Amana – ECE 4007
Technical Review
Hydrophone Sensors
Introduction
Hydrophones are electrical devices used to listen to sounds underwater. Low cost,
highly sensitive hydrophones are of interest in homeland security and ocean research [1].
Underwater hydrophone arrays towed by small autonomous submarines are being
investigated as a possible way to monitor ocean-going threats in America’s waterways
[1]. Most hydrophones are made of piezoelectric transducers, which are materials that
create electrical signals when exposed to changes in pressure. Hydrophones can be
characterized by their frequency response, sensitivity, and maximum operating depth.
This paper focuses on the underlying technologies, types of hydrophones and recent
developments for hydrophones.
Underlying Technology
Hydrophones are built from piezoelectric materials. These materials have the
ability to generate an electrical potential in response to an applied mechanical stress [2].
In hydrophones, the piezoelectric transducer produces an electric signal when subjected
to a change in pressure. Sound is a pressure wave in water; therefore it can be detected
by a piezoelectric element [2]. There are both naturally occurring and man-made
piezoelectric materials. Some examples are Quartz, Berlinite, and Tourmaline. Apart
from hydrophones, piezoelectric materials are used in electric cigarette lighters,
piezoelectric transformers and actuators.
In medical procedures involving ultrasound, an alternate to the piezoelectric
hydrophone has been proposed. The conventional piezoelectric material used to detect
pressure in the hydrophone is replaced with an inexpensive conductive material [3]. The
conductive hydrophones depend on a bias current flowing through the conductive
material [3]. In accordance with the acoustoelectric effect, a voltage is generated when a
pressure wave interacts with a current field. This effect is exploited to test and design
disposable hydrophones used in clinical applications.
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Amana – ECE 4007
Technical Review
Types of Hydrophones
Hydrophones are of two types, omnidirectional and directional. The
omnidirectional type detects sounds from all directions with equal sensitivity. Directional
hydrophones have higher sensitivity to signals in a particular direction [4]. Directional
hydrophones are usually built from several omnidirectional hydrophones, but can also be
created from hydrophone arrays. Hydrophone arrays are created by connecting several
hydrophones in known locations. Sounds reach the hydrophones at different times
depending on the origin of the source. The difference between the sound arrival times can
be used in calculations to determine the direction that the sound is coming from [4].
Simple arrays with only two hydrophones can be used to detect the origin of a sound.
Arrays are more efficient than single hydrophones because they are able to filter out noise
from other directions. This arrangement also increases the signal-to-noise ratio, which
allows for hard to detect signals to be heard. A hydrophone array is used in most sensors
to detect sounds.
New Technologies
Fiber optic sensors have become one of the most promising acoustic detection
devices for future operational sonar systems [5]. Fiber optic sensors are favorable
because of their high sensitivity, wide dynamic range, immunity to electromagnetic
interference and feasibility in multiplexing [5]. However, the multiplexing in fiber optic
sensors is complicated. The solution to this complication is a fiber laser sensor, which
gives an alternate approach to fiber optic sensors. As shown in Zhang et al [5], an ultrathin fiber laser hydrophone is tested. This hydrophone achieves an enhanced sensitivity
while the frequency response is flattened. The enhanced sensitivity is achieved by using a
thin metal cylinder and a piston-like diaphragm [5]. A sensitivity of 7 nm/MPa was
achieved.
Another major type of hydrophone for underwater acoustics is the vector
hydrophone. Because of its inherent directivity and unique vector-phase processing, the
vector hydrophone possesses potential advantages in the detection of quiet underwater
acoustic targets emitting low frequency and ultra-low frequency noise [6]. Recent
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Technical Review
developments in vector hydrophones have produced a MEMS (microelectromechanical
systems)-based piezoresistive vector hydrophone [7]. The advantage of piezoresitive
effect is that it can be used to detect at low frequencies, even at zero hertz. Using MEMS
technology produces smaller, more reliable, robust, mass-produced, and cheaper devices
[7]. Overall this design has several advantages, these include: low-frequency sensitivity,
small volume and lightweight.
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Technical Review
Sources
[1]
A. Lee. (2005, May 23). Underwater Ears. The Engineer Online
[Online].Available:http://www.theengineer.co.uk/Articles/290861/Underwater+ea
rs.htm
[2]
“Piezoelectric Materials,” July 21, 2007. [Online]. Available: http://www.
piezomaterials.com/index.htm [Accessed: Aug. 29, 2008].
[3]
R.S. Witte, T. Hall, R. Olasfsson, S.W. Huang, and M. O’Donnell, “Inexpensive
Acoustoelectric Hydrophone for Measuring High Intensity Ultrasound Fields,” in
2007 IEEE Ultrasonics Symp., New York, NY, 2007, pp. 737 – 740.
[4]
“Discovery of sound in the sea,” [Online]. Available: http://dosits.org/science/
measurng/1.htm [Accessed: Aug. 29, 2008].
[5]
W. Zhang, Y. Liu, F Li, and H. Xiao. (2008, May). Fiber Laser Hydrophone
Based on Double Diaphragms: Theory and Experiment. Lightwave Technology
[Online]. 26(10), pp. 1349–1352. Available: http://ieeexplore.ieee.org.library.
gatech.edu:2048/iel5/50/4542899/04488230.pdf?tp=&arnumber=4488230&isnum
ber=4542899
[6]
S.G. Qing , L.Q. Hu, Y.X. Ting, and S.C. Yu. (2006, Aug.). A novel fiber optic
hydrophone and vector hydrophone. Wuli [Online]. 35(9), pp. 645–653.
Available: http://www/engineeringvillage2.org.www.library.gatech.edu:2048/
controller/servlet/Controller?SEARCHID=3f96ee11c0f2cd6d3M1e7aprod3data1
&CID=quickSearchAbstractFormat&DOCINDEX=1&database=131139&format
=quickSearchAbstractFormat
[7]
S. Chen, C. Xue, B. Zhang, B. Xie, and H. Qiao “A Novel MEMS Based
Piezoresistive Vector Hydrophone for Low Frequency Detection,” in
Mechatronics and Automation, 2007, ICMA 2007, Int. Conf., Harbin, 2007, pp.
1839 – 1844.
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