On a connection between amplitude fluctuations, phase fluctuations, and processing gain
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Weston et al. [Philos. Trans. R. Soc. London, Ser. A 265, 595 (1969)] show the remarkable agreement between the spectra for the coefficients of variation of amplitude and phase, i.e., amplitude and phase fluctuations, for a hydrophone in shallow water. Wagstaff [J. Acoust. Soc. Am. 112, 2422 (2002)] showed the functional similarity between phase angles and pseudo-phase angles for an outdoor microphone. Pseudo-phase angles are amplitude fluctuations that have been scaled appropriately to have a similar functionality in temporally coherent signal processing as phase angles. The concepts of the two previously mentioned references are merged to exploit the similarity of phase fluctuations and amplitude fluctuations to achieve multiplicative pseudo-coherent gain. Gains in excess of 20 log(N) have been achieved (N is the number of samples averaged). 10 log(N) is considered ideal for vector averaging. It is seldom achieved, because of coherent attenuation and cancellation associated with the use of real phase angles. Results are included for wind noise in outdoor measurements. [Work supported by U.S. Army Armament Research Development and Engineering Center.]Keywords:
Hydrophone
Similarity (geometry)
Phase angle (astronomy)
SIGNAL (programming language)
A bstract The design of a standard hydrophone with a maximally flat (Butterworth) response in the frequency range 8.0 Hz‐1.0 kHz is described. The standard hydrophone has been developed primarily for calibrating line hydrophone arrays (seismic streamers) and marine seismic sources. The standard hydrophone has been used successfully during the past eight years for monitoring the output of a single air gun. It can be used for the calibration of a marine seismic streamer.
Hydrophone
Vertical seismic profile
Line (geometry)
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The purpose of this study was to develop and experimentally verify a practical spatial averaging model for frequencies up to 40 MHz. The model is applicable to focused sources of circular geometry, accounts for the effects of hydrophone probe finite aperture, and allows calibration by substitution to be performed when the active elements of reference and tested hydrophone probes differ significantly. Several broadband sources with focal numbers between 3 and 20 were used to produce ultrasound fields with frequencies up to 40 MHz. The effective diameters of the ultrasonic hydrophone probes calibrated in the focal plane of the sources ranged from 150 to 500 /spl mu/m. Prior to application of the spatial averaging corrections, the hydrophones with diameters smaller than that of the reference hydrophone exhibited experimentally determined absolute sensitivities higher than the true ones. This discrepancy increased with decreasing focal numbers and increasing frequency. It was determined that the error was governed by the cross-section of the beam in the focal plane and the ratio of the effective diameters of the reference and tested hydrophone probes. In addition, the error was found to be reliant on the frequency-dependent effective hydrophone radius. After applying the spatial averaging correction, the overall uncertainty in the hydrophone calibration was on the order of /spl plusmn/1 dB. The model developed is being extended to be applicable to frequencies beyond 40 MHz, which are becoming increasingly important in diagnostic ultrasound imaging applications.
Hydrophone
Aperture (computer memory)
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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%.
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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 calibrator has been developed by the Naval Research Laboratory to calibrate hydrophones as large as 12 cm in diameter by the comparison method in the frequency range 25–1000 Hz at sound pressure levels from 130 to 190 dB re 1 µPa. Thus, it is possible to calibrate a hydrophone immediately before deployment and immediately after recovery. The calibration can include the complete electronic measuring system as well as the hydrophone. Vertical stabilization of the calibrator for ship roll and pitch of 10° and isolation from ship's noise are provided.
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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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Most commercially available ultrasonic transducers exhibit finite amplitude distortion in water during hydrophone measurements needed to comply with regulatory requirements. The frequencies observed due to finite amplitude distortion can easily exceed ten times the transducer center frequency and 100 MHz. Typically, hydrophone calibrations are supplied only up to 15 or 20 MHz and do not exhibit a flat response. The frequency response above 15 MHz should be known to accurately represent the acoustic information, especially for high-frequency transducers ranging between 7.5 and 15 MHz. A new hydrophone calibration technique has successfully predicted the frequency response of hydrophones up to 100 MHz. A circular source transducer was first characterized and then modeled using the KZK wave propagation model. This model accounts for diffraction, absorption, and nonlinearity. The transducer frequency response was measured with a hydrophone and compared to the simulation. This difference characterized the frequency response of the hydrophone and was used to estimate the hydrophone calibration. The estimated calibration at 20 MHz was checked and provided good agreement with the manufacturer calibration supplied. Acoustic measurement accuracy will be improved if the hydrophone frequency response is deconvolved from the actual acoustic transducer response.
Hydrophone
Distortion (music)
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A fibre-optic hydrophone consisting of a polarisation-maintaining fibre carrying light from a laser source has been used to measure the acoustic output parameters of a single focused hyperthermia transducer and a six-transducer assembly. Beam profiles of the transducers were measured using the fibre-optic hydrophone and the results compared with those obtained using a PVDF hydrophone. The acoustic power output from the hyperthermia transducer was measured using a radiation force balance. It was observed that the root mean square voltage of the fibre-optic hydrophone output is proportional to the square root of the acoustic power up to more than 80 W. It was also observed that, under continuous-wave operation, the fibre optic hydrophone can stand up to very high power (more than 200 W) without being damaged. As its sensing element is the fibre itself, whose diameter is considerably narrower than the width of the ultrasonic beam, it can provide resolution of about 80 microm in beam profile measurement. The fibre is a line sensor and a computer tomographic technique is used to recover the pressure profile from the hydrophone output voltage. In typical clinical operations, the six-transducer assembly is driven with less than 150 W of electrical power input. In such cases, each individual transducer receives less than 25 W of input power and non-linearity and generation of high frequency harmonics at the focus is not a significant problem.
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