P.M. Rajeshwari et al.: Development of Hydrophone for Sonar Applications 115

978-1-4673-0266-1/11/$26.00 ©2011 IEEE

Development of Hydrophone for Sonar Applications

P.M. Rajeshwari1, C. Kannan2, R. Dhilsha3 and M.A. Atmanand4 Marine Sensor Systems Group, National Institute of Ocean Technology, Chennai–600 100, India

1pmr@niot.res.in; 2kannan@niot.res.in; 3krd@niot.res.in; 4atma@niot.res.in

Abstract: This paper demonstrates the development of a sub-array hydrophone in a frequency range 2–24 kHz, which can be used for detection of buried objects or signals from targets. The array consists of 4 sub-array channels and each sub array consists of 5 elements wired in series. The inter element spacing between elements is 50 mm. Sub array hydrophone in series connection works as a single element there by maintaining low frequency of interest and at the same time enhancement in receiving sensitivity as well. A receiving sensitivity of –185 dB re 1 V/µPa @ 1m has been achieved at 9 kHz which can be improved to any suitable level using a suitable preamplifier.

Keywords: Directivity, Hydrophone, Sonar, Sub-array. 1. Introduction Hydrophones are devices meant for listening to underwater sound. Most hydrophones are based on a piezoelectric transducer and piezoelectricity is a property where strain/charge is generated when subjected to a pressure change and vice versa. When hydrophones are grouped and arranged in a fashion, it is termed as an array. It is necessary to assemble the elements into an array to achieve the desired source level and directivity. Depending upon the arrangement it can be classified as line, cylindrical, concentric, circular, hexagonal arrays etc. Array interactions can be studied using numerical and analytical methods. In this paper, an attempt has been made to design a hydrophone (single element) using Finite element software ATILA and constructed a hydrophone array which consists

of 5 elements with an inter element spacing of 50 mm covering a wide frequency range of 2–24 kHz. The spacing between sub-array plays a major role which has been detailed in the design section. 2. Design of Sub-array A linear array comprises of individual transducers connected in series or parallel is designated as elements. Spacing between elements is a prime criteria which is essential that the recorded energy is fully sampled in space as well as in time, i.e., Spatial aliasing of the data is avoided [1]. To avoid spatial aliasing the spacing between sub-arrays need to be displaced by not more than λ min/2 [2], hence the spacing between sub-arrays worked out to be 25 cm. The phase difference between sub-arrays provides input for the deviation of sonar from straight trajectory [3].

P.M. Rajeshwari et al.: Development of Hydrophone for Sonar Applications 116

Figure 1 shows the photograph of the developed array.

For a single element, receiving sensitivity was obtained from the finite element software ATILA. Array performance was calculated using the equation:

Array Sensitivity = Sensitivity of single element + 20 log(M)

where M = no. of elements in the array

For a linear array the directional pattern is given by the equation.

Array factor = sin (Nψ/2)/N sin (ψ/2)

Where ψ = kd cosθ + δ, k = wave number, d = element separation and N = No. of elements, δ = progressive phase shift for elements. Figures 2, 3 and 4 shows the theoretical calculation of directional responses at 12 kHz, 15 kHz and 24 kHz respectively, for a separation of half wavelength.

Fig. 1: Snap Shot of the Array

Fig. 2: Beam Pattern @12 kHz, Beam Width = 26°

A preamplifier with a gain of 20–30 dB has been designed with 2 pole Butterworth filter and an impedance matching circuit between the hydrophone and the data acquisition system. Figure 5 shows the schematic of the preamplifier diagram.

Fig. 3: Beam Pattern @15 kHz, Beam Width = 22°

P.M. Rajeshwari et al.: Development of Hydrophone for Sonar Applications 117

Fig. 4: Beam Pattern @ 24 kHz, Beam Width = 14°

Fig. 5: Schematic of Preamplifier Circuit

3. Element Design Using ATILA software, a single cymbal transducer element has been modeled and an element with device diameter 19 mm, cap sheet thickness of range 180–320 microns were used for the study. After several iterations an optimized sheet thickness of 180 microns was arrived. Water radius was modeled for half wavelength and principle of axi-symmetry is used while modeling to reduce the computation time. Unstructured mesh pattern was used and model was generated comprising of 1668 nodes, 768 elements. Figure 6 shows an unstructured mesh model of a cymbal transducer element.

Fig. 6: Mesh Model of Cymbal Element

Fig. 7: Admittance Graph (ATILA simulation)

Figure 7 shows the admittance graph obtained by simulation. It was observed from the graph that at 8 kHz fundamental resonance occurred. The Naval Brass was used as material for end caps and PZT-5A was used as active element.

Two different brass sheet thicknesses were modeled and results were obtained as shown in the Figures 8 and 9. The reason for lowering of resonant frequency is due to the change in stiffness of the end caps. From the analysis of the transmitting and receiving response, it is apparent that frequency can be lowered by changing the thickness of the brass material cap. From 12 kHz frequency reduced to 9 kHz which clearly shows a gain of 3 kHz by varying the thickness.

Transmitting Voltage Response

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Frequency(Hz)

dB r

e 1

uPa/

1V@

1m

Thickness = 0.18 mm

Thickness = 0.25 mm

Fig. 8: Transmitting Sensitivity (ATILA simulation)

Receiving sensitivity

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dB re

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/uPa

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m

thickness=0.18 mmthickness =0.25 mm

Fig. 9: Receiving Response (ATILA simulation)

3.1 Fabrication of Elements Brass caps were blanked and formed with the use of a combination die setup and a hydraulic press to form the caps with truncated cone. The dimensions are reported elsewhere [5]. Ceramic is bonded to the end caps using a two part 3M-glue epoxy and cured at room temperature for 24 hours with the use of a specially designed jig. If the bonding is not proper, it will cause debonding during testing which lead to low d33 values [6]. Identical elements having similar d33

P.M. Rajeshwari et al.: Development of Hydrophone for Sonar Applications 118

values were selected using a digital Piezometer and the procedure is reported elsewhere [7].

3.2 Measurement of Piezoelectric Coefficients The characteristics of piezoelectric properties depend on their orientation to the poling axis. This orientation determines the direction of the action or response associated with the property. One of the important parameter among the piezoelectric constants is piezoelectric charge constant. The piezoelectric charge constant, d is the polarization generated per unit of mechanical stress applied to a piezoelectric material or, is the mechanical strain experienced by a piezoelectric material per unit of electric field applied. The first subscript to ‘d’ indicates the direction of polarization generated in the material when the electric field, E is zero. The second subscript is the direction of the applied stress or the induced strain. The subscript nota- tions define the axes of a component in terms of orthogonal axes: 1 corresponds to the x-axis, 2 correspond to the y-axis, and 3 correspond to the z-axis. d33 is the longitudinal piezoelectric coefficient and corresponds to strain induced in direction 3 per unit electric field applied in direction 3. d31 is the transverse piezoelectric coefficient produce strain in direction 1 per unit electric field applied in direction 3. Conven- tionally, the direction of polarization is defined as the 3 axis.

Table 1: Measured d33 Values

PZT 5 A (d33) pC/N

Thickness (0.18 mm), d33 pC/N

PZT 5 A (d33) pC/N

Thickness (0.25 mm) d33 pC/N

411 26177 401 20816 374 24292 383 17441 407 23989 403 16081 402 24426 402 16630

It is clearly evident that as the sheet thickness reduces d33 value increases. Measured d33 values for Piezo ceramic crystals as well as cymbal devices are shown in Table 1. Piezometer PM-200, a digital Piezometer has been used for

measuring the dynamic strain coefficient d33 values. A highest d33 value of 30,000 pC/N for 19 mm cymbal transducer has been reported earlier by the Marine Sensor Systems group at NIOT [4].

4. Experimental Results The Hydrophone array was characterized at Acoustic Test Facility, NIOT, NABL (National Accreditation Board of Laboratories) accredited. The tank dimensions were 16 m × 9 m × 7 m depth. Massa transducer (TR 1025) was used as an acoustic projector. Receiver was an Omni directional hydrophone type 8104 Bruel and Kjaer. The Hydrophone was characterized for the following parameters (1) Transmitting Response (2) Receiving Response and (3) Directional response.

5. Transmitting Response Even though design was made for Hydrophone, from the ATILA simulation result it was well understood that it can perform as a transmitter as well. Hence determination of Transmitting response experiment was carried out and it exhibited good transmitting sensitivity.

Transmitting Voltage Response

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70.0

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dB re

1uP

a/V@

1m

ch4 simulated single element

Fig. 10: TVR Comparison of Simulated Single

Element with Single Channel Sub-array (measured)

Figure 10 shows the transmitting sensitivity of simulated and response of single channel. The response was identical for other 3 channels. The reason why single element is having enhanced sensitivity is, element is modeled without any

P.M. Rajeshwari et al.: Development of Hydrophone for Sonar Applications 119

encapsulation. The hydrophones constructed were potted using 3M 2140 U Scotchcast resin. Damping occurs because of the potting com- pound. It is perceptible from Figure 10, the peak amplitude was made flat/even by damping matrix.

6. Receiving Response The Hydrophone was immersed at 3.5 m depth and the distance between separation between transmitter and receiver was 4.5 m. Calibration was performed by substitution method. Figure 11

Receiving Sensitivity

-230.0-220.0-210.0-200.0-190.0-180.0-170.0-160.0

2 5 8 11 14 17 20 23Frequency(kHz)

dB re

1 V

/uPa

@ 1

m

Channel 4

Simluated single element

Fig. 11: Receiving Sensitivity Comparison of Simulated Single Element with Experimental Values of Sub-array

shows the receiving sensitivity response for 4th channel without preamplifier and a comparison is made between simulated and measured. An identical performance was noticed for other 3 channels also.

7. Directional Response Directivity measurements were carried out for 2 kHz, 12 kHz and 24 kHz. The directivity response for 12 kHz and 24 kHz are shown in Figures 12 and 13 respectively. At 2 kHz directivity was Omni directional since it is low frequency. Theoretical calculation of beam width at 12 kHz worked out to be 26 degree. But practical measurement gave a bandwidth of 20 degree at 12 kHz and 14 degree at 24 kHz respectively.

(a)

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-40 -30 -20 -10

(b)

Fig. 12: (a) Calculated Beam pattern (b) Measured Beam Pattern @ 12 kHz, Beam Width = 20°

8. Discussion and Conclusion The development of hydrophone array in the frequency range 2–24 kHz, using cymbal type transducers has been completed. Simulations were carried out using ATILA software for two different brass thickness sheets. Lowering of

(a)

P.M. Rajeshwari et al.: Development of Hydrophone for Sonar Applications 120

0

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-30 -20 -10

1 FFT 24064 Hz

(b)

Fig. 13: (a) Calculated Beam Pattern (b) Measured Beam Pattern @ 24 kHz, Beam Width = 14°

resonance frequency was observed due to stiffness change in metal caps. Cymbal transducer elements with device diameter 19 mm and sheet thickness of 0.15 mm were used in the development of array. Though the simulated resonant frequency was 9 kHz, a resonance frequency of 8 kHz was observed in experiments. This variation may be due to the changes in the material properties of the transducer components, effective fabrication and assembly procedures, and use of potting compound. A good agreement is obtained between simulated and measured results. Directional response beam width at 12 kHz and 24 kHz are in good agreement.

Acknowledgments The authors are grateful to other members of Marine Sensor Systems group for fabrication and assembly of hydrophones. The authors also thank A. Malarkodi, Ocean Acoustics group, for her immense patience and skilful expertise in calibrating transducers. C. Dhanraj, Ocean Acoustics group is thanked for his professional practical assistance in carrying out the experiments. This work is funded completely

by National Institute of Ocean Technology (NIOT), MoES, Govt. of India.

References [1] Bull, Jonathan M. et al., “Design of a 3 D

Chirp Sub-bottom Imaging System”, Marine Geophysical Researches, Springer, 2005, pp. 157–169.

[2] Hayes, Michael P. and Gough, Peter, IEEE Journal of Oceanic Engineering, Vol. 17, pp. 80–94, 1992.

[3] Raven, R.S., “Electronic Stabilization for Displaced Phase Center Systems,” US Patent 42244036, Jan 1981.

[4] Rajapan, Dhilsha, Rajeshwari, P.M., Sankar, M., Trinath, K., Prasad, N.S., “Miniaturization of Underwater Sensors for the Realization of Conformal Arrays”, OCEANS’06, May 16–19, Asia Pacific, Singapore, pp 1–7, 2006.

[5] Dhilsha, R., Rajeshwari, P.M. and Sankar, M., “Underwater Performance of 5 × 10 Cymbal Array for Oceanographical Applications”, Proceedings of International Conference in Ocean Engineering, ICOE’09, Feb. 1–5, IIT Madras, Chennai, India, 2009

[6] Ochoa, P., Pons, J.L., Villegas, M. and Fernandez, J.F., “Effect of Bonding Layer on the Electromechanical Response of the Cymbal Metal-Ceramic Piezo Composite”, Journal of the European Ceramic Society 27, pp. 1143–1149, 2007.

[7] Kannan, C., Rajeshwari, P.M., Jacob, Shibu, Malarkodi, A., Dhilsha, R. and Atmanand, M.A., “Effect of Manufacturing Procedure on the Miniaturized Flextensional Transducers (Cymbals) and Hydrophone Array Per- formance”, IEEE/MTS, OCEANS’11, June 6–9, Santander, Spain, 2011.