A novel method for the quantitative measurement of acoustic emission is worked out. The performance of the method is analyzed with regard to transducer loss and ultrasonic attenuation in the specimen. The magnitude of the error in the measurements is shown to be negligible for purposes of the present discussion. This method is actually applied to tension tests of pure aluminium specimens. The power spectrum of the acoustic emission for plastic deformation of the specimen is obtained quantitatively over a wide range from 100 kHz to 4 MHz. The total acoustic emission power attains a peak of 5 pW at the beginning of plastic deformation and decreases to 0. 3 pW as strain increases. The autocorrelation function, which is the Fourier transform of the power spectrum, is given as a monotonically decreasing function of τ. This shows that the elastic energy of the acoustic emission is radiated not in oscillatory form but in the form of random pulses. The mean value of the pulse width is estimated to be about 0. 6 μsec in the early stages of deformation and is observed to decrease gradually to less than 0. 2 μsec with deformation. This value is considered to correspond to the average dwell time of the elementary source events producing the acoustic emission. The change of the dwell time is attributed to an increase of the density of dislocations in the material. It is concluded that this quantitative method is a powerful tool for research on acoustic emission mechanisms in connection with dislocation kinetics.
The neuroral response of the collicular auditory neurons of the cat were investigated using the connected speech and the vowel /a/ segmented from the connected speech. Using a CAT B400 computer, PST (poststimulus time) histograms are made from the spike data obtained for quantitative measurements and the correspondence between the components of speech sound are discussed. In the PST histograms, the abscissa is time and the height along the ordinate represents the number of spikes which occurred in a particular time bin over the 64 repetitions of the speech sound. (1) The collicular auditory neurons responding to the voiced sound carry the information concerning the fundamental frequency of the voice by means of spike train time patterns regardlees of the response type, CF or stimulating intensity level (Fig. 2). (2) When the intensity level of the stimulus is increased, the neuroral activity increases and the duration of response to the sound becomes long due to the response of slow adaptation-type neurons. The response pattern on the histograms for on-type neurons is different from that for slow adaptation-type neurons (Fig. 3). (3) The relation between the frequency components of speech sound and the response patterns for on-type neurons with various CF is diverse and complex. (4) The neuroral responses for a vowel segmented from the connected speech differs from the responses for the same vowel in connected speech. In many cases, the neuroral activity for the segmented vowel increases (Figs. 6 and 7). There appears to be a particular inhibition having a delay extending over 100 msec in the auditory neural network, and thus neuroral response th the segmented vowel increases due to the reduction of such an inhibition which is produced by the preceding syllable.
In case of an isolated vowel or a CV syllable, a close relation can be found between the phonetic quality of a certain phoneme and its acoustic feature. This relation, however, is not always preserved to the phonemes in an ordinary connected speech. In our previous study, it was found that the perception of a vowel in a connected speech was seriously impaired by the complete removal of its phonetic environment and that two syllables, one preceding and one following, were necessary and sufficient to provide a perceptual environment for their correct identification. From this point of view, an identification experiment concerning the dynamic aspect of speech perception has been performed on synthetic vowels in terms of phoneme boundary location. As a typical example of phonetic environment vowels, symmetrical nonsense words /uVu/ were chosen for the perceptual experiment. The formant patterns assigned to the point of closest approach to target were selected from a set of 16 points spaced equally on a straight line in the F_1-F_2 plane (Fig. 1). Synthetic stimuli were presented to listeners under two conditions, steady-state and /uVu/ condition. Listeners were asked for identifying each synthetic vowel under the steady-state condition and the middle vowel in each /uVu/ word with one of Japanese five vowels. It was found that the phoneme boundary locations of vowels under the /uVu/ condition shifted largely, about 4 to 5 steps in stimulus number, compared to those of vowels under the steady-state condition (Fig. 6, Fig. 7 and Table 2). The result suggests that there exists a certain function in human auditory system so as to compensate the formant frequency undershoot associated with vowel reduction.