Active Sonar Calibration Standards
Definitions: A sonar, or sonar system, may be defined as a system consisting of an electroacoustic transducer and a package of electronics to control the transmission and/or reception of sound. The transducer converts an electrical signal to acoustic vibrations in the contact or immersion medium, e.g., seafloor or water column. The transducer acts reciprocally, converting acoustic vibrations to an electrical signal. A hydrophone may describe any transducer used underwater, but sometimes has the narrower meaning of a transducer used only to receive underwater sound. In all cases when the subject is transducer calibration, an accompanying set of electronics to control transmission and/or reception is presumed, corresponding to an active or passive sonar system, respectively.
Performance characteristics: A sonar calibration is undertaken to determine performance characteristics of the system. For transducers used in reception, as well as passive sonar systems, a major quantity of interest is the free-field voltage sensitivity. For transducers used in transmission, major quantities of interest are the transmitting current response and transmitting voltage response. For a sonar system used in the traditional active mode to transmit sound and receive echoes resulting from its transmissions, the combined transmitting and receiving response is typically sufficient. In all cases, the directional response of transducers and sonars in transmission and/or reception is typically also of interest.
Standard procedures for calibrating electroacoustic transducers used in transmission or in reception are presented in the following standard:
ANSI/ASA S1.20-2012, “American National Standard: Procedures for calibration of underwater electroacoustic transducers” (Acoustical Society of America, Melville, NY, 2012).
This is available at http://webstore.ansi.org/RecordDetail.aspx?sku=ANSI%2fASA+S1.20-2012 (last viewed 29 July 2013).
Principles for calibrating active sonars used for echo measurement are presented in Appendix F of the same reference. This is the standard-target method developed initially for calibrating scientific echo sounders at sea (Foote 1982) and wideband echo measurements in a laboratory tank (Dragonette et al. 1981). Guidelines for calibrating scientific echo sounders by the standard-target method are presented in:
K. G. Foote, H. P. Knudsen, G. Vestnes, D. N. MacLennan, and E. J. Simmonds, “Calibration of acoustic instruments for fish density estimation: A practical guide,” ICES Coop. Res. Rep., 144, (1987), 69 pp.
This is available for free download at: http://www.ices.dk/sites/pub/Publication%20Reports/Cooperative%20Research%20Report%20%28CRR%29/crr144/crr144.pdf (last viewed 29 July 2013).
The method has been extended to a variety of echo sounder systems, including those of a multiple-frequency echo sounder system (Vagle et al. 1996) and an acoustic backscattering sensor (ABS) (Foote and Martini 2010), among others.
The method has also been extended to calibration of multibeam sonars that provide the water-column signal (Foote et al. 2005). The method is also being used to calibrate parametric sonars (Westervelt 1963) of a particular type ordinarily used for profiling the sub-bottom (Dybedal 1993). Significantly, the difference frequency of this system is in the low-kilohertz band. Applications of the standard-target method span the total frequency range from 1 kHz to more than 1 MHz.
Range compensation: It is presumed that gain within the receiver electronics is controlled according to a quantitative function, formerly called time-varied gain, but now more generally called range compensation. Readily accessible references on this subject include the following: Medwin and Clay (1998), Foote (2012).
L. R. Dragonette, S. K. Numrich, and L. J. Flax, “Calibration technique for acoustic scattering measurements,” J. Acoust. Soc. Am. 69, 1186–1189 (1981).
J. Dybedal, “TOPAS: parametric end-fire array used in offshore applications,” in Advances in Nonlinear Acoustics, edited by H. Hobæk (World Scientific, Singapore, 1993), pp. 264-275.
K. G. Foote, “Optimizing copper spheres for precision calibration of hydroacoustic equipment,” J. Acoust. Soc. Am. 71, 742-747 (1982).
K. G. Foote, “Range compensation for backscattering measurements in the difference-frequency nearfield of a parametric sonar. J. Acoust. Soc. Am. 131, 3698-3709 (2012).
K. G. Foote and M. A. Martini, “Standard-target calibration of an acoustic backscatter system,” in Oceans 2010 MTS/IEEE Seattle Conference Proceedings [doi: 10.1109/OCEANS.2010.5664362], 5 pp.
K. G. Foote, D. Chu, T. R. Hammar, K. C. Baldwin, L. A. Mayer, L. C. Hufnagle, Jr., and J. M. Jech, “Protocols for calibrating multibeam sonar,” J. Acoust. Soc. Am. 117, 2013-2027 (2005).
H. Medwin and C. S. Clay, Fundamentals of Acoustical Oceanography (Academic Press, Boston, 1998), 712 pp.
S. Vagle, K. G. Foote, M. V. Trevorrow, and D. M. Farmer, “A technique for calibration of monostatic echosounder systems,” IEEE J. Oceanic Eng. 21, 298-304 (1996).
P. J. Westervelt, “Parametric acoustic array,” J. Acoust. Soc. Am. 35, 535–537 (1963).