| 摘要 |
A first transducer 7, having a centre frequency f min 2, has a frequency characteristic for transmission T2 min and a frequency characteristic for reception R2 min . A second transducer 8, having a centre frequency f1, has a frequency characteristic for transmission T1, and a frequency characteristic for reception R1. T1 partly overlaps with R2 min . Therefore, the first transducer 7 could receive reflected signals caused by ultrasonic transmission from the second transducer 8. To eliminate this crosstalk an electrical filter having a frequency characteristic LF is provided, for example, on the receiving side of the first transducer 7. …<??>Thus, the frequency characteristics of the transducers 7 and 8 have an inherent filtering function, and this is combined with the filtering function of an electrical filter provided for example on the receiving side of the first transducer 7. Electrical filters may be provided on the receiving side of the second transducer, or on the transmission sides of the first and second transducers. |
