1991 年 57 巻 539 号 p. 2217-2222
This research paper presents a new method of measuring the length of a pipe by using sound frequency. A sweep signal is radiated from the speaker toward the inside of the pipe. The sound pressure at the resonant frequency of the air column in the pipe indicates the largest value. The length of the pipe to be measured corresponds well to the resonant frequency. The phase. locked loop device is adopted to obtain the stabilities of resonation and frequency. A frequency counter with a reciprocal mode is adopted for higher resolution within a shorter time period because of low frequency.