Experimental Investigation on a Fibre-Optic Hydrophone with a Cylindrical Helmholtz Resonator
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A novel mechanical anti-aliasing filtering fibre-optic hydrophone with a cylindrical Helmholtz resonator is constructed and tested. The experimental results show that the hydrophone has a function of low-pass filtering. The low frequency acoustic sensitivity is about −160 dB (1 rad/μPa), and the response curve has a resonance determined by the Helmholtz resonator. Theoretical and experimental results both show that the resonant frequency moves towards high frequency with the increasing orifice diameters. The sensitivity attenuation of high frequency is larger than 10 dB. This new fibre-optic hydrophone is a prototype device for a class of sensors used to eliminate the aliasing in future sonar systems.Keywords:
Hydrophone
Helmholtz resonator
Aliasing
A novel mechanical anti-aliasing filtering fibre-optic hydrophone with a cylindrical Helmholtz resonator is constructed and tested. The experimental results show that the hydrophone has a function of low-pass filtering. The low frequency acoustic sensitivity is about −160 dB (1 rad/μPa), and the response curve has a resonance determined by the Helmholtz resonator. Theoretical and experimental results both show that the resonant frequency moves towards high frequency with the increasing orifice diameters. The sensitivity attenuation of high frequency is larger than 10 dB. This new fibre-optic hydrophone is a prototype device for a class of sensors used to eliminate the aliasing in future sonar systems.
Hydrophone
Helmholtz resonator
Aliasing
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Sonar operation in coastal waters is challenging due to high false alarm rates and strongly varying sonar conditions. Optimal choices for sonar design and pulse characteristics depend strongly on target location and velocity, as well as the present environment. Given a description of the target and environment, acoustical models may estimate sonar performance for different sonar parameters. Updating sonar parameters to best meet shifting sonar conditions impose an unnecessary workload on operators and must be automated for unmanned systems. We suggest an optimization approach that takes into account both a variable environment and a random target. An acoustic ray trace model is run in all directions for a large number of different environment, target, and sonar realisations. Target parameters such as Doppler and aspect are modelled, and optimal sonar parameters are determined. The method is demonstrated for a littoral test case, where both the sonar design and its pulse parameters are optimized. The design takes into account whether the sonar is towed or hull-mounted, and its frequency. The pulse parameters include pulse length and pulse repetition time. The method can easily be extended to other sonar parameters, but the main intent here is to demonstrate the approach.
Synthetic aperture sonar
False alarm
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The frequency response of a polymer film Fabry-Perot fibre-optic hydrophone developed for the characterisation of ultrasound fields, has been investigated. The transduction mechanism of the hydrophone is based upon the detection of acoustically and thermally induced thickness changes in a polymer film Fabry-Perot interferometer deposited at the tip of a single mode optical fibre. The frequency response of the original sensor has been found to be significantly non-uniform over the 50 MHz operating bandwidth. A finite difference simulation of acoustic interactions with the sensor has successfully been used to predict the response and investigate the origin of the non-uniformities. Additionally, the model has been used to predict the response of a sensor with modified tip geometry in order to find a design capable of providing an improved response. Sensors with a hemispherical tip have now been fabricated, characterised experimentally and found to provide a significantly improved response. Measurements were made on a shockwave toneburst (f c = 1 MHz), using both types of fibre-optic hydrophone and a 0.4 mm PVdF membrane hydrophone. Deconvolution was used (following IEC62127 - 1) in order to produce accurate pressure waveforms from the measured data. It was found that, in addition to providing an improved frequency response, the modification to the hydrophone geometry improved the efficacy of the deconvolution process.
Hydrophone
Response time
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Sonar sequences were introduced by Golomb and Taylor in 1982. We introduce the concept of extended sonar sequences, which is similar to that of sonar sequences except that blank columns are permitted. Several constructions for extended sonar sequences are offered here. Some of these are close to constructions for ordinary sonar sequences, but they provide improvements to the list of best sonar sequences with up to 100 symbols.
Synthetic aperture sonar
Golomb coding
Sequence (biology)
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Calibration of ultrasonic hydrophone probes in the frequency range from 250 kHz to 1 MHz is required to sufficiently assess the peak rarefactional pressure (pr) and the Mechanical Index (MI) of medical ultrasound imaging devices. However, the ultrasonic hydrophone calibration in this low frequency is barely conducted. Therefore, the objective of this research was to develop a calibration technique for ultrasonic hydrophone probes in the frequency range from 250 kHz to 1 MHz. Two ultrasonic hydrophone probes, one membrane hydrophone and one needle hydrophone, were calibrated using a substitution method combined with time-delay spectrometry (TDS). The calibration results are presented in term of end-of-cable voltage sensitivity as a function of frequency. The calibration data show that the membrane hydrophone exhibit a very flat frequency response, to within ±1 dB for the entire investigated frequency range, whereas the needle hydrophone demonstrates a relative large variations in sensitivity of about 5 dB. These results are in good agreement with the limited data previously reported. Therefore, the substitution calibration technique with Time Delay Spectrometry (TDS) is capable of calibrating the ultrasonic hydrophone probes in the frequency range from 250 kHz to 1 MHz.
Hydrophone
Substitution method
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The directivity model of hydrophone describes the frequency response of the hydrophone at different incident angles, which can be used for estimation of the effective diameter of the hydrophone. This parameter is very important because of the correction of the spatial average effect and the accurate measurement of the sound field parameters. At present, the nominal diameter of most commercial hydrophones is difficult to meet the requirement that the effective radius of hydrophone should be less than or equal to 1 / 4 of the acoustic wave wavelength, which may result in large errors because of spatial averaging. To solve this problem, this paper studies an effective diameter measurement method based on three kinds of hydrophone directivity models. In this method, the received signals of the hydrophone at different angles are measured, and the directional response model of hydrophone is established by least square method according to rigid baffle (RB), un-baffled (UB) and soft baffle (SB) model. The influence of directional models on effective diameter measurement is evaluated at different frequencies. The experimental results show that the directivity response data of hydrophone are not only matched with one model at different frequencies, but the directivity model closest to the data points should be selected to estimate the effective diameter of hydrophone.
Hydrophone
Directivity
Baffle
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A substitution calibration technique for piezoelectric ultrasonic hydrophones is presented that uses an optical multilayer hydrophone as the reference receiver. Broadband nonlinearly distorted focused pulses are first measured with the reference hydrophone and then with the hydrophone to be calibrated. By Fourier transformation of the time wave forms and division of the frequency spectra, the complex-valued frequency response of the hydrophone under test is obtained in a broad frequency range in a very fast and efficient way and with high frequency resolution. The results obtained for a membrane hydrophone and a needle-type hydrophone are compared with those obtained by independent calibration techniques such as primary calibration using optical interferometry and secondary calibration using time-delay spectrometry, and good agreement is found. The calibration data obtained are apt to improve the results of ultrasound exposure measurements using broadband voltage-to-pressure conversion. This is demonstrated for standard pulse parameter determination from exemplar exposure measurements on a commercial diagnostic ultrasound machine. For the membrane hydrophone, the evaluation method commonly used leads to an overestimation of the positive peak pressure by up to 50%, an underestimation of the rarefactional peak pressure by up to 11%, and an overestimation of the pulse intensity integral by up to 28%.
Hydrophone
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The principle of simple ultrasonic hydrophone is briefly introduced.The structural design and fabrication method are discussed in detail as well.The experiment results show that the ultrasonic hydrophone has the characteristic of high sensitivity,simple and practical.It has strong anti-jamming ability while the cost is merely 3 % of hydrophone.The sensitivity of simple subaqueous ultrasonic transducer is 95 % of hydrophone in 40 kHz.The working frequency range is 10~100 kHz.It is calibrated by the CS—3 hydrophone.Ultrasonic hydrophone could be widely used as subaqueous ultrasonic pressure sensor in simple measurement.
Hydrophone
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A hydrophone away is used to measure spatial distribution in underwater sound field and to detect incoming direction of sound waves in the ocean. It is not usually convenient to handle the hydrophone away because of its extensive scale. And it is not easy to purchase the hydrophone away because of expensive price. A hydrophone logger combined with a hydrophone and data logger was developed to consist conveniently of a hydrophone away for use to receive underwater sound waves. And a hydrophone array system with the hydrophone loggers was developed. Main configurations of the hydrophone 1o99er and the hydrophone array system are introduced in this paper. Also we present some measurement results by the hydrophone logger in a water tank and measurement examples on ambient noise in the sea by the hydrophone away system. And we discuss some advantages in use of the hydrophone array system.
Hydrophone
Data logger
Underwater Acoustics
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