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A pvdf membrane hydrophone for operation in the range 0.5 MHz to 15 MHz
A pvdf membrane hydrophone for operation in the range 0.5 MHz to 15 MHz K.C. SHOTTON,
D.R. BACON and R.M. QUiLLlAM
A new form of hydrophone for measuring the spatial and temporal distributions of pressure within the fields from medical ultrasonic equipment is described. The device comprises an acoustically transparent plastic membrane with a small, central region activated to provide a freely-suspended piezoelectric element. This form of detector offers several advantages over conventional hydrophone probes employing ceramic elements, in particular a flat frequency response. The performance characteristics of a prototype membrane hydrophone and some possible developments and applications are described.
Introduction
Conventional
There is an increasing demand for quantitative measurements to be performed on the fields emitted by medical ultrasonic equipment. As no method is available for characterizing fields within the patient’s body, such measurements are normally carried out on the fields generated by medical transducers radiating into a water-filled test tank. Total time-averaged output powers from the transducers can then be measured by reference to any of a number of physical properties of ultrasonic fields, for example the radiation force exerted on a reflecting or absorbing target, but measurements of the spatial and temporal distributions of acoustic pressure within the fields are in general more difficult.
Miniature hydrophones currently available, either commercially or in the form of design and construction details in the technical literature, comprise typically a disc of piezoelectric ceramic mounted on the end of a rod or cone, the latter providing both mechanical support and acoustic backing. The disc is mounted with its surfaces perpendicular to the intended direction of propagation of incident acoustic waves. To achieve adequate spatial resolution and good directional characteristics, a small disc must be used, typically one of less than 1 mm in diameter. Further, to achieve the required frequency response, the disc must be no thicker than a few hundred pm, ensuring that the fundamental thickness mode resonance is at a frequency beyond the range of interest, a typical resonance frequency being between 20 MHz and 30 MHz. Unfortunately the constraint imposed on the diameter of the disc by the need to limit the directionality of the hydrophone response leads to the presence of a fundamental radial mode resonance at a frequency of a few MHz, and this may well affect the overall dependence of the hydrophone’s acoustic sensitivity on frequency.
The most widely accepted method of quantifying these distributions is the use of miniature piezoelectric hydrophone probes. To obtain accurate information on the fields from medical ultrasonic equipment by this means, however, hydrophones are needed that exhibit a flat response in terms of pressure sensitivity against frequency over the output spectrum likely to be encountered, say 0.5 MHz to 15 MHz, and possess a response characteristic that is insensitive to the propagation direction of the incident wave and free from reverberation effects. Although most medical equipment has an operating frequency of between 2 MHz and 8 MHz, a hydrophone frequency response flat to 15 MHz is required to obtain accurate representations of acoustic waveforms in the short-pulse fields used, for example, in most imaging equipment. K.C. Shotton and D.R. Bacon are at the Division of Radiation Science and Acoustics, The National Physical Laboratory, Teddington, Middlesex, UK; R.M. Quilliam is at GEC-Marconi Electronics Ltd. Marconi Research Laboratories, Great Baddow, Essex, U.K. Paper received 22 October 1979. Revised 28 November 1979.
0041-624X/80/0301 ULTRASONICS.
MAY 1980
hydrophone
design
Some of the physical characteristics of the piezoelectric ceramic materials themselves lead to unavoidable defects in the performance of conventional probes. The high acoustic impedances of such materials (2.9 x 10’ kg m”s-’ for PZTSA' , for example, compared with 1.5 x lo6 kg m-*sW1 for water) mean that the probe perturbs the acoustic field it is used to measure. Secondly, to reduce acoustic reflection from the back face of the piezoelectric disc and so obtain a broad frequency response, a matching high impedance backing material must also be used. This again interferes with the acoustic field, and energy may well reach the active
23-04 $02.00 0
1980 IPC Business Press 123
element as longitudinal or shear waves propagating within the backing material itself, giving rise to undesirable reverberations in the received signal. Further, the brittle nature of ceramics, and the difficulty likely to be experienced in bonding them reproducibly to other materials, makes the manufacture of probes from small ceramic elements an exacting and time-consuming procedure. Finally, good longterm stability in the sensitivity of ceramic hydrophones, particularly when immersed in water for prolonged periods, has proved difficult to achieve.
The membrane hydrophone In this paper we describe an alternative approach to hydrophone design proposed at the National Physical Laboratory, and report some of the initial results obtained with prototypes constructed under a contract placed by the Metrology and Standards Requirements Board with GEC-Marconi Limited in Chelmsford, UK. The design exploits the properties of polyvinylidene fluoride (pvdf), a piezoelectric plastic polymer which has already proved to be useful as a transducer material over a wide range of frequencies and operating modes; see for example the references2”. In the new form of hydrophone, a prototype of which is shown in Fig. 1, the pvdf is used as a thin, acoustically transparent membrane, stretched over an annular frame large enough to allow the entire ultrasonic beam to pass through the central aperture. A small central region of the membrane is coated on both surfaces with metal film electrodes, and is poled to induce piezoelectric properties only within that region. The device, therefore, may be regarded as a small sensing element, suspended freely in the ultrasonic field, which responds to the local pressure fluctuations associated with the passage of ultrasonic waves. Metal film contacts are necessary to carry the signal generated by the piezoelectric element out of the region of the field, but it is believed that these may be made sufficiently thin not to affect significantly the acoustic transmission characteristics of the membrane, This form of detector offers a number of potential advantages over the conventional device based on ceramic piezoelectric elements. Firstly pvdf can be fabricated as film with thicknesses down to a few pm, offering devices with fundamental mode resonances at frequencies far above the range of interest. Secondly, the frequencies of transverse or radial modes of the membrane are related to the diameter of the entire device rather than that of the active element alone, and will be well below the 0.5 MHz lower limit of interest. Thirdly, since the acoustic impedance of pvdf (4.1 x lo6 kg rne2se1 from our own and published values of density6 and velocity7) is more closely matched to that of water than is the impedance of ceramic materials, the acoustic reflection coefficient at the surface of the membrane will be low and the device will exhibit only low Q resonance effects and hence a broad frequency response. Further the piezoelectric properties of pvdf are such that the sensitivity of the device as a detector is likely to be comparable with, or better than, that normally achieved with ceramic elements, typically 0.1 /.LVPa-’ to 1 E.IVPa-‘. Finally, there can be no problems associated with the presence of an acoustic backing material, and it should be possible to reduce any field distortion due to the presence of the device to a negligible level. Membrane hydrophones should possess, therefore, a broad, flat frequency response free from the effects of reverberation associated with conventional backing and mounting configurations.
124
Fig. 1
The prototype
membrane
hydrophone
A possible disadvantage of this form of detector is the relatively low capacitance of a pvdf element made sufficiently small to provide good directional characteristics and spatial resolution. Since it is essentially charge that is generated by pressure on the active element, the voltage signal will be reduced by lead, cable and amplifier input capacitances. Another disadvantage is that for broadband response characteristics the membrane thickness should be much less than half the wavelength of ultrasound at the highest component frequency, not only to avoid interference from thickness mode resonances, but also to ensure that the membrane appears acoustically transparent over the required frequency range, and that it effectively senses the pressure at a point within the acoustic waveform. An extremely thin membrane, however, is likely to puncture or stretch in routine use and may not be suitable as the basis of a simple, convenient and robust device. Finaily, in its simplest form the membrane hydrophone requires thin film leads to carry the electrical signal across the membrane to a point on the supporting frame outside the field where connections to a conventional screened cable may be made. With such exposed leads in the water of the test tank it is likely to be difficult to avoid some electrical pick-up problems. The prototype
design
In drawing up the specifications of a prototype hydrophone, it was our intention to design a unit suitable for assessing the extent to which the potential advantages of a membrane device could be realized in practice and to obtain information on the likely importance of the disadvantages, rather than to attempt to produce immediately a hydrophone with optimum performance characteristics. The first prototype uses pvdf sheet of 25 pm thickness with a 4 mm diameter active element, a design chosen to provide a rugged device with an active element of reasonable sensitivity and capacitance. The membrane is supported on a perspex ring of approximately 100 mm diameter, and the metal film leads to the central electrodes on either side of the membrane are well separated to minimize load capacitance and to prevent the region between the leads becoming piezoelectrically active during the poling process. The poling itself is carried out at a temperature in excess of 100°C and with a field of approximately 1MV cm-‘. Finally, the metal film leads are connected to a 50 fi coaxial lead with conducting epoxy adhesive, and the connection potted in epoxy resin for insulation and to provide good mechanical strength.
ULTRASONICS
. MAY 1980
During initial assessment trials, prototypes of this new form of hydrophone have performed well. When used in pulsed fields, the devices gave a clean signal free from reverberation effe:cts (Fig. 2). Electrical pick-up experienced mainly at 50 Hz and in the MHz region was reduced to a few mV by ensuring that the coaxial screen of the signal lead was connected to a conductor in good contact with the distilled water in the test tank, and sensitivities of approximately 1 r.lV Pa-’ were achieved. No evidence was observed of any marked or sudden change in sensitivity throughout the frequency range over which it is hoped to use the hydrophone. Preliminary calibration at discrete frequencies by means of secondary standard transducers, themselves calibrated by self-reciprocity, yielded the frequency response data given in Table 1. These results are subject to the estimated maximum uncertainties indicated. Allowing for the effects of the finite thickness of the membrane and the acoustic reflection coefficients at the two surfaces, the sensitivity of the membrane should be greater at a frequency of 10 MHz than at 1 MHz by approximately 5%, whereas the experimental values showed a fall in sensitivity of the order of 20%. This may perhaps be accounted for by the greater difficulty of achieving the required alignment accuracy at the higher frequencies when using a device employing a relatively large active element (the 4mm diameter element being about 25 wavelengths across at 10 MHz), an effect which would systematically reduce the higher frequency result. Additional uncertainties due to the variation in the power of the higher frequency transducers available tests, and to the imperfections in the measurement itself, were included in the estimates of uncertainty in the table.
output for these procedure quoted
The present prototypes have a thickness mode resonance frequency at approximately 50 MHz, and component frequencies of up to 40 MHz were readily observed in the output signals obtained when detecting short-pulse fields. The generally robust and serviceable nature of the device, however, suggests that considerably thinner membranes could be used with ease, offering resonance frequencies well over 100 MHz. The measured plane wave transmission characteristics of the device are shown in Fig. 3, and compared with those calculated on the basis of the acoustic characteristics of pvdf given in the literature. 4,5 While satisfactory for many applications, the use of thinner membranes is again indicated as a means of improving performance: a 12 pm thick membrane, for example, would have an amplitude transmission coefficient of 0.98 at 5 MHz and 0.93 at 10 MHz. If membrane hydrophones are to be developed with the spatial resolution and directional characteristics necessary for the full evaluation of medical equipment, much smaller active elements, of the order of a few acoustic wavelengths or less in diameter, are required. Preliminary measurements on various pvdf devices with elements of 1 mm and 2 mm in diameter have indicated that membrane hydrophones exhibit a sensitivity that is largely independent of the size of the active element, and with a preamplifier positioned on or near the support ring, the performance of smallelement hydrophones need not be affected adversely by their relatively low capacitance.
ULTRASONICS.
MAY
1980
Fig. 2 Signal obtained in the field from a 2 MHz transducer (excited by a single-cycledrive voltage) with: a -a membrane hydrophone; b -a conventional hydrophone probe employing a small ceramic element
Frequency response data
Table 1. Frequency
[MHz]
1.0
2.0
5.0
7.5
10.0
Hydrophone sensitivity [PV Pa-’
I
1.05
1.01
0.93
0.85
0.88
+0.14
+0.15
+0.26
+0.26
+0.31
-0.14
-0.13
-0.21
-0.18
-0.22
Estimated maximum uncertainty [fiV
Pa-‘]
Conclusion The membrane hydrophone should have a usefulness beyond that of conventional probes. It may, for instance, be used to calibrate a hydrophone of unknown sensitivity without the need to complete lengthy reciprocity procedures. If the membrane probe’s sensitivity is known, the probe to be calibrated can be placed immediately behind the membrane in the field of a transducer, and the voltages from the two devices
125
ment should facilitate the convenient measurement of ultrasonic beam profiles, especially when used in conjunction with appropriate electronic sampling circuitry.
IO-i-f-___
‘i-_,\ OB-
f -.
E
The investigations described in this paper are part of a continuing programme at the NPL, aimed at the provision of standards and calibration services for the measurement of the output characteristics of medical ultrasonic equipment manufactured or used in the UK.
t-
!!
E
8
oti-
5
c $
s
04-
+
Expermentalresults
Acknowledgement
; ---
Q
Theoreltcol values
This paper is Crown Copyright.
0.2-
0/ 0
References 2
4
6 a Frequency [MHz1
IO
12
Fig. 3 Transmission against frequency characteristics of a 25 pm thick membrane of pvdf as used in the present hydrophone prototype
compared. Secondly it should be possible to measure the field strength within an ultrasonic cleaning bath by means of a suitable membrane device constructed without a heavy support ring. Such baths contain a superposition of waves travelling in several different directions, and the resulting field cannot normally be measured by conventional probes because of the distortion they introduce. Finally, it is planned to develop the basic design by constructing a device with an array of active elements. This arrange-
126
Wells, P.N.T., Biomedical Ultrasonics, Academic Press (1977) 52 Bui, L., Shaw, H.J., Zitelli, L.T., Experimental Broadband Ultrasonic Transducers Using PVF? Piezoelectric Film, Electron. Lett. 12 (1976) 393 Woodward, B., The suitability of Polyvinylidene I:luoride as an Underwater Transducer Material, Acusticu 38 (1977) 264 Sullivan, T.D., Powers, J.M., Piezoelectric Flexural Disc Hydro phone, J. Acousf. Sot. Am. 63 (1978) 1396 Woodward, B., Chandra, R.C., Underwater Acoustic Measurements on Polyvinylidene Fluoride Transducers, Electrocompo-
nent Science & Technology 5 (1978) 149 Murayama, N., Nakamura, K., Obara, H., Segawa, Strong Piezoelectricity in Polyvinylidene Fluoride
M., The (PVDF),
Ultrasonics 14 (1976) 15 Ohigashi, H., Electromechanical Properties of Polarised Polyvinylidene Fluoride Films as Studied by the Piezoelectric Resonance Method, JAppl. Phys. 47 (1976) 949
ULTRASONICS.
MAY 1980
D.R. BACON and R.M. QUiLLlAM
A new form of hydrophone for measuring the spatial and temporal distributions of pressure within the fields from medical ultrasonic equipment is described. The device comprises an acoustically transparent plastic membrane with a small, central region activated to provide a freely-suspended piezoelectric element. This form of detector offers several advantages over conventional hydrophone probes employing ceramic elements, in particular a flat frequency response. The performance characteristics of a prototype membrane hydrophone and some possible developments and applications are described.
Introduction
Conventional
There is an increasing demand for quantitative measurements to be performed on the fields emitted by medical ultrasonic equipment. As no method is available for characterizing fields within the patient’s body, such measurements are normally carried out on the fields generated by medical transducers radiating into a water-filled test tank. Total time-averaged output powers from the transducers can then be measured by reference to any of a number of physical properties of ultrasonic fields, for example the radiation force exerted on a reflecting or absorbing target, but measurements of the spatial and temporal distributions of acoustic pressure within the fields are in general more difficult.
Miniature hydrophones currently available, either commercially or in the form of design and construction details in the technical literature, comprise typically a disc of piezoelectric ceramic mounted on the end of a rod or cone, the latter providing both mechanical support and acoustic backing. The disc is mounted with its surfaces perpendicular to the intended direction of propagation of incident acoustic waves. To achieve adequate spatial resolution and good directional characteristics, a small disc must be used, typically one of less than 1 mm in diameter. Further, to achieve the required frequency response, the disc must be no thicker than a few hundred pm, ensuring that the fundamental thickness mode resonance is at a frequency beyond the range of interest, a typical resonance frequency being between 20 MHz and 30 MHz. Unfortunately the constraint imposed on the diameter of the disc by the need to limit the directionality of the hydrophone response leads to the presence of a fundamental radial mode resonance at a frequency of a few MHz, and this may well affect the overall dependence of the hydrophone’s acoustic sensitivity on frequency.
The most widely accepted method of quantifying these distributions is the use of miniature piezoelectric hydrophone probes. To obtain accurate information on the fields from medical ultrasonic equipment by this means, however, hydrophones are needed that exhibit a flat response in terms of pressure sensitivity against frequency over the output spectrum likely to be encountered, say 0.5 MHz to 15 MHz, and possess a response characteristic that is insensitive to the propagation direction of the incident wave and free from reverberation effects. Although most medical equipment has an operating frequency of between 2 MHz and 8 MHz, a hydrophone frequency response flat to 15 MHz is required to obtain accurate representations of acoustic waveforms in the short-pulse fields used, for example, in most imaging equipment. K.C. Shotton and D.R. Bacon are at the Division of Radiation Science and Acoustics, The National Physical Laboratory, Teddington, Middlesex, UK; R.M. Quilliam is at GEC-Marconi Electronics Ltd. Marconi Research Laboratories, Great Baddow, Essex, U.K. Paper received 22 October 1979. Revised 28 November 1979.
0041-624X/80/0301 ULTRASONICS.
MAY 1980
hydrophone
design
Some of the physical characteristics of the piezoelectric ceramic materials themselves lead to unavoidable defects in the performance of conventional probes. The high acoustic impedances of such materials (2.9 x 10’ kg m”s-’ for PZTSA' , for example, compared with 1.5 x lo6 kg m-*sW1 for water) mean that the probe perturbs the acoustic field it is used to measure. Secondly, to reduce acoustic reflection from the back face of the piezoelectric disc and so obtain a broad frequency response, a matching high impedance backing material must also be used. This again interferes with the acoustic field, and energy may well reach the active
23-04 $02.00 0
1980 IPC Business Press 123
element as longitudinal or shear waves propagating within the backing material itself, giving rise to undesirable reverberations in the received signal. Further, the brittle nature of ceramics, and the difficulty likely to be experienced in bonding them reproducibly to other materials, makes the manufacture of probes from small ceramic elements an exacting and time-consuming procedure. Finally, good longterm stability in the sensitivity of ceramic hydrophones, particularly when immersed in water for prolonged periods, has proved difficult to achieve.
The membrane hydrophone In this paper we describe an alternative approach to hydrophone design proposed at the National Physical Laboratory, and report some of the initial results obtained with prototypes constructed under a contract placed by the Metrology and Standards Requirements Board with GEC-Marconi Limited in Chelmsford, UK. The design exploits the properties of polyvinylidene fluoride (pvdf), a piezoelectric plastic polymer which has already proved to be useful as a transducer material over a wide range of frequencies and operating modes; see for example the references2”. In the new form of hydrophone, a prototype of which is shown in Fig. 1, the pvdf is used as a thin, acoustically transparent membrane, stretched over an annular frame large enough to allow the entire ultrasonic beam to pass through the central aperture. A small central region of the membrane is coated on both surfaces with metal film electrodes, and is poled to induce piezoelectric properties only within that region. The device, therefore, may be regarded as a small sensing element, suspended freely in the ultrasonic field, which responds to the local pressure fluctuations associated with the passage of ultrasonic waves. Metal film contacts are necessary to carry the signal generated by the piezoelectric element out of the region of the field, but it is believed that these may be made sufficiently thin not to affect significantly the acoustic transmission characteristics of the membrane, This form of detector offers a number of potential advantages over the conventional device based on ceramic piezoelectric elements. Firstly pvdf can be fabricated as film with thicknesses down to a few pm, offering devices with fundamental mode resonances at frequencies far above the range of interest. Secondly, the frequencies of transverse or radial modes of the membrane are related to the diameter of the entire device rather than that of the active element alone, and will be well below the 0.5 MHz lower limit of interest. Thirdly, since the acoustic impedance of pvdf (4.1 x lo6 kg rne2se1 from our own and published values of density6 and velocity7) is more closely matched to that of water than is the impedance of ceramic materials, the acoustic reflection coefficient at the surface of the membrane will be low and the device will exhibit only low Q resonance effects and hence a broad frequency response. Further the piezoelectric properties of pvdf are such that the sensitivity of the device as a detector is likely to be comparable with, or better than, that normally achieved with ceramic elements, typically 0.1 /.LVPa-’ to 1 E.IVPa-‘. Finally, there can be no problems associated with the presence of an acoustic backing material, and it should be possible to reduce any field distortion due to the presence of the device to a negligible level. Membrane hydrophones should possess, therefore, a broad, flat frequency response free from the effects of reverberation associated with conventional backing and mounting configurations.
124
Fig. 1
The prototype
membrane
hydrophone
A possible disadvantage of this form of detector is the relatively low capacitance of a pvdf element made sufficiently small to provide good directional characteristics and spatial resolution. Since it is essentially charge that is generated by pressure on the active element, the voltage signal will be reduced by lead, cable and amplifier input capacitances. Another disadvantage is that for broadband response characteristics the membrane thickness should be much less than half the wavelength of ultrasound at the highest component frequency, not only to avoid interference from thickness mode resonances, but also to ensure that the membrane appears acoustically transparent over the required frequency range, and that it effectively senses the pressure at a point within the acoustic waveform. An extremely thin membrane, however, is likely to puncture or stretch in routine use and may not be suitable as the basis of a simple, convenient and robust device. Finaily, in its simplest form the membrane hydrophone requires thin film leads to carry the electrical signal across the membrane to a point on the supporting frame outside the field where connections to a conventional screened cable may be made. With such exposed leads in the water of the test tank it is likely to be difficult to avoid some electrical pick-up problems. The prototype
design
In drawing up the specifications of a prototype hydrophone, it was our intention to design a unit suitable for assessing the extent to which the potential advantages of a membrane device could be realized in practice and to obtain information on the likely importance of the disadvantages, rather than to attempt to produce immediately a hydrophone with optimum performance characteristics. The first prototype uses pvdf sheet of 25 pm thickness with a 4 mm diameter active element, a design chosen to provide a rugged device with an active element of reasonable sensitivity and capacitance. The membrane is supported on a perspex ring of approximately 100 mm diameter, and the metal film leads to the central electrodes on either side of the membrane are well separated to minimize load capacitance and to prevent the region between the leads becoming piezoelectrically active during the poling process. The poling itself is carried out at a temperature in excess of 100°C and with a field of approximately 1MV cm-‘. Finally, the metal film leads are connected to a 50 fi coaxial lead with conducting epoxy adhesive, and the connection potted in epoxy resin for insulation and to provide good mechanical strength.
ULTRASONICS
. MAY 1980
During initial assessment trials, prototypes of this new form of hydrophone have performed well. When used in pulsed fields, the devices gave a clean signal free from reverberation effe:cts (Fig. 2). Electrical pick-up experienced mainly at 50 Hz and in the MHz region was reduced to a few mV by ensuring that the coaxial screen of the signal lead was connected to a conductor in good contact with the distilled water in the test tank, and sensitivities of approximately 1 r.lV Pa-’ were achieved. No evidence was observed of any marked or sudden change in sensitivity throughout the frequency range over which it is hoped to use the hydrophone. Preliminary calibration at discrete frequencies by means of secondary standard transducers, themselves calibrated by self-reciprocity, yielded the frequency response data given in Table 1. These results are subject to the estimated maximum uncertainties indicated. Allowing for the effects of the finite thickness of the membrane and the acoustic reflection coefficients at the two surfaces, the sensitivity of the membrane should be greater at a frequency of 10 MHz than at 1 MHz by approximately 5%, whereas the experimental values showed a fall in sensitivity of the order of 20%. This may perhaps be accounted for by the greater difficulty of achieving the required alignment accuracy at the higher frequencies when using a device employing a relatively large active element (the 4mm diameter element being about 25 wavelengths across at 10 MHz), an effect which would systematically reduce the higher frequency result. Additional uncertainties due to the variation in the power of the higher frequency transducers available tests, and to the imperfections in the measurement itself, were included in the estimates of uncertainty in the table.
output for these procedure quoted
The present prototypes have a thickness mode resonance frequency at approximately 50 MHz, and component frequencies of up to 40 MHz were readily observed in the output signals obtained when detecting short-pulse fields. The generally robust and serviceable nature of the device, however, suggests that considerably thinner membranes could be used with ease, offering resonance frequencies well over 100 MHz. The measured plane wave transmission characteristics of the device are shown in Fig. 3, and compared with those calculated on the basis of the acoustic characteristics of pvdf given in the literature. 4,5 While satisfactory for many applications, the use of thinner membranes is again indicated as a means of improving performance: a 12 pm thick membrane, for example, would have an amplitude transmission coefficient of 0.98 at 5 MHz and 0.93 at 10 MHz. If membrane hydrophones are to be developed with the spatial resolution and directional characteristics necessary for the full evaluation of medical equipment, much smaller active elements, of the order of a few acoustic wavelengths or less in diameter, are required. Preliminary measurements on various pvdf devices with elements of 1 mm and 2 mm in diameter have indicated that membrane hydrophones exhibit a sensitivity that is largely independent of the size of the active element, and with a preamplifier positioned on or near the support ring, the performance of smallelement hydrophones need not be affected adversely by their relatively low capacitance.
ULTRASONICS.
MAY
1980
Fig. 2 Signal obtained in the field from a 2 MHz transducer (excited by a single-cycledrive voltage) with: a -a membrane hydrophone; b -a conventional hydrophone probe employing a small ceramic element
Frequency response data
Table 1. Frequency
[MHz]
1.0
2.0
5.0
7.5
10.0
Hydrophone sensitivity [PV Pa-’
I
1.05
1.01
0.93
0.85
0.88
+0.14
+0.15
+0.26
+0.26
+0.31
-0.14
-0.13
-0.21
-0.18
-0.22
Estimated maximum uncertainty [fiV
Pa-‘]
Conclusion The membrane hydrophone should have a usefulness beyond that of conventional probes. It may, for instance, be used to calibrate a hydrophone of unknown sensitivity without the need to complete lengthy reciprocity procedures. If the membrane probe’s sensitivity is known, the probe to be calibrated can be placed immediately behind the membrane in the field of a transducer, and the voltages from the two devices
125
ment should facilitate the convenient measurement of ultrasonic beam profiles, especially when used in conjunction with appropriate electronic sampling circuitry.
IO-i-f-___
‘i-_,\ OB-
f -.
E
The investigations described in this paper are part of a continuing programme at the NPL, aimed at the provision of standards and calibration services for the measurement of the output characteristics of medical ultrasonic equipment manufactured or used in the UK.
t-
!!
E
8
oti-
5
c $
s
04-
+
Expermentalresults
Acknowledgement
; ---
Q
Theoreltcol values
This paper is Crown Copyright.
0.2-
0/ 0
References 2
4
6 a Frequency [MHz1
IO
12
Fig. 3 Transmission against frequency characteristics of a 25 pm thick membrane of pvdf as used in the present hydrophone prototype
compared. Secondly it should be possible to measure the field strength within an ultrasonic cleaning bath by means of a suitable membrane device constructed without a heavy support ring. Such baths contain a superposition of waves travelling in several different directions, and the resulting field cannot normally be measured by conventional probes because of the distortion they introduce. Finally, it is planned to develop the basic design by constructing a device with an array of active elements. This arrange-
126
Wells, P.N.T., Biomedical Ultrasonics, Academic Press (1977) 52 Bui, L., Shaw, H.J., Zitelli, L.T., Experimental Broadband Ultrasonic Transducers Using PVF? Piezoelectric Film, Electron. Lett. 12 (1976) 393 Woodward, B., The suitability of Polyvinylidene I:luoride as an Underwater Transducer Material, Acusticu 38 (1977) 264 Sullivan, T.D., Powers, J.M., Piezoelectric Flexural Disc Hydro phone, J. Acousf. Sot. Am. 63 (1978) 1396 Woodward, B., Chandra, R.C., Underwater Acoustic Measurements on Polyvinylidene Fluoride Transducers, Electrocompo-
nent Science & Technology 5 (1978) 149 Murayama, N., Nakamura, K., Obara, H., Segawa, Strong Piezoelectricity in Polyvinylidene Fluoride
M., The (PVDF),
Ultrasonics 14 (1976) 15 Ohigashi, H., Electromechanical Properties of Polarised Polyvinylidene Fluoride Films as Studied by the Piezoelectric Resonance Method, JAppl. Phys. 47 (1976) 949
ULTRASONICS.
MAY 1980