Characterization of very high frequency transducers withwire target and hydrophone

Abstract

In this paper, the wire-target technique will be shown to be a useful alternative for beamprofile measurements in very high frequency range (30–60 MHz). A 9 cm long tungsten wire with adiameter of 8 µm was used as the pulse-echo target to measure the lateral beam profiles at the focalpoints of two transducers, a spherically focused 40 MHz Panametrics transducer with an aperturesize of 6.35 mm and a geometrical focal length of 12.7 mm and a lense-focused in-house lithiumniobate (LiNbO3) 60 MHz transducer with an aperture size of 2 mm and a geometrical focal lengtharound 6.5 mm. For comparison, measurements on the same transducers were performed by threesmall-aperture hydrophones. The first one is a polyvinylidene fluoride trifluoroethylene (PVDF-TrFE)membrane hydrophone developed by Hewlett-Packard, which has a geometric diameter of 37µm, a measured effective diameter of less than 100 µm and a –3 dB bandwidth of more than 150MHz. The second one is a needle-type PVDF hydrophone from Precision Acoustics, which has a 9µm-thick PVDF element, a 40 µm geometrical aperture and a measured effective diameter of lessthan 100 µm. The third one is a needle-type PVDF hydrophone from Onda, which has a 150 µmgeometrical aperture and an effective diameter of about 180 µm. Experimental results show that the–6 dB two-way beam widths measured by this 8 µm wire-target are in agreement with –3 dBtransmitted beam widths measured by small-aperture hydrophones. Compared to small-aperturehydrophones, the wire-target technique is simpler and more cost-effective. Its major advantagehowever is in the frequency range above 100 MHz in which commercial hydrophones are not yetavailable.