Hydrophone Line Array Calibration Using Acoustic Field Modeling
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Solution Overview
Problem
Existing methods for calibrating towed hydrophone line arrays in open water face challenges due to the large size of the arrays, limitations in simulating free field propagation, and the introduction of errors from boundary reflections, which restrict the low frequency limit for accurate calibration.
Innovation Solution
A method that uses a continuous Gaussian noise waveform to create an empirical model of the acoustic field, accounting for spatial variations and reflections, and employs vibration isolation to reduce errors, allowing for calibration below the frequency limit of free field propagation, using multiple calibrated reference standard hydrophones to compute the free field voltage sensitivity of hydrophone channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If acoustic free field or gated continuous wave calibration techniques are used for towed line arrays, then calibration can be performed in open water, but a low frequency limit is imposed by the requirement for reflection free signals
Solution Approach 1:
The patent converts the harmful effect of boundary reflections into a beneficial feature by using multiple microphones to capture reflected acoustic energy and computationally removing these reflections from the calibration data. This allows low frequency calibration below the traditional free field limit where reflections cannot be gated out, thereby extending the usable frequency range while maintaining calibration accuracy.
2Measurement precision
If a large number of hydrophones are distributed over the length of the towed line arrays, then comprehensive coverage is achieved, but the array size increases making controlled laboratory calibration infeasible
Solution Approach 1:
The patent replaces the need for a physically large array in a controlled laboratory environment with a computational approach. By using cross-spectral density analysis and empirical Green's function methods, the system can calibrate large distributed arrays in situ in open water, eliminating the mechanical constraint of transporting and setting up large arrays in laboratory facilities.
3Shape
If acoustic projector is located at the center of the cylindrical test fixture, then geometric symmetry is achieved, but vibrations from the projector corrupt the received acoustic data
Solution Approach 1:
The patent extracts and removes the harmful vibration component from the calibration data. By using multiple microphones distributed around the cylindrical fixture and applying cross-spectral density analysis, the system can identify and remove vibration-induced signals from the acoustic measurements, allowing the projector to remain at the geometric center while preventing vibration contamination.
4Ease of manufacture
If acoustically non-reflective test tank surfaces are assumed to exist, then simplified calibration calculations can be performed, but such tanks are not known in the prior art and reflections introduce errors
Solution Approach 1:
The patent uses feedback from multiple microphone measurements to iteratively improve calibration accuracy in the presence of reflections. By measuring the acoustic field from multiple locations and using cross-spectral density analysis, the system can identify reflected energy components and computationally remove them, providing accurate calibration results without requiring specially designed anechoic facilities.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method extends the low frequency calibration limit and improves precision and accuracy by accounting for boundary reflections and vibrations, providing accurate calibration of hydrophone channels across a broader frequency range.
Implementation Method 1
The acoustic projector transmits acoustic signals with known properties
Implementation Method 2
The lifting device includes a vibration isolator for reducing the amplitude of vibrations that are transmitted from the acoustic projector
Data Source
AI summary
A method is disclosed for calibration of a towed line array. In a low frequency band, calibration is performed using an acoustic field observed by reference standard hydrophones. The observations form a model of a complex acoustic field throughout a space occupied by a measurement apparatus. The array sensitivities are computed by comparing output voltages of the array with the acoustic field estimated at the locations occupied by hydrophones of the array. Variations in the acoustic field that cannot be accounted for by free field propagation theory are included in the calculation of array channel sensitivities. The method extends the low frequency limit for the calibration to less than the minimum frequency at which free field propagation conditions can be approximated. Boundary reflections and spatial variations in the acoustic field are recognized. The spatial distribution of acoustic energy is used to provide low frequency calibration with improved precision.