FM信号を利用した相関法による水中音響計測

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The pulse and the standing wave methods and mainly applied to under water acoustic measurements. But in most cases where they are disturbed by external noise, it is effective to apply the correlation technique. The main advantages of the correlation method which are well known, are as follows:(1)The sound intensity can be measured without the influence of external noise. (2)If there are multiple sound paths from the emitter to the receiver, each sound ray can be measured by using continuous waves as distinctly as by the pulse method. From the cross-correlation we can detect the traveling time and the relative intensity of the sound. This paper deals with FM signals(frequency modulated waves)as a continuous signal having properties which are common to the white Gaussian noise. In the case of high modulation index, it is well known that the spectrum density of the FM signals is approximately proportional to the probability density function of the frequency-modulating process. Thus, it is expected that we can get the various spectra of the FM signal by changing the frequency-modulating process and the modulation index. Then, as a modulating signal we examined the use of a binary Poisson process(a binary random process whose successive zero-crossing intervals are Poissonly distributed). In this case the FM signal has the spectrum density as shown in Fig. 3. Therefore this method enables the equivalent bandwidth of the receiver output to be large as shown in Fig. 2, when the mechanical Q of the transducer has a high value, as in under water acoustics. Consequently, by selecting a suitable frequency-modulating process and modulation index according to the properties of the measuring system, it is possible to get a smaller separable interval of signals arriving at different times than by the pulse method in which the pulse length is determined by the value of Q. This enables us to take measurements in a small water tank. As mentioned above, using FM signals has the following additional advantages:(3)It is possible to select the spectrum density function, the center frequency and the equivalent bandwidth of the measuring signal, voluntarily. (4)Accordingly, the separable time intervals of the receiving signals can be made equivalent irrespective of the value of the mechanical Q of the transducer. (5)As the FM signal has a constant amplitude, it is easy to polarize the signal in the polarity coincidence correlation detection.
1973-02-01

FM信号を利用した相関法による水中音響計測

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