which together provide a horizontal field of coverage of 45°. Each transducer array spreads the ultrasonic energy into its spectrum, as indicated by the “rainbow” colours here. The far field begins roughly 2.25 m away from the sonar face. The strongest fields occur in the near field, near the face of the sonar.
2 Metrics for Ultrasonic Exposure Risk The sound field metrics used here are taken from the sonar [10, 11] and ultrasonic literature in references [13] to [19]. The ultrasonic metrics deal mainly with the ability of the field to disrupt and heat the material through which the ultrasound propagates. x Disruption by sound is mainly through cavitation—the production and excitation of energetic small bubbles whose collapse and oscillation can be destructive to biomaterial. This disruption is correlated with the instantaneous peak under pressure (peak rarefaction) of the ultrasonic field [13, 16]. x Heating is through the absorption of ultrasonic energy—its conversion from sound into heat energy. Absorption is proportional to the intensity (energy density) of the ultrasonic field [12, 15]. Field metrics related to the peak rarefaction, peak and average intensity, and their risk of harm are of interest here.
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Figure 4 Example ultrasonic field as a function of time observed from the P450- 45 ultrasonic imaging sonar. The acoustic pressure is shown above, and the acoustic intensity below. The inset illustrates the ultrasonic (high-frequency) content of the field. The field metrics used for intensity in diagnostic ultrasound are indicated by subscripts ࢀࡼ for temporal peak, ࡼ for pulse average, ࢀ temporal average, and (not shown here) ࡿࡼ spatial peak and ࡿ spatial average (see Table (1) below).
2.1 RMS Pressure and Sound Pressure Level Sonars generally transmit band-limited signals, and the pressure is the time-averaged pressure—more precisely, the root-mean squared (RMS) pressure. In sonar, but not in ultrasonics, it is usual for sonar engineers to report pressure as the sound pressure level ܵܲܮ expressed in decibels (dB) relative to one micro-Pascal, 1-μPa = 10-6 Pa,
ଶ (1) ܵܲܮ ൌ ͳͲ ቀ ௦ ቁ ൌʹͲ ቀ ௦ ቁ ͳɊǤ ଵ ͳͲି ଵ ͳͲି
ܵܲܮ can be estimated in the far field directly using the sonar equation, and it can be measured by experiment, through the measurement of ௦ for individual sonar pulses. ௦ is the pulse RMS in Fig. (4), which illustrates many of the metrics required in this work.
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2.2 Peak Rarefaction (Under) Pressure The risk of harmful mechanical effects due to ultrasonic waves is correlated with the peak rarefaction pressure (see Fig. (4)). In band-limited ultrasonic fields of the kind observed for the P450-45 sonar, consisting of a slowly changing envelop of high-frequency time variation, the magnitude of the temporal ି ା peak rarefaction and peak compression are equal to each other within the accuracy of repeatable measurement,
ି ା ( ൌ ൌ ൌሺݐሻǤ (2 ௧௨௦
The instantaneous will of course be greater than the pulse average ௦. For oscillatory signals, It can be shown that
ሺௌΤሻ ଶ ି ξʹ௦ ൌ ξʹ ൈͳͲ ൈͳͲ Ǥ (3)
More particularly, for a narrow-band sonar, characterized by a single frequency ݂ and bandwidth ο݂ ሺௌΤሻ ଶ equal to one over the pule length ߬, ο݂ ൌ ͳΤ ߬, it can be shown that ൎʹǤͳൈͳͲ ൈ ି ͳͲ . This cannot be used here owing to the wide bandwidth of the P450-45 sonar. was therefore measured directly. By measuring the ultrasonic field of the P450-45 sonar, it was found that, within 1 m distance from the sonar face,
ሺௌΤሻ ଶ ି ൌ ሺʹǤͷͺ͵ േ ͲǤ͵͵ʹሻ ൈͳͲ ൈͳͲ ǡ (4) in which ͲǤ͵͵ʹ is the standard deviation for a set of 408 observations, and the pulse length is ߬ ൌ ͲǤͲͲͳ s. All near-field observations fell below
ሺ୫ୟ୶ሺௌሻΤሻଶ ି ൏ͶൈͳͲ ൈͳͲ Ǥ (5)
This upper bound will be used in Section (5) to characterize an upper bound to the risk posed by the P450-45 sonar. 2.3 Ultrasonic Intensity The intensity ܫ of the ultrasonic field is its power density on a surface perpendicular to the direction of wave travel, expressed in units of acoustic power per unit area, such as W/cm2. In both sonar and ultrasound it is usual to infer the intensity ܫ from the observed acoustic pressure field using the well- established [11,12] far-field equation,
ଶ (6) ܫൌ ൈͳͲିସ ǡ ߩܿ ଶ in which the acoustic pressure (measured) is in units of Pascals (Pa), and for sonar, ߩ ൎ ͳͲͲͲ kg/m3 is the density of water and ܿ ൎ ͳͷͲͲ m/s is the speed of sound in water, and ͳͲିସ is a conversion factor from m2 to cm2.
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Equation (6) applies in the far field, where the acoustic particle velocity (a vector quantity) is aligned with the direction acoustic energy flow. But equation (6) is used by convention in the assessment of exposure levels in the near field of a transducer in medical diagnostic ultrasound [12]. This convention will be used here as well. As in ultrasound generally [14], a number of intensity properties are used here to characterize the ultrasonic field as summarized in Table (1).
Table 1 List of ultrasonic intensity metrics adapted from [14]. Each has a role in characterizing the strength of the ultrasonic field. Intensity Metric Description
Spatial-peak ܫௌ் highest intensity measured at any point in the ultrasound beam and at any temporal-peak time
Spatial-peak ܫௌ highest intensity measured at any point in the ultrasound beam averaged pulse-average over the time duration of the pulse
Spatial-peak ܫௌ் highest intensity measured at any point in the ultrasound beam averaged temporal- over the pulse repetition period average
Spatial- ܫௌ average intensity over a selected area, such as the transducer face, averaged average pulse- over the time duration of pulse average
Spatial- ܫௌ் average intensity over a selected area, such as the transducer face, averaged average over the pulse repetition period temporal- average
The spatial-peak pulse-average intensity ܫௌ occurs at the position in the sonar beam where ܵܲܮ is a maximum,
ͳ (7) ܫ ൌ ͳͲሾ୫ୟ୶ሺௌሻΤሿଶ ൈͳͲିଵ ௌ ߩܿ ଶ
This is a key metric for ultrasonic exposure assessment insofar as it characterizes the maximum possible exposure to ultrasonic wave energy. The average diver exposure is mitigated significantly by the pulse (ping) rate of the sonar. In imaging sonar, the maximum frame rate of the imaging sonar equals the maximum ping rate, which is greater than or equal to the two-way travel time for sound to go from the sonar, out to the maximum range scale setting ܴ௫ of the sonar, and back to the sonar; ݐ ʹܴ௫Τܿ. The duty cycle of active transmission is
ܦൌ݂߬ǡ (8)
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in which ݂ is the ping frequency of the sonar ݂ ൌͳΤ ݐ. The spatial-peak temporal-average intensity over an extended time is then
ܫௌ் ൌ݂߬ܫௌǤ (9)
The spatial-peak temporal-average intensity ܫௌ் is closely related to the thermal index. The Canadian Ministry of Health recommendations for diagnostic ultrasound equipment is that the maximum 2 attainable values for the derated spatial peak time average intensity ܫௌ்Ǥଷ not exceed 720 mW/cm (see Table (3) and (4) below).
ܫௌ் is the highest, localized, momentary intensity in the ultrasonic field. It is used to characterize the field at the focus of an ultrasonic field produced by transducers that have a notable focus in their beam. Unlike many ultrasonic transducers, however, the blazed-array sonar transducer is a linear phased array that is neither designed to produce a narrow focus in its beam, nor does it produce a narrow focus incidentally. One can still assess the ܫௌ், either direct observation through a systematic search for it in the near-field, or indirect inference from measurements and/or modelling. Since ܫௌ் is unrelated to a particular design feature and without notable prominence or expected location, it is less important than for focused ultrasonic transducers.
The ܫௌ and ܫௌ் are the average intensity of a given area, such as the face of the transducer, for the duration of a pulse (the pulse average PA) and the time average across pulses (temporal average TA) as in Table (1) and [14]. Here they are defined as ratio of the pulse-average acoustic power ܹ in mW by the area of the transducer face ܣ in cm2,
ܹ (10) ܫ ൌ ǡ ௌ ܣ ଶ
ܹ ݂߬ (11) ܫ ൌܫ ݂߬ ൌ ǡ ௌ் ௌ ܣ ଶ where ߬ is the pulse length in s, and ݂ is the sonar ping (pulse) frequency in Hz. These are not given explicit recommended limits in the diagnostic ultrasonic medical community, but they provide a simple way to estimate the exposure level at the face of the transducer, where, for imaging blazed-array imaging sonars, the strongest field can be expected, and where the fields are difficult to measure, owing to the strength of the fields and the spatial and temporal variation is relatively high. 2.4 Derating of Intensities and Indices In medical diagnostic ultrasound it is usual to derate the thermal and mechanical indices (see Annex D.2.6 in [13]). Derating is the reduction of an index in proportion to the attenuation that the ultrasonic intensity undergoes with travel into the human body relative to its travel outside of the body, through the water in which the index was assessed. Derating allows for stronger ultrasonic sources than when derating is not used. Derating is not used here because the ultrasonic field is not focused by the transducer to points inside the body, and because the illumination of the diver is largely uncontrolled and unpredictable. This makes the results of the present assessment slightly more conservative than in most medical diagnostic ultrasound assessments.
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2.5 Effective Beam Area, ࢋࢌࢌ, and Effective Cross-Section Beam Dimensions The effective beam area is defined as the ratio
்ܹ ܹ ଶ (12) ܣሺܴሻ ൌ ൌ ǡ ܫௌ்ሺܴሻ ܫௌሺܴሻ in which ܹ and ்ܹ are the pulse-average and temporal average acoustic power in mW, ܴ is the distance from the sonar face along the axis of sonar, and ܫௌሺܴሻ and ܫௌ்ሺܴሻ are assessed at the distance ܴ. The effective beam area gives an indication of the size of the beam where the intensities are highest. Given a field of view that is nominally ܪ ⁰ X ܸ ⁰, the cross-sectional area of the beam is approximately elliptical with major (horizontal) axis ݀ that is ܪȀܸ times the minor (vertical) axis, and area ߨ݀ଶܸܪΤ Ǥ Equating that area with ܣሺܴሻ, gives the effective diameter ݀,
(13) ܪܣ ሺܴሻ ܪ ܹ ݀ሺܴሻ ൌ ඨ ൌ ඨ Ǥ ߨܸ ߨܸ ܫௌሺܴሻ
2.6 The Ultrasonic Thermal Index Ultrasonic waves are potentially harmful through the heating delivered to biomaterial, which depends on the intensity of the ultrasonic sound waves, on the absorption of that energy by the biomaterial, and on the time duration of exposure. The risk of harm is correlated with the intensity of the ultrasonic waves, particularly with the spatial-peak temporal-average intensity ܫௌ். Guidelines for the safe use of ultrasonic waves in ultrasonic diagnostics set maximum limits on the Thermal Index (ܶܫ) defined [12, 20-26] as the energy ܧ of the ultrasonic waves delivered to a location in the body, divided by the energy ܧଵͼ that, under the same test conditions, would raise the temperature of bone or tissue by 1 ⁰C,
ܧ (14) ܶܫ ൌ Ǥ ܧଵͼ
The greatest thermal risk in diagnostic ultrasound is to the developing fetus, whose imaging is often the motivation for ultrasound. The high value of diagnostic imaging during pregnancy has motivated considerable research into the risks posed to the fetus. The National Council on Radiation Protection and Measurements (NCRP, USA) warns that the risks of ultrasound may offset its benefits when the temperature rise at the focal point of the ultrasound beam is calculated to be over 3 degrees Celsius for ten minutes or more [12]. Some diagnostic ultrasound machines used in other applications can cause temperature rises of 6 ⁰C, but this is generally avoided by frequent movement of the ultrasound probe. The bioeffects of heating of adults by ultrasonic exposure are mainly those of hyperthermia (elevated body temperature [24]). Hyperthermia would presumably be unlikely for a calm diver in cold waters, though it may be a concern for divers under heavy exertion or in warm water. The safety recommendations adopted here from diagnostic ultrasound (Section (5) below) are that the maximum
Ultrasonic Exposure Safety Recommendations for High-Frequency Hand-Held Diver Sonars Page 14 of 35 allowed temperature rise due to exposure to ultrasound be less than 1.5 ⁰C. This temperature rise is considered to be safe across a very wide spectrum of elevated body temperatures [26]. Diagnostic ultrasound generally uses three different classes of thermal indices [13, 20]: ܶܫܵ for soft tissue based on three different models of thermal effects arising from three different scanning modes; ܶܫܤ for bone; and;ܶܫܥ for cranial bone. Each is based on a different thermal model of vulnerability (absorption and specific heat) for the biomaterial exposed, different scanning modes, and transducer design. The set covers the cases routinely faced in medicine under very controlled, repeatable conditions. Departures from those conditions require a new model of heating for computing the thermal index, such as the largely uncontrolled conditions of diver exposure to ultrasonic sonar. The method of thermal modelling was developed at length for diagnostic ultrasound in [12, 16]. The method was adapted by MWS to diver exposure in a companion publication [27] in which it is shown that a very conservative estimate of an upper bound on the thermal index due to sonar exposure is given by
ܫ (15) ܶܫ ൏ ௌ ǡ ܫௌ̴ଵι where
ͳ (16) ܫௌ̴ଵι ൌ ଶǡ ሺ ͳͲሻ݂ெு௭݂߬οݐ in which ݂ெு௭ is the ultrasonic frequency expressed in MHz, ߬ is the pulse length in seconds, ݂ is the .(ௌ̴ଵι are given in Table (2ܫ ping frequency in Hz, and οݐ is the exposure time in s. Example values of Equation (16) is a conservative estimate inasmuch as its derivation assumed that: 1. The exposed biological material has an artificially elevated susceptibility to ultrasonic heating, combining the relatively high energy capture (high ultrasonic attenuation) of bone together with the relatively high temperature sensitivity (low heat capacity) of fat. 2. Significant mitigating factors against ultrasonic thermal effects were furthermore ignored, namely: a. the protection against ultrasonic waves that would be provided by a diver’s wet-suit or dry suit and other equipment such as mask and flotation vest; b. the cooling of the diver by immersion in cool or cold water; c. the reduction of exposure level and dwell time by the relative motion of the hand-held sonar beam and the exposed diver; and d. the diffusion and perfusion of heat away from heated areas by conduction and blood circulation in the body. 3. In effect, the exposed diver is assumed to (1) consist entirely of biomaterial that is particularly susceptible to ultrasonic heating, is (2) motionless in a stationary sonar beam, is (3) without a dive suit or equipment, while nevertheless being (4) well insulated (no cooling).
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ι۱ for existing kinds of ultrasonic sonars to which divers may be exposed̴ۯ۾۾܁Table 2 Example values of ۷ (taken from [27]).
2 Center Pulse Ping ࡵࡿࡼࡼ̴ι (W/cm ) Frequency Length Rate
ࢌ ࣎ ࢌ Representative ࡹࡴࢠ Sonar Type (MHz) (s) (Hz) ο࢚ ൌ s ο࢚ ൌ s ο࢚ ൌ s
Acoustic Lens 1.10 0.0005 15 0.877 0.175 0.088 Imaging Sonar [28]
Blazed Array 0.500 0.001 20 0.724 0.145 0.072 Imaging Sonar [29]
Diver Detection 0.070 0.040 0.5 0.517 0.103 0.052 Sonar [30]
2.7 The Ultrasonic Mechanical Index Ultrasonic waves are potentially harmful through the mechanical effects that they may have, which include (1) macrostreaming (induced flow) in vessels filled with a moderately absorbing liquid due to the transfer of momentum from the propagating wave to the liquid, and (2) harmful energy released in the collapse of transient gas bubbles by means of cavitation, causing capillary hemorrhaging in soft tissues [13, 15, 31]. The risk of mechanical harm is correlated with the mechanical index [13, 14, 15, 20]
ି (17) ܯܫ ൌ ̴ெ ඥ݂ெு௭
ି in which ̴ெ is the peak rarefaction (under) pressure from Section (2.2) above, expressed in MPa, and ݂ெு௭ is the ultrasonic frequency expressed in MHz. The safety recommendation ܯܫ ൏ ͳǤͻ from diagnostic ultrasound will be used here to delimit safe ultrasonic exposure so far as mechanical effects are concerned (see Table (3) and (4) below). The mechanical bioeffects of ultrasound generally increase with the presence of bubbles and air cavities [31]. Gas contrast agents (micro-bubbles) are sometimes deliberately used in diagnostic ultrasound to enhance the acoustic signature. For divers, there is a risk of bubble formation in the blood during decompression. The recommendation followed here (from Section (13.13.2) of [13]) is that non-thermal, biological effects are not expected for the fetus and for tissues with stabilized gas bodies when ܯܫ ൏ ͲǤͷ. 3 Safety Recommendations from Diagnostic Ultrasound Leading safety recommendations for diagnostic ultrasound are summarized in Table (3). The table illustrates the plausible range of recommendations that might be used here insofar as, in the absence of other safety recommendations, diagnostic ultrasound recommendations are adopted for diver exposure to ultrasonic sonar. Some recommendations, notably from GBR (BMUS [21]), tabulate safe exposure
Ultrasonic Exposure Safety Recommendations for High-Frequency Hand-Held Diver Sonars Page 16 of 35 levels and exposure times for obstetric and other diagnostic applications. Such refinements might be required if the simpler conservative safety recommendations for diver sonar are deemed to be too restrictive when translated into safe stand-offs during operations (Section (5) below).
Table 3 Selection of Diagnostic Ultrasound Safety Recommendations Reference Recommendations Description
Ahmadi et al [25] ܶܫ ͲǤͶ No observable thermal effects
CAN MoH [20] ܶܫ ͳ Recommended maximum attainable values for diagnostic equipment ophthalmic devices or FDA [22] ܯܫ ͲǤʹ͵ applications. 2 ܫௌ் ͷͲ mW/cm
NCRP, Section ܯܫ ͲǤͷ Non-thermal, biological effects are not expected for (13.13.2) of [13] the fetus and for tissues with stabilized gas bodies.
2 CAN MoH [20] ܫௌ் ʹͲ mW/cm For fetal heart-rate monitor at the transducer face. FDA [22]
CAN MoH [20] ܶܫ ͳ Currently available evidence indicates that the risk of an injury due to ultrasonic heating is negligible. ܯܫ ͳ If ܯܫ ͳ there is a small risk of capillary hemorrhaging in the lung during ultrasound examinations involving exposure of the neonatal and infant chest. If these ܶܫ and ܯܫ are exceeded in training, demonstration or research, then the subject should be informed of the anticipated exposure and how it compares in safety with conditions for normal diagnostic practice.
CAN MoH [20] ܶܫ ͳǤͷ Diagnostic ultrasound of fetus (the most vulnerable of human biomaterial) may be used clinically without Obstetrician & above normal body- reservation on thermal grounds. Gynaecologist [26] temperature levels
CAN MoH [20] ܯܫ ͳǤͻ Maximum attainable values recommended for
2 diagnostic equipment. FDA [22] for ܫௌ்Ǥଷ ʹͲ mW/cm peripheral vessels If ܯܫ ͳ, then there is a small risk of capillary ܫ ͳͻͲ mW/cm2 ௌǤଷ hemorrhaging in the lung during ultrasound examinations involving exposure of the neonatal and infant chest.
FDA [22] ܯܫ ͳǤͻ For cardiac imaging
2 ܫௌ்Ǥଷ Ͷ͵Ͳ mW/cm 2 ܫௌǤଷ ͳͻͲ mW/cm
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FDA [22] ܯܫ Maximum upper limit on diagnostic exposure unless justified by clear reasons. BMUS [21] (general abdominal, peripheral vessel, unlisted applications)
The subset of Canadian Ministry of Health (CAN MoH [20]) recommendations listed in Table (4) will be used here because they are conservative for diver exposure (they focus on the safety of the most vulnerable fetus), and because they have official sanction.
Table 4 The set of Canadian Ministry of Health Diagnostic Ultrasound recommendations [20] for ࢀࡵǡ ࡹࡵ and ࡵࡿࡼࢀ adopted here for the ultrasonic exposure of divers, plus additional recommendations for ࡹࡵ for tissues with gas bodies (bubbles) [13] and for ࡵࡿࡼࡼ from the USA FDA [22]. Thermal Index ܶܫ ͳǤͷ
Mechanical Index ܯܫ ͳǤͻ generally ܯܫ ͲǤͷ when gas bodies present
2 Spatial-Peak Temporal-Average Intensity ܫௌ் ʹͲ mW/cm
2 Spatial-Peak Pulse-Average Intensity ܫௌ ͳͻͲ W/cm
4 Ultrasonic Field of the P450-45 4.1 Experimental Setup The test plan used for characterizing the ultrasonic field of the P450-45 is given in [32]. The equipment, set up, and main results are summarized here. The experimental equipment used is summarized in Table (5). Table 5 Equipment used to characterize the ultrasonic field of the P450-45 imaging sonar. Sonar Under Test Teledyne BlueView P450-45 Imaging Sonar firmware ver 6262, Ser # 1636, controlled using the Teledyne Blueview ProViewer 4 Version 4.3.0.9526.
DRDC –Atlantic DRDC test facilities specially designed for controlled measurements of sound Calibration Barge, fields produced by acoustic sources in sea water under anechoic conditions. Halifax The working well size inside the barge is 9 X 18 m. The depth of the water below the barge is 42 m. The barge is equipped with positioning equipment for stepping through the field produced by a transducer in Cartesian coordinates and cylindrical coordinates around the transducer. Measurements were carried out over a two week time period in Jan 2017.
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Hydrophone Reson TC4038-4, -236.6 dB re 1V/uPa, together with Reson CCA1000 Conditioning Charge amplifier at the hydrophone.
Oscilloscope Tektronix Model 3054C digital oscilloscope, 8 bit resolution (20*log10(2^8) = 48 dB dynamic range "Verniered" for best dynamic range, 10k record length on all channels. A sampling frequency fs = 5 MHz was used throughout.
MatLab Post analysis of recorded signals, including computation of RMS pressure, peak pressure, and frequency-domain analyses using FFTs.
Measurements were carried out (1) in the near field (distances less than 2 m) by mounting the sonar at a depth of 2 m near the middle of the barge well, with the sonar pointing directly downward, and (2) in the far field (distances 2 m to 10 m) by mounting the sonar at a depth of 4 m near one end of the barge well, with the sonar pointing horizontally as shown in Fig. (5).
Figure 5 Schematic of near-field measurements (left) in Cartesian coordinates less than 2 m distance from the sonar, with the sonar looking downward in the calibration barge well; and far-field measurements (right) in cylindrical coordinates greater than 2 m distance, with the sonar looking horizontally at 4 m depth in the calibration barge well.
Measurement uncertainties are summarized in Table (6).
Table 6 Measurement Uncertainties ± ;Positioning Accuracy Cartesian: ± (0.5, 0.5, 0.5) cm relative stepping in ሺݔǡ ݕǡ ݖሻ cm absolute positioning ሺݔǡݕǡݖሻ (0.25 ,0.25 ,0.25) Cylindrical: ± 0.5 cm range; ± 0.1 ⁰ bearing; ± 0.5 cm depth
Hydrophone Calibration ± 0.5 dB
Variability of Acoustic ± 10 % Pressure
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Peak Rarefaction Pressure ± 12 % Measurement Uncertainty
Intensity and Acoustic ± 24 % or ± 1 dB Power Measurement Uncertainty
Background Noise Levels Below the least significant bit across the sonar operating frequency band.
4.2 P450-45 Ultrasonic Pulses Observations of the ultrasonic field (time series of the acoustic field pressure) are shown Fig. (4) and (6). The main features to note here are that (1) the ultrasonic field is a slowly varying envelope of high frequency, ultrasonic content, and (2) that the field is not impulsive (without prominent peaks or spikes in the acoustic pressure; hence typical of band-limited signals). Features of interest for ultrasonic exposure are especially (see Fig. (4)) the: 1. Peak rarefaction (under pressure), which, is assessed in ultrasonic exposure safety owing to the risk of harm by the cavitation that strong rarefactions can cause; and 2. Mean intensity of the pulse, averaged over the duration of the pulse. The average exposure over extended time depends in part on the ping-rate of the sonar. The ping rate of the P450 series sonar can be software adjusted. It was reduced for the purposes of experimentation to about 10 Hz. The specified frame-rate for the commercial model is 12 Hz, indicating that its ping rate its 12 Hz, but a more conservative (for ultrasonic exposure) ping rate of 20 Hz will be assumed here where necessary. The pulse length for the sonar is fixed at roughly 1 ms (milliseconds). The pulse uses frequency modulation, going from 600 kHz at the start of the pulse to 300 kHz at the end (see on-axis far-field spectrogram in Fig. (6)). In the far field, at 4 m radius from the sonar, the bandwidth of the central frequency directed along an angular beam changes somewhat with center frequency (beam angle), but is was observed to be roughly 20 kHz, corresponding to an effective pulse length for the frequency in an angular beam of 0.05 ms.
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Figure 6 The spectrogram and time series for a single pulse observed at 4 m along the axis of the sonar. The units are relative, in dB intensity for the spectrogram (-40 dB to 0 dB) in the upper figure, and in volts proportional to the acoustic pressure in the time series in the lower figure.
4.3 P450-45 Far-Field Beam Pattern The far-field horizontal and vertical beam patterns observed for the sonar are shown in Fig. (7) and Fig. (8). In sonar, as elsewhere, the beam width is usually determined by the angles at which the relative beam strength is 3 dB below its maximum. Thus the beam width is roughly 6 ⁰ H X 5 ⁰ V. Most of the ultrasonic power lies in this beam. Note that the sonar field of view 45 ⁰ H X 10 ⁰ V is wider than the beam width. Unlike the total field of view of the sonar, then, the energy of the ultrasonic beam lies in an almost circular beam cross section. Most important for the present analysis is the fact that, in the far field, the beam is strongest on its central axis. The highest ultrasonic exposures will be experienced by divers along the axis of the sonar.
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Figure 7 The horizontal beam pattern for the P450-45 at a distance of 4 m from sonar. The sonar is at 4 m depth. The lighter curves are the beam pattern along 4 m circular arcs displaced vertically in 2 cm steps off the axis of the axis of the sonar.
Figure 8 The vertical beam pattern of P450-45 at a distance of 4 m from the sonar. The sonar is at 4 m depth.
4.4 P450-45 Beam at Close Range (0.85 m) The beam was characterized in the near field at a distance of 0.85 m from the face of the sonar. Fig. (9) shows observed time series, and Fig.(10) shows the pulse-average ultrasonic intensity ܫ at a regular grid of points (X,Y) in a plane perpendicular to the axis of the sonar. A pulse length of ߬ ൌ ͲǤͲͲͳ s was assumed in the computation of ܫ. 2 The spatial-peak pulse-average intensity ܫܵܲܲܣ ൌ ʹͳͲ mW/cm at this range occurs at the brightest 2 central point. The effective beam area is ܣ݂݂݁ ൌ ͳ͵ͺ cm (see equation (12)), and the effective beam diameter is 13 cm (± 6.5 cm in the figure).
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For a sonar ping rate of ݂ ൌʹͲ Hz, at 0.85 m on axis from the sonar face, the spatial-peak temporal- 2 average intensity ܫܵܲܶܣ ൌܫܵܲܲܣඥ݂߬ ൌ͵Ͳ mW/cm . The value will generally be lower at distances greater than 0.85 m from the sonar face. The value should be compared with the medical diagnostic 2 ultrasound recommendation in Section (3) above that ܫௌ் ʹͲ mW/cm . Section (5) below will treat the value of ܫௌ் in the near field more generally, at distances less than 0.85 m. Figure (11) shows the mechanical index ܯܫ, computed from measurement of the peak rarefaction pressure at points across the beam of the sonar, 0.85 m from the face of the sonar. At 0.85 m on axis from the sonar face, the highest value is ܯܫ ൌ ͲǤʹͺ. The ܯܫ will generally be less than this at larger distances from the sonar. This value should be compared with the recommendations (Section (3)) that ܯܫ ൏ ͳǤͻ. Section (5) will treat the value of MI in the near field more generally, at distances less than 0.85 m.
Figure 9 Time series of acoustic pressure at a distance of 0.85 m from the sonar face (near-field set up in Fig. (5)). “Nadir” is on the central axis of the sonar where the acoustic intensity is highest. “Off Axis” are from points 2 to 4 cm off the axis of the sonar.
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2 Figure 10 The pulse-average acoustic intensity ࡵࡼ in mW/cm across the sonar beam, 0.85 m from the face of the sonar (near-field set up in Fig. (5)). Left is ࡵࡼ across a grid of evenly spaced points across the beam. Right is ࡵࡼ along the horizontal line through the peak.
Figure 11 The mechanical index ࡹࡵ computed from direct measurement of the peak rarefaction pressure at points across the beam of the sonar, 0.85 m from the face of the sonar. The highest value is ࡹࡵ ൌ Ǥ ૡ
4.5 P450-45 Field on Axis (sonar face to 10 m) All of the measured, on-axis RMS acoustic pressures are plotted in Fig. (12) for a pulse length of ߬ൌ ͲǤͲͲͳ s, when the pressure is converted to sound pressure level ܵܲܮ in dB per μPa (see equation (1)). In sonar terms, viewed from along the axis where the ultrasonic field is strongest, the sonar has source level ܵܮ of ܵܮ ൌ ʹͳʹ dB ±1 dB re 1-μPa at 1 m. At axial distances ܴ greater than 1 m, the field is that of a point source
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ܵܲܮ ൌ ܵܮ െ ʹͲ ଵ ܴ െ ߙܴͳɊǡ (18) in which ߙ is the attenuation by sea water, ߙ ൎ ͲǤͳͲ dB/m at 450 kHz. The attenuation is on the order of 1 dB at 10 m, which is small enough to ignore here. By inspection of Fig. (12), the model fits the measured data down to an axial distance of 0.4 m where the model gives ܵܲܮ ൌ ʹʹͲͳɊ.
Figure 12 Measurements of the sound pressure level (SPL) along the axis of the P450-45 imaging sonar as a function of distance from the sonar face.
4.6 P450-45 Pulse-Average Acoustic Power ࢃࡼ and Average Intensity at the Sonar Face
The pulse-average acoustic power ܹ ൌʹͻ W ± 7 W was calculated from the acoustic intensity over a large, dense regular grid of measurement points in the far field, at 4 m from face of the sonar (not shown here).
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The projected area of the two blazed arrays inside the sonar onto the sonar face is 12 cm horizontally X 3 cm vertically. Thus the spatial-average pulse-average intensity at the face of the sonar (see (10) above) is
ܹ ʹͻ (19) ܫௌ ൌ ൌ ଶ ൌ ͶͶ ଶǤ ܣ௬௦ ͳͷ ൈ ͵
The pulse length was taken to be ߬ ൌ ͲǤͲͲͳ s for these measurements. For a ping rate of ݂ ൌʹͲ Hz, this gives
(20) ܫ ൌ݂߬ ܫ ൌ ͲǤͲͲͳ ൈ ʹͲ ൈ ͶͶ ൌ ͳʹǤͻ Ǥ ௌ் ௌ ଶ
This spatial-average temporal-average intensity at 0.85 m is significantly less than the recommendation 2 in medical diagnostic ultrasound ܫௌ் ʹͲ mW/cm (Table (1) above). It is also less than allowable 2 limits on a fetal heart rate monitor ܫௌ் ൎ ʹͲmW/cm . 4.7 P450-45 Exposure Model A computer model of the acoustic field from twin-blazed array transducers in the P450-45 was developed to investigate the near field of the sonar more widely than could be done experimentally. A ray model widely used in sonar modelling (BELLHOP) was considered. It was found to be suitable for distances beyond 1 m, in effect replicating the ܵܲܮ in Fig. (13) for ܵܮ ൌ ʹͳʹ dB, giving ܵܲܮ ൌ ʹͳʹ െ ʹͲ ଵ ܴ dB re 1-μPa, hence with no need to include its modelling results here. But the ray model fails in the near field because it always treats acoustic sources as ideal point sources, whose near field ܵܲܮ exhibits the unbounded growth (ଵ ܴ singularity) as ܴ՜Ͳ, which is not realistic for array transducers. Another model was implemented using the Boundary Element Method (BEM) [33, 34]. BEM models estimate the acoustic radiation from transducers of any shape in three dimensions, by first of all estimating the field directly on the surface of the transducer (here the two blazed arrays), and secondly estimating the field then at any points off the transducer (in the ultrasonic beam for instance). The BEM method therefore operates throughout the near field, up to the transducer face, and throughout the far field. Each blazed array was modelled using the echelon transducer geometry for this sonar. The left- and right-directed arrays were stacked one on top of the other, and each was given a horizontal rotation about its center, to direct its 500 kHz beam along the axis of the sonar (see Fig. (2) and (3)). The acoustic normal velocity uniformly assigned to all staves was adjusted so that the total power radiated into the ultrasonic beam equaled that observed for the P450-45. The frequency band 300 kHz to 600 kHz was modelled, and the time-domain signals were modelled using synthesis across the frequency band for frequency-modulated transmit pulses of the kind shown in the spectrogram for the P450-45 (Fig. (6) above). The model was used to explore the ultrasonic field in more detail, confirming (1) that the maximum field (intensity and peak rarefaction) lies on the axis of the sonar, and (2) there is no focusing of the ultrasonic field onto a point. The measurements suffice, without augmentation through further modeling when these conditions are true. The model could be applied to other sonars whose ultrasonic exposures are to be evaluated alongside, or in lieu of experimental measurement.
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5 P450-45 Ultrasonic Exposure Summary Measurements with the sonar indicate that the exposure throughout the field of field of the sonar can be conservatively modelled by two exposure zones: 1. Near zone: Ͳ൏ܴ൏ͲǤͶ m, where the sound pressure level is conservatively taken to be
ܵܲܮ ൌ ʹʹͲ dB re 1-μPa; and (21)
2. Mid zone: ͲǤͶ ൏ ܴ, where the sound pressure level is conservatively taken to be the axial
ܵܲܮ ൌ ʹͳʹ െ ʹͲ ଵ ܴ dB re 1-μPa. (22)
The zones are indicated in Fig. (12). Table (7) characterizes the ultrasonic field in the two zones for exposure purposes. Table (8) assesses the exposure level in the two zones. It is clear from Table (8) that, at any distance from the sonar, the Mechanical Index ܯܫ falls within the safety recommendations ܯܫ ൏ ͳǤͻ adopted from medical diagnostic ultrasound for tissues without gas bodies (bubbles). For tissues with gas bodies, however, the recommendation ܯܫ ൏ ͲǤͷ is, by the conservative estimates applied in this exposure model, is not met at distances less than 60 cm from the sonar face. Some care regarding exposures during diver decompression is therefore advisable, namely, to avoid exposures at less than 60 cm from the sonar face. For exposure durations shorter than 1 minute, moreover, the thermal index ܶܫ falls within the safety recommendations ܶܫ ൏ ͳǤͷadopted from medical diagnostic ultrasound in both the near and mid zones of the sonar. The conservative safety recommendations are exceeded for longer exposures, however, such as for 10 minute exposures in the near zone (ܴ൏ͲǤͶ m). Figure (13) plots the maximum exposure time on the axis of the sonar without exceeding the conservative safety recommendation for the thermal index.
Table 7 Summary of ultrasonic field characterization for the P450-45 imaging sonar.
Ref. in Near Zone Mid Zone this Metric Report ࡾ൏Ǥ m Ǥ ࡾ m Safe Values
Nominal Bandwidth Section 300 to 600 kHz Center frequency ranging N/A between 300 to 600 kHz kHz (1.1) across horizontal field of view, 500 kHz on axis, with roughly 20 kHz bandwidth
Pulse Length Figure 1 1 1 N/A ms Figure 6 Section (4.2)
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Effective Source Figure N/A ܵܮ ൌ ʹͳʹ N/A Level for Pulse 1 12 dB re 1μPa at 1 m
Average Sound Section ܵܲܮ ൌ ʹʹͲ ܵܲܮ ൌ ʹͳʹ െ ʹͲ ଵ ܴ N/A Pressure Level in (2.1) Pulse 1 dB re 1μPa
Pulse-Average Section ܹ ൌ ʹͻͲͲͲ ܹ ൌ ʹͻͲͲͲ N/A Acoustic Power 1 (4.6) mW
Temporal-Average ்ܹ ൌ ͷͺͲ ்ܹ ൌ ͷͺͲ N/A Acoustic Power 2 mW
ሺௌିଵଶሻ Beam: Effective Section ି N/A ܣ ൌߩܹܿͳͲ ଵ Cross-Section Area (2.5) ଶ ܣ ൌ ʹͶܴ 2 ܣ ൌ45 cm Eq. (12)
Beam: Effective Section N/A ܣ Diameter (2.5) ݀ൌඨ ߨ cm Eq.(13) ݀ൌͻǤ͵ܴ ݀ൌ͵Ǥͺ
Spatial-Peak Pulse- Section ௌΤ ଵ ܫ ൏ ͳͻͲ ͳͲ ିଵଷ ௌ Average Intensity 1 (2.3) ܫௌ ൌ ൈͳͲ ߩܿ W/cm2 ܫௌ ൌ ͲǤ ଶ ܫௌ ൌ ͲǤͳͲΤ ܴ
Spatial-Peak Section ܫௌ் ൌܫௌ݂߬ Temporal-Average (2.3) ܫ ൌʹǤͳʹܴΤ ଶ ܫ ൏ ʹͲ Intensity 2, 3 ܫௌ் ൌͳ͵Ǥ͵ ௌ் ௌ் mW/cm2 Notes: 1 1 ms pulse length assumed 2 ping rate of 20 Hz assumed 3 2 compare typical fetal heart rate monitor delivering ܫௌ் ൎʹͲ mW/cm .
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Table 8 Summary of thermal and mechanical indices for the P450-45 imaging sonar.
Ref. in Near Zone Mid Zone this Safe Metric Report ࡾ൏Ǥ m Ǥ ࡾ m Values
Thermal Index Section ܫ ܶܫ ௌ (2.6) ܫ 1 minute exposure ௌభι
ͳ ܫௌ̴ଵι ൌ ൌ ʹͶ ଶ ሺ ͳͲሻ݂ெு௭݂߬οݐ
ܶܫ ͲǤͻʹ ͲǤͳͶ ܶܫ ൏ ͳǤͷ ଵ ܶܫ ଵ ܴଶ
Thermal Index Section ͳ ܫ ൌ ൌ ʹǤͶ (2.6) ௌ̴ଵι ଶ minute exposure ሺ ͳͲሻ݂ெு௭݂߬οݐ 10
ܶܫ ͻǤʹ ͳǤͶ ܶܫ ൏ ͳǤͷ ଵ ܶܫ ଵ ܴଶ
Mechanical Index Section ܯܫǤ଼ହ 0.28 N/A ܯܫ ൏ ͳǤͻ ܯܫ ܴ ൌ ͲǤͺͷ m (2.7) generally ܯܫ ൏ ͲǤͷ gas bodies present
Mechanical Index Section ͶൈͳͲௌΤ ଶ ൈͳͲିଵଶ ܯܫ at other ranges (2.7) ܯܫ ൏ ඥ݂ ܴ ெு௭
ͶൈͳͲௌΤ ଶ ൈͳͲିଵଶ ܯܫ ܯܫ ൏ ͳǤͻ ܯܫ ൏ ͶൈͳͲሺௌିଶ ୪୭ ோሻΤଶ ൈͳͲିଵଶ generally ඥ݂ெு௭ ൏ ଶଶΤ ଶ ିଵଶ Ͷ ൈͳͲ ൈͳͲ ඥ݂ெு௭ ܯܫ ൏ ͲǤͷ ሺଶଵଶିଶ ୪୭ ோሻΤଶ ିଵଶ gas bodies ξͲǤ͵ͲͲ Ͷ ൈͳͲ ൈ ͳͲ ൏ present ͲǤ͵Ͳ ξͲǤ͵ͲͲ ͲǤʹͻͳ ܴ
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Figure 13 The safe exposure time and distance, conservatively estimated, for a diver on the axis of the P450-45 sonar. Ultrasonic induced heating of the exposed diver is certain to be less than 1.5 °C for exposure times equal to or less than the ࢀࡵ ൌ Ǥ curve in this figure. ࡹࡵ ൏ Ǥ ૢ at all distances from the sonar as generally recommended in diagnostic ultrasound. ࡹࡵ ൏ Ǥ at distances greater than 60 cm from the sonar as recommended when gas bodies are present. The upper bound on the maximum at distances less than 60 cm is ࡹࡵ ൏ Ǥ ૠ.
6 Conclusions and Recommendations for Use Tables (7) and (8) and Fig. (13) place the P450-45 sonar field into the context of ultrasonic exposure risk in diagnostic ultrasound. The following conclusions can be drawn: 1. The main result is Fig. (13). It shows that: a. at all distances from the sonar, the ultrasonic effects fall within the safety recommendations for mechanical effects (mechanical index ܯܫ ൏ ͳǤͻ), and for thermal effects under continuous exposure of 1.6 minutes or less (thermal index ܶܫ ൏ ͳǤͷ);
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b. at distances greater than 1 m from the sonar, the thermal effects for exposure times up to 10 minutes fall within the safety recommendations (thermal index ܶܫ ൏ ͳǤͷ); and c. in the event of bubble formation in the diver body during decompression, at distances greater than 0.60 m from the sonar, the ultrasonic mechanical effects fall within the safety recommendations (ܯܫ ൏ ͲǤͷ) (no change in thermal effects due to bubble formation) 2. The mechanical effects of the ultrasonic field produced by the P450-45, as assessed by the mechanical index ܯܫ, are significantly less (ܯܫ ൏ ͲǤ͵Ͳሻ at all distances from the sonar face than the recommended maximum in diagnostic ultrasound (ܯܫ ൏ ͳǤͻሻ. The risk of mechanical modes of harm from individual single sonar pulses is therefore less than or comparable to the acceptably low risk that an obstetric patient faces during diagnostic ultrasound. 3. The mechanical effects of ultrasound increase when gas bodies are present in the body, which may occur during times of diver decompression. When gas bodies are present, the recommendation is ܯܫ ൏ ͲǤͷ, which is true for distances greater than 60 cm from the sonar face. Diver exposure at closer ranges should be avoided during decompression. 4. The thermal effects of the ultrasonic field produced by the P450-45, as conservatively assessed by the thermal index ܶܫ [27], leads to the maximum exposure times as a function of distance for ܶܫ ൌ ͳǤͷ in Fig. (13) as summarized here: a. The exposure is highest close to the sonar face, at distances less than 40 cm, where a ͵ൌ ͳǤͷ is οݐ ൌ ͳǤ ܫܶ conservative estimate of the maximum safe exposure time for minutes. b. The exposure decreases quickly with increasing distances ܴ from the sonar face, with .ሺܴǡ οݐሻ ൌ (1 m, 10 min) and (2 m, 40 min) for instance 5. The strongest ultrasonic exposures occur along the axis of the sonar beam, with higher exposures closer to the sonar face. 6. The intense part of the P450-45 sonar beam is much smaller than the field of view of the sonar. The cross-sectional diameter of the intense portion of the sonar beam is given by the effective diameter ݀ in Table (7). At a distance of 10 m, for instance, the effective diameter of the sonar beam is 93 cm, corresponding to ±2.6 ⁰ beam width. 7. The eye is known to be vulnerable to ultrasonic exposure (see ophthalmic imaging in Table (3)). Direct exposure of the eye was not considered to be a safety risk for divers properly wearing a diver’s mask. The safety recommendations should be revisited if exposures without a mask can be expected during operations. Divers should be cautioned to avoid direct exposure of the eyes to the sonar beam close to the sonar. The safety assessment of the thermal index is considered to be conservative (erring on the side of caution) inasmuch as: 8. The safe exposure regime (ܶܫ ൏ ͳǤͷ) was taken from medical diagnostic ultrasound applied to the particularly sensitive developing fetus during pregnancy. 9. The model [27] of the power required to raise temperature ͳ ⁰C assumes relatively large attenuation (highest energy capture) for bone and the relatively low heat capacity (highest temperature sensitivity) for fat, in order to conservatively set an upper bound on ܶܫ with no
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need then to consider (as in diagnostic ultrasound) where the ultrasonic illumination falls and passes through the body; 10. Significant mitigating factors against ultrasonic waves and their thermal effects were ignored in the model [27] of the power required to raise temperature ͳ ⁰C, namely: a. the protection against ultrasonic waves that will be provided by a diver’s wet-suit or dry suit and other equipment such as mask and flotation vest; b. cooling of the diver by immersion in water; and c. the time variation, and hence reduced average exposures, that can generally be expected owing to the motion of both the hand-held ultrasonic sonar beam and of the exposed diver. These were excluded here owing to (1) the lack of data on the protection that different suits and equipment provide, (2) the significant analytic complexity that these mitigating factors introduce, and (3) the large scope that the present conservative safety recommendations already afford to operations. 11. In effect, then, the sonar and exposed diver were very conservatively assumed to be stationary, and the diver is furthermore assumed to be d. diving without a suit or equipment, and all ultrasonic energy enters the diver body without reflection; e. the diver body consists of an artificial biological material whose thermal sensitivity to ultrasonic waves is somewhat higher than any part of the human body, and f. the water is at body temperature and perfectly still.
The safety assessment of the mechanical index ܯܫ is considered to be conservative (erring on the side of caution) inasmuch as: 12. In the construction of Fig. (13), the observed (measured) relationship between the peak rarefaction pressure and the RMS pressure for the ultrasonic sonar field was conservatively applied in the form of the upper bound (5). 13. Mitigating effects of the diver suit and equipment were ignored in the estimates. Some final remarks are: 14. One could take steps to remove the conservative assumptions used here in the thermal index, but there is no need to do so if the present constraints on exposure level and time do not interfere with dive operations with a sonar. If the conservative safety limits do interfere with diver operations, then the safety limits should be revisited and adjusted, taking into account of the mitigating that cooling by water temperature difference and flow will have, as well as the protection that a wet suit or dry suit provide. 15. Very close proximity of a diver to the face of the sonar will certainly ruin the sonar image of a more distant scene. No scene imaging or navigation could be carried out with a diver within 1 m of the sonar face, for instance. There is no reason to operate the sonar with a diver continuously within that proximity for an extended period of time.
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16. The P450-45 has no ability to image scenes closer than 1 m and therefore cannot be used to image anything at closer distances to the sonar face. Thus the P450-45 sonar could not be used to help one diver assist or work with another diver at distances less than 1 m. 17. The developing fetus is most vulnerable to ultrasonic exposure, but ultrasonic diagnostics are nevertheless judiciously used to directly expose and image the fetus. The same safety recommendations used in that diagnostic ultrasound were applied here to adult divers. Much as ultrasonic exposure of the fetus for any purpose other than diagnostic imaging is not recommended [20], so too the exposure of divers who may be pregnant is not recommended. 18. Fresh water is less attenuating of ultrasound than seawater. Slightly stronger fields are expected in freshwater, but these effects will be negligibly small over the small distances, close to the sonar face, where the ultrasonic field poses its risk. 19. Detailed modelling (Section (4.7)) assisted understanding of the blazed-array transducer operation, its beams, frequency content and time-domain signal. The detailed model results were not included here owing to the completeness of the measurements. The model could be used for other ultrasonic sonar technologies and makes. 20. Ultrasonic fields can be reflected from hard flat surfaces, redirecting the beam, reflecting back toward the diver using the sonar for instance. Strong reflections of this kind will be evident in the sonar imagery of a scene. The divers should be made aware of the possibility of redirection of the sonar beam by reflection. 21. The ultrasonic field was tested at points behind the sonar, confirming that the diver using the sonar is not exposed to rearward ultrasonic exposure.
7 Acknowledgements The manager, Ricky Vienneau, of the DRDC – Atlantic Calibration Barge, where the field of the imaging sonar was measured, provided excellent support to the collection of the data used in this report.
8 References 1. M.A. Ainslie, "Review of published safety thresholds for human divers exposed to underwater sound", TNO Rreport TNO-DV 2007 A598, April 2008. 2. NATO Undersea Research Centre Human Diver and Marine Mammal Risk Mitigation Rules and Procedures, NURC-SP-2006-008, September 2006. 3. Effects of Intense Water-Borne Sound on Divers. Prepared by Naval Submarine Medical Research Laboratory, Groton, CT, Department of the Navy, 1996. 4. S.J. Parvin, E.A Cudahy & D.M Fothergill, "Guidance for diver exposure to underwater sound in the frequency range 500 to 2500 Hz", Proceedings of Underwater Defence Technology (UDT) (2002). 5. E. Cudahy and S. Parvin, "The effects of underwater blast on divers", Naval Submarine Medical Research Laboratory, NSMRL Report 1218, 08 February 2001
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