This paper describes a new type of underwater imaging system which displays an acoustic image formed by an acoustic lens in real time. The rod-like transducer constructed by arranging 500 narrow strips of a piezoelectric material is used in the system. This transducer with an acoustic lens radiates a fan-shaped sound beam. Scanning of the focused sound beam to illuminate some objects in water is performed by varying the sound frequency periodically. Measured values of this transmitting part are as follows: 1. Horizontal beam patterns of the transducer with a cylindrical lens having 10m focal length are measured. The beam width and focal depth are 6cm and 4m respectively (Fig. 7). 2. Direction of sound radiation varies at 30° with the change of frequency from 1. 0 to 1. 9MHz (Fig. 6). Back scattered sound waves from the objects are detected by a receiving equipment which consists of an aspherical solidliquid compound lens and piezoelectric receiving plate. Closely-spaced shallow cuts are made across the front face of the plate, to provide electrical and acoustical isolation between there adjacent line elements. The following are the characteristics of the receiving equipment: 3. Field of view of the compound lens is 20° with reduction of 3dB of the sound pressure (Fig. 15). 4. Effective value of the ratio of aperture-focal length of the compound lens is 1. 0. 5. Acoustical coupling of the adjacent line element of the receiver plate is estimated to be -8 dB (Fig. 15). Acoustic imaging at 10-m distance is achieved by supplying DC power of 30 watts into the driving amplifier, while the imaging speed is 37 frames per sec. The results of investigation show that this imaging system is available for the long-range and high-speed imaging in water.
This paper presents a method to estimate the attenuation of a band noise by a thick barrier. In this method, the attenuation is assumed to be effected by two factors, one is the effect of a virtual thin barrer placed on the receiving side of the barrier with the same height and the other is the effect of thickness (see Eq. (1)). The charts to estimate the attenuation by a thin barrier are discussed in Section 3, and the validity of Meakawa's chart is verified by Macdonald's solution (see Figs. 2(a) and 2(b)). A new chart based on Macdonald's solution is offered to estimate the attenuation by a thin barrier more accurately (see Fig. 3) and the accuracy of this chart is checked numerically (see Figs. 4 and 5). The effect of thickness is discussed in Section 4. The effect of thickness is defined by the difference of sound pressure level in the shadow zone resulting from the replacement of a thin barrier by a thick barrier. The effect of thickness for a pure tone _p[ET] varies periodically along the frequency axis (see dashed curve in Fig. 6). For a band noise, this effect of thickness must be averaged over the frequency range of the noise. Then, from the practical point of view, a simplified chart to estimate the effect of thickness is offered withe a reasonable approximation when the thickness of barrier is larger than half a wave-length (see Fig. 9). The validity of the method presented here is verified by experimental results (see Fig. 10).
Behavior of an adaptation-type neuron to the connected vowel speech sound has been investigated using an electronic model of the auditory nervous system. Samples of the speech sound are /aija/, /auwa/ and /aja/ of the synthesized speech (Fig. 1) and /maiasa/ of the human speech. An essential function of the adaptation-type neuron is a remarkable temporal sharpening effect due to the delayed lateral inhibition (Figs. 2, 3, 4, 13), so that the response to the connected speech sound is in many cases segmented temporally at each valley of the envelope of the speech sound (Figs. 6, 12). Furthermore, the additional segmentations to the secondary and third formants in the response occur for the frequency change (Figs. 5, 6, 7, 10(I), 12) and amplitude change (Figs. 8, 9, 10(II)) following the formant transition. Owing to these functions the response tends to be devided into a transitional part and a steady part of the connected speech sound. Adaptation-type neuron also behaves a stronger spatial sharpening than the primary and secondary neurons of the sustained-type. Therefore, it is suggested that the connected speech sound which a continuous signal is converted to the spatio-temporally discrete signals in the auditory nervous system.