Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 7586
Title: Calibration of a broadband acoustic transducer with a standard spherical target in the near field
Author: D. Chu, G. Eastland
Publication Year: 2015
Journal: Journal of the Acoustical Society of America
Volume: 137
Issue: 4
Pages: 2148-2157
Keywords: acoustics,calbration,broadband,nearfield

 This paper investigates the applicability of calibrating a broadband acoustic system in the near field. The calibration was performed on a single transducer with a mono-static, i.e., backscattering, configuration using a standard target, a 25-mm tungsten carbide sphere, in the nearfield of both the transducer and the sphere. A theoretical model to quantify the nearfield effect was developed. Numerical simulations revealed that although the shape of the frequency responses of the received echoes at different distances varied significantly, the null positions were essentially invariant, a unique  characteristic that was used to determine the compressional and shear wave speeds in the calibration sphere. The calibration curves obtained at different ranges in the near field by taking into account the nearfield effect were consistent when nearfield effects were accounted for. Since the transducer was located in the near field, the signal-to-noise ratio was high, resulting in a much wider useable bandwidth than the nominal bandwidth. The resultant calibration uncertainty, i.e. root-mean-square (rms) uncertainty, over the entire usable frequency band is 1.05 dB and reduces to 0.33 dB when the frequency regions corresponding to nulls are excluded. The methods reported here could potentially be applied to the calibration of multibeam echosounder/sonar systems.


 The paper provides the theory and experiment for a nearfield calibration of an acoustic pulse-echo system.  Numerical simulations based on the nearfield theory agrees with the experimental data. 

Theme: Ecosystem approach to improve management of marine resources
Foci: Provide scientific support for the implementation of ecosystem-based management