Acoustic pollution of our environment by a great number of sources is now one of important social problems. In the present paper, noises emitted by a lot of sources are considered statistically. At first, noises from a flow of random point sources simulating a road traffic are discussed. Time and space correlatios of an acoustic power in the radiated field as well as the mean and variance of it are derived and examined in several cases. Then the above mentioned random sound field is treated as a stochastic time process by applying the theory of shot noise. These considerations may give some information on the propagation of noise emitted by freely flowing traffic and be useful to estimate highway traffic noise. Further investigations are required, however, to develop a more realistic model for traffic noise in city which is a complex mixture of deterministic and non-deterministic properties.
When the medium of propagation is dispersive, the pulse method is used for the measurement of the group velocity because the pulse is propagated by energy-transmission, while the standing wave method is used for the measurement of the phase velocity. The correlation technique with some advantages has also been reported to be useful for analyzing propagation of energy. In this case a band-limited white Gaussian noise or AM signal where the Gaussian noise is modulated by M-sequence signal is applied as the measuring signal, and this method can be qualitatively explained as a system applying the envelope detection of the noise signal provided with information of the group propagation. In this paper we examine the use of FM signal which is effective when the mechanical Q of the transducer has such a high value as in under water acoustics, as reported in a preceding paper in detail, because its spectrum can be changed to compensate for the influence of the mechanical Q by using the random signal named binary Poisson process as a modulating signal. However, as the FM signal has a constant amplitude, it is inevitable to adopt a different method from the envelope detection mentioned above and so the system applying the heterodyne detection is investigated in Section 2 (Fig. 2). The following facts are recognized: (1) It is theoretically analyzed that the FM correlation technique applying the heterodyne detection enables us to measure the group velocity, and it is confirmed experimentally that the measurement of the group velocity in a fluid cylinder with a free boundary is consistent with the theoretical curve (Fig. 8). (2) Additionally, even if a signal of a high frequency range is applied, a correlator operating in a low frequency range can be used by changing the frequency of the local oscillator. In Section 3, we investigate the polarity coincidence correlation function in which a nonlinear operator is added. The results obtained are as follows: (3) If the arriving time interval of each receiving signal propagating through multiple sound paths is longer than a certain value, the group-transmission time can be measured discriminately from the cross-correlation curve as in the case without a hard limiter. This is also confirmed experimentally by measuring the sound propagation through a brass pipe (Fig. 5). (4) But, as a strong signal suppresses a weak signal in this case, the amplitude of each receiving signal is not in proportion to the corresponding correlation coefficient (Fig. 7). Accordingly we cannot directly measure the relative intensity of each receiving signal from the cross-correlation coefficient.
The frequency modulated tone (FM-tone) plays an important role in the intonation of conversation and the melody of music. Furthermore, it is generally accepted that FM-components in the earlier part of a consonant are essential for speech recognition. There have been, however, very few reports on the sensation produced by FM-tone by mean of psychophysical experiments. In this paper, the sensation of the FM-tone, especialy loudness, was studied by evaluating the masking value. As seen in Fig. 1, a short burst tone (signal) was applied to FM-tone (masker) repeatedly at 5sec-intervals and the critical recognition level of the signal by the normal ear was measured. The signal was superimposed to the masker at various phases so that the masking pattern was obtaind as a function of the time difference between the onset of masker and that of signal. To get the highest time resolution of masking pattern, a 3kHz-signal with 5msec-duration was chosen and the frequency of masker was changed trapezoidally from about 0. 5 to 2kHz with various modulation intervals. The following experiments were carried out : (1) masking patterns produced by ascending and descending FM-tone, (2) monoaural and binaural maskings by FM-tone, (3) effect of modulation interval, (4) masking pattern produced by two FM-tones, (5) simulation of FM-masking, and (6) masking pattern produced by consonant [ra]. From the experimental results, it was concluded that the masking began at about 80msec prior to the onset of the frequency change and increased markedly at the FM-interval and continued 100msec there after (Fig. 3). These phenomena were observed clearly in binaural experiments at both ascending and descending FM-tones (Fig. 4 (a), (b)). In order to confirm this fact, we compared the binaural masking by the FM-tone with that by the alternating AM-tone which was seen in the sonogram of Fig. 5. From this result, it seemed quite clear that the binaural masking depends only on frequency change. This indicated the possibility of the effect of FM-neurons found in the superior olivary on the binaural FM-masking. The masking value depended on also the modulation time. As seen in Fig. 6 the masking value increased markedly in the range of 10 to 100 msec modulation interval for both ascending and descending frequency changes. It should be noted that the modulation time of 10〜100msec is almost the same as that of FM-components in the earlier part of consonant. In the case of consonant, two or three FM-components are present simultaneously or successively. As seen in Figs. 7 and 8, when two FM-tones were applied simultaneously or successively, a masking pattern was obtaind almost as the sum of two independent masking patterns due to respective FM-components. From the results described above, we derived an empirical equation which was applicable in the binaural FM-masking within 0. 5 to 2kHz frequency change. This was a frequency change/df/dt/of the first order linear system and its time constant was about 0. 1sec. Next, in place of FM-tone, we used a voice [ra] which was filtered and gated as seen in a sonogram in Fig. 9 (a). As is evident from Fig. 9 (b), the masking pattern of [ra] increased markedly at the formant transition and separated clearly two formant transitions. The masking pattern seemed to be a valuable method to analyzed the vocal feature extracting process.