| 摘要 |
PROBLEM TO BE SOLVED: To enhance the accuracy and resolution of measurement by an arrangement wherein a circuit for driving one of a pair of ultrasonic oscillator has first and second different driving frequencies. SOLUTION: A control section 12 controls a drive circuit 7 to drive an ultrasonic oscillator (hereinafter, oscillator) 2 to transmit an ultrasonic pulse (hereinafter, pulse). An operating section 11 operates a propagation time t1 from the pulse received by the oscillator 3. Subsequently, the oscillator is switched between the drive circuit 7 and a receiving searching circuit 9 and when a transmission start signal is delivered from the control section 12, a transmission pulse from the oscillator 3 is received by an oscillator 2 and a propagation time t2 is operated at the operating section 11. A current velocity V is determined from t1 and t2. When the frequencies of first and second transmission circuits 4, 5 are set at about 70 kHz and 100 kHz, respectively, the oscillator 2 resonates at the first frequency but when it is driven subsequently at the second frequency, resonance at the first frequency is prevented and the oscillator 2 does not resonate at the second frequency. Consequently, the receiving pulse of the oscillator 3 has a short reverberation time.
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