23 Experimental Mapping of the Acoustic Field Generated by Ultrasonic Transducers 245 23.3 Testing Performed In this work, a non-contact plane type circular ultrasonic transducer Ultran NCG50-D50 [14] was tested, as shown in Fig. 23.2. The transducer is air-coupled and designed to generate plane traveling wave in both air and solids. It has an effective diameter of 50 mm and a nominal operating frequency of 50 kHz. To characterize the ultrasonic transducer, a laser scanning vibrometer, Polytec PSV-400 [15], was used to measure the vibration characteristics for the transducer’s surface. In Fig. 23.2, a circular pattern of 217 numbered grid points (for instance, point #7 on the inner most ring) including the one at the center (Point #1) was defined to represent the transducer surface. Initially, a test was performed to determine the FRFs of the transducer surface velocity with respective to the drive voltage. In the test, a periodic chirp signal generated by the Polytec laser vibrometer’s function generator was used as the excitation signal, which went through a power amplifier PZD700 [16] and finally was applied to the test transducer; surface vibration was recorded by the Polytec system. The testing frequency used was from 0 to 80 kHz with a sampling frequency of 204.8 kHz and frequency resolution of 6.25 Hz. Then, the vibration velocity profile of the transducer surface was measured by using the Polytec scanning laser vibrometer at different frequencies within the effective operating frequency range determined by the above test. Experiments were also performed to measure the acoustic FRFs for the ultrasonic transducer over the same frequency range identified by vibration test. In these experiments, an array of ten 10-mm spaced precision microphones (Model 378C01) [17] was located in the acoustic field before the ultrasonic transducer, as shown in Fig. 23.3a. Three finite vertical measurement planes (in XY plane), P1, P2 and P3, and one finite horizontal plane (in XZ plane) P4 were tested, as illustrated in Fig. 23.3b. The distances between planes P1, P2, and P3 and the transducer surface were 160, 140 and 180 mm, respectively. Plane P4 was located at the center of the transducer with the near and far end limits of the plane located 110 and 210 mm from the transducer, respectively. Note that distance was measured from the front of the microphone protection grid to the surface of the ultrasonic transducer plane. In each plane, a rectangular measurement pattern of 441 grid points evenly distributed within a range of 100 mm 100 mm was employed, as seen in Fig. 23.3c, a local coordinate system x0 (represents a column) and y0 (represents a row) was set to define the measurement points in a plane. The distance between two adjacent points along either x0 or y0 direction was 5 mm. In performing the acoustic FRF measurement, the Polytec laser system was used to provide the periodic chirp excitation signal. A multi-Channel LMS Test.Lab Spectral Analyzer [18] was used to measure the data and process the FRFs. The frequency resolution for both Polytec and LMS system was 6.25 Hz. Ultrasonic transducer Number of points: 217 Vibration isolation Point #1 Point #7 Fig. 23.2 Ultrasonic transducer and pattern of grid points for measuring the ultrasonic transducer surface vibration
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