Since it takes about several seconds to obtain an image pattern in mechanical scan ultrasonodiagnostic tomography, high speed electronical scanning is required to obtain image patterns of moving human organs like a heart. Since the high speed sector scan system designed by J. C. Somer required many electrical circuits connected to each of many transducer elements, the system should be complicated and expensive. We planned to use a small number of transducer elements. However, the reduction in number of elements caused an enlargement of side lobes. In order to suppress the side lobes, the signal processing technique was employed to process the output signals of the receiver. In this paper is described a designing method of an array transducer for electronic sector scan ultrasonic tomography. An array transducer which was designed by this method was produced and its directivity was measured. When each element of the array transducer arranged with an equal interval of d is excited with a suitable delay time, large lobes which equal to main lobe in amplitude should appear at every angle θ satisfing Eq. (3). However, with the maximum interval d_<max> satisfing Eq. (4), these large lobes disappear within ±90°. Then, the overall sensitivity in the direction φ, the deflected angle of the main lobe, decreases due to the directivity of each element. Accordingly, if this sensitivity reduction at the maximum deflection angle φ_<max> is allowed down to α, the width of each element 2B is limitted by Eq. (6). The farfield directivity of the array is given by Eq. (2). Therefore, by designing the transmitter and receiver suitably, overall directivity of the transducer shown in Fig. 6 is given by Eq. (7). When the second zero angle in D_1(θ) equals to the first zero angle in D_2(θ) or 2Md equals to Nd/2, both of the first and the second side lobes in D(θ) become considerably small. Employing the signal processing technique as shown in Fig. 7 to process the output signals from the receiver, the width of the main lobe is narrowed and the side lobes decrease. In this case, the first zero angle θ_<f0> of the overall directivity is given by Eq. (9). We designed the array transducer by the above method on the specifications that the receiver consists of 4 elements, width of the main lobe is ±1 cm at a distance of 10cm from the array, every side lobe smaller than -20dB of main lobe and maximum deflected angle is ±45°. The calculated directivity of the designed array transducer are shown in Figs. 11 and 12, where frequency is 1. 5MHz and α is 0. 7. Figure 8 shows the construction of the transducer consisting of PZT bars of 1mm in thickness, 0. 4mm in width ans 15mm in length. Block diagram of the measuring system is shown in Fig. 9. Figure 10 shows the overall directivity of one element. Figures 11 and 12 show the overall directivity of the arry transducer when the deflected angle φ are 0° and 45° respectively. As a result of the measurement, the transducer had a main lobe with 13° in angular width and side lobes with the lebel less then -20dB in amplitude, when the signal processing technique is employed to process the output signals from the receiver.
Data of sound attenuation constant in sand is nesessary for designing detection apparatus for underground buried pipes by the sound echo method. The measurement of the sound absorption constant in sand with a continuous or pulsed sinusoidal wave would be difficult to perform due to obstruction by electrical induction directly from the transmitting system to the receiving system. The authors devised the method to measure the frequency property of attenuation constant in sand with an impulsive sound wave. In this method, the impulsive sound is radiated from the electromagnetic induction type sound source placed on the ground surface and the sound pressure propagated in sand is received by, a piezoelectric microphone buried at a depth of 1 m just below the sound source, and the received output signals are analyzed with 1/3-Oct. band pass filters. Similar measurements are performed varing the distance from the sound source to the microphone by digging down into the sand. The distance was varied from 1 m to 20 cm. By the variety of each frequency spectrum component of recieved output signals at the varied distance, the sound attenuation constant in sand is determined. In this study, the experiment to obtain the sound attenuation constant in the model sand bath (11 m × 8 m × 2 m) filled with wet mountain sand was performed by the method mentioned above. Soil tests to define the fundamental property of wet mountain sand were carried out. The results of the soil tests are shown in Fig. 1 and Table 1. Fig. 2 shows the block diagram of the measuring apparatus of radiated sound pressure. The received signals were recorded by a magnetic tape recorder. In this experiment, the driving current frequency f_c was increased from 200 Hz to 950 Hz by decreasing the condenser capacitance from 180 μF to 10 μF, and the input energy was kept at 250 joule by increasing the charging voltage as the capacitance is decreased. Fig. 3 shows the block diagram of the apparatus for frequency analysis. The received output signals recorded by a magnetic tape recorder were processed using 1/3-Oct. band pass filter during reproduction. In order to obtain an accurate sound attenuation constant, it is necessary to use received output signals with large S/N ratios. Accordingly, we adopt the result obtained by frequency analysis in case of three kinds of f_e in each frequency band, as shown in Table 2. Fig. 4 shows, as an example, the attenuation constant in frequency band at 355 to 450 Hz. Because measurement points are fairly scattered in the figure, the attenuation constant was obtained by the method of least squares. Besides, each measurememt value was normalized with the value in the case where the distance from the sound source to the microphone was 20 cm, or the sound attenuation constant in sand was the smallest. Attenuation constants of other frequency bands were obtained by a similar method. The results are shown in Fig. 5. We also found by the experiment in a model sand bath that the attenuation constant in wet mountain sand (Particle size: 0. 4 mm, Specific gravity of particle: 2. 45, Porosity: 34. 45%, Water content in percentage of dry weight: 11. 6%) is expressed as α≃3. 1×10^<-5>×f^2 (dB/m) where f is the frequency in Hz.
Ultrasonic power applications sometimes require morevibrational energy than a single transducer can provide. In such cases, the ultrasonic power density canbe increased if concentration of energy from severaltransducers onto one load is achieved by a certaindevice. The authors devised energy concentrators fortorsional vibrations utilizing flexural modes based onthe idea that there is no transverse displacements butonly angular displacements at the nodal point of aflexurally vibrating bar. If we use this system inversely, vibrational energy of a single transducer canalso be divided onto plural loads. The authors alsodevised the cross-type concentrator/divider in whichtwo flexurally vibrating bars are crossed at a nodalpoint.
The paper describes an analysis on the fundamental resonant frequency and the vibration mode of the plate tuning fork resonator and gives a design guide of the resonator. There are two types in the flat plate type resonator, the uniform arm type and the stepped arm type, as shown in Fig. 1 (a)〜(c). The stepped arm type resonator in Fig. 1 (b) is analyzed, since the other type resonators are asumed as the modifications of that of Fig. 1 (b). The half section of the resonator drawn by thick line in Fig. 2 can be expressed with the mechanical equivalent network shown in Fig. 3. If the external force P_<a1> is applied at a tree end of the stepped arm in Fig. 2, admittance matrices of the parts A_B, B_B, C_B are given by Eqs. (1)〜(3) respectively, and admittance Y_s and Y_f are given by Eqs. (6) and (7). Therefore, the driving point impedance z_i(=P_<a1>/V_<a1>) is derived as Eq. (14), and the equation to determine resonant frequency is given as Eq. (15) from resonant condition z_i=0. Similaly, the resonant frequency equetion of the uniform arm type resonator in Fig. 1 (a) is given as Eq. (17). The calculated and the experimental values of resonant frequencies against base width ω_c of the resonator are shown in Fig. 4. The differeces between both values are less than 3. 4% in the range of 2<ω_c<8 mm. The resonant frequency for the arm length 1_b is shown in Fig. 5, in which the resonant frequency of a cantilever with same arm dimensions as one of the plate tuning fork is shown by doted line for reference. The relation of the normalized resonant frequency ratios versus the rength ratio (l_a/l_<ab>) for the stepped arm type is shown in Fig. 6. The differences in the experimental values between the symmetric type and the unsymmetric type are less than 1. 3% as shown in Fig. 6; and the differences between the calculated and the experimental values inn two types mentioned above are within 1. 5%. It is clear that the stepped arm type resonator gives lower resonant frequency by about 35% as compared to the uniform type one. The resonant vibration mode of the resonator is determined by the combination of both flexural and tosional vibration displacements. The respective velocities (v_m and φ_m) at an arbitral point of the resonator are determined from Eqs. (18) and (19), and then the resultant displacement η_m is calculated by substituting the v_m and φ_m into Eq. (21), in which plus sign in the right side of Eq. (21) indicates that it lies outward by (ω'_n/2) from neutral line, and the minus sign means it lies inward by (ω'_n/2). The vibrational displacement on the neutral axis of the uniform arm type is shown in Fig. 7, and the location of nodal line against the base width ω_c is shown in Figs. 8 and 9. The displacement on the neutral axis against the length ratio (l_a/l_<ab>) of the stepped arm type is shown in Fig. 10, and the location of nodal line shifts, as shown in Fig. 11, as l_a increase. The analytical and experimental results mentioned above are useful for the design of the plate tuning fork in low frequencies below several hundred Hz.
Detection and visualization techniques for ultrasonic fields and vibrations by means of cholesteric and nematic liquid crystals are first examined. Ultrasonic fields of a circular vibrator in a sectional plane perpendicular or parallel to the vibrator are visualized on the detective cholesteric liquid crystal plate inserted (Fig. 4, 6 and 8). The patterns (change of the color) corresponding to the sound pressure or temperature distributions are measured by a hydrophone or a thermocouple with a small sound absorbing ball (Fig. 5 and 7). Simple ultrasonic camera is demonstrated by making use of this cholesteric liquid crystal plate(Fig. 9). A visualization technique for the vibrational modes of a viblator is developed. A cholesteric liquid crystal sheet is attached to the surface of the vibrator by means of electron grease as a loose coupling means. the pattern appeared corresponds to its verocity distribution. On the other hand, when α-cyano acrylate (alone alpha) is used for tight attachment, the pattern corresponds to the temperature distribution. That is, the color is just in reverse to the former (Fig. 10). Similar experiments are carried out for nematic liquid crystal as a detector. Ultrasonic field patterns of a circular vibrator in a sectional plane parallel to the vibrator are visualized on the nematic liquid crystal layer sandwiched by glass plates (Fig. 11). Two treatments were tested for orientating the crystal molecular axis perpendicular to glass substrates. One is to paint lecithin on the substrates and the other to dope a small amount of Cetyltrimethylammonium bromide into the liquid crystal. Detection sensitivity is found to be far better for the latter than for the former (Fig. 12). It is also found that the sensitivity is increased by several times with reasonable electric field superposed (Fig. 13). It is shown that the nematic liquid crystal can be used for visualizing the vibrational modes of a vibrator. On the vibrator surface, a nematic liquid crystal layer is formed, which is then covered by a thin glass plate. Similar treatments are necessary for molecular orientation. THe modal pattern can be observed by bare eyes, which is due to the dynamic scattering mode (Fig. 14). The cholesteric liquid crystal detector provides better resolution than the nematic, while the sensitivity is rather same.