Optical methods of the visualization of ultrasonic waves have been classified in accordance with the mechanism of the image formation and also in accordance with the object (purpose), and the relations among these various methods have been discussed together in this paper. The methods existed so far were classified according to the mechanism of the image formation into the following groups: (A) the Schlieren method, (B) polarization method, (c) phase-shift method, (D') geometrical optical image, (D) Fresnel diffraction image, (E) shadow photograph method, (F) Fraunhofer diffraction image, (G) phenomenon of the fringe shift in the Mach's (or Jamin's) interferometer, and (H) method of the optical mirage. On the other hand, the existing methods were classified according to the object (purpose) of the image as, (1) image of the sound wave surfaces by instantaneous illumination, rotating mirror, and stroboscopic illumination ( other than the ultrasonic stroboscope employing progressive ultrasonic wave), (2) image of the sound wave surfaces by ultrasonic stroboscope employing progressive ultrasonic waves, (3) image of standing ultrasonic waves, (4) combination waves (fringes formed by two crossing ultrasonic waves under continuous illumination), and (5) sound amplitude fields of progressive ultrasonic waves. The classifications (A)-(H) and (1)-(5) are independent and many combinations like A(1), C(5) are available, though not all of the methods (A)-(H) are applicable to each of the images of the classes (1)-(5).
Mechanism of the image formation by the optical methods of the visualization of ultrasonic waves has been discussed in relation to the characteristics of the images obtained. The images obtained by the various methods, such as (A) the Schlieren method, (B) polarization method, (C) phase-shift method, (D') geometrical optical image, and (D) Fresnel diffraction image, have different characteristics in regards to the plane of focus of the image, and the position of the wave-surfaces, and these are also dependent on the experimental arrangements, sound intensity, and the nature of the waves-progressive or standing waves-in some cases.
Speech band was divided into 8 channels by filters of which midfrequencies were 350, 523, 783, 1174, 1760, 2637, 3591 and 5916 c/s. From the statistical results of analysis obtained from vowels of 100 male students, Japanese vowels can be expected to be discriminated with considerably high accuracy with only first five filters. An experimental vowel discriminator was constructed and examined on 20 men's vowels, whose age ranging from 16 to 60 years. Good results were obtained as expected.
In the present study it was clarified that barium titanate transducers, smaller in dimensions than the wave lengths of elastic waves in solids, transmit simultaneously longitudinal, shear, and surface waves of ultrasonic frequencies when subjected to an electrical pulse. Directional characteristics of longitudinal and shear waves generated by small transducers in aluminium semi-cylinder was also measured by a pulse method. Experimental values show qualitative agreements with the theoretical predictions of Hirono (1948), and Miller & Pursey (1954). It was made clear from the results of investigation of the propagation of elastic waves in aluminium plate that a transmitter and receiver should be placed in proper positions and directions on surface of solids when small transducers are applied for the detections of sear waves in solids.
In many experiments conducted in water tank such as the measurement of the sensitivity and the directivity of ultrasonic transducers or the measurement of sound field distribution, we experience sometimes severe attenuation anomaly. The anomaly is due to the bubbles which are generated naturally on the surface of the transducers or in the water. The authors discussed the condition of the bubble generation from the view point of supersaturation of dissolved air in the water due to temperature rise of the water. A chart was made representing air content in water and the solubility versus temperature and total pressure. The degassifying treatment of the water by heating, and the same by pressure reduction which are used in this country as the prevention of the attenuation anomaly are explained with this chart. The authors invented and tested an apparatus for the measurement of the air content in water or the degree of saturation. The accuracy is not so high, although the apparatus is sufficiently useful for the determination of whether the degassifying treatment is required or not. As the example of the attenuation anomaly, the transmission through the water in an anechoic tank was observed under heating condition by an electric heater and during natural cooling. The result showed clearly the fact that the considerable increase of attenuation takes place in the period of temperature rise and no increase in the cooling period for 50 kc. However, the abnormal attenuation was very little for 100 kc. Concerning the duration of the effectiveness of degassifying after the treatment, the author calculated the diffusion of air in the water, and concluded that the duration must be mainly due to the convection of the water in the tank. In addition, the causes of bubble generation other than the temperature rise such as the assimilation by plants in water or planktons and the decomposition of organic substances deposited on the bottom are discussed. And the reason that the water in the depths of a large tank has few chance to generate bubbles is also described.
We have had reported the results of our study on the propagation characteristics of ultrasound, from 100 kc to 400 kc. Lately, we have made experiments on the ultrasound of 1 Mc as described below. Taking the universality into consideration, this experiment was made simultaneously with the frequency of 28 kc, data of which had been collected for years. The 1 Mc fish-finder have been checked in comparison with the 50 kc fish-finder, applying them to fish school, seaweed, etc. The results of the above experiment have been summarized as follows: (1) The absorption constant in horizontal propagation is from 500 db/km to 560 db/km at the mouth of the Tokyo Bay. (2) The absorption constant in vertical propagation is 400 db/km there. (3) The reflection loss at the sea bottom is 30 db in case of mud and 15 db in case of coarse sand including pieces of shells. (4) The result of experiment of fish finding by 1 Mc made in comparison with that by 50 kc shows that the densities of fish schools were found more clearly by the former, while the absorption by bubbles in the wake was hardly noticeable.
A new system of the low frequency ultrasonic apparatus for bacteria destruction was developed for laboratory use. The plot of this system is not to produce an intense ultrasonic field in liquid but to vibrate a small part of the bottom of a metallic spherical flask violently. The driving system consists of a 20 kc rectangular magneto-strictive transducer and a resonant metallic exponential horn of half wave length, and they are soldered together. The vessel containing the liquid for treatment, made of stainless steel sheet of 0. 7 mm thickness, is placed on the narrow end of the metallic horn and is pressed against it with adequate pressure. Hence, the vessel is detachable and can be pasteurized conveniently. The introduction of the horn as a mechanical impedance transformer improved the efficiency. The voltage proportional to the velocity or the displacement of the horn end is indicated on a voltmeter as the measure of the destroying activity. This voltage is produced by means of "motional impedance bridge" which is inserted between the power amplifier and the transducer. This voltage must be the more direct measure of the activity than the sound pressure in the vessel or the driving current of the transducer. The frequency of 20 kc was selected so as to avoid the deterioration of the contents in the cell and the discomfortable sustained noise. The actual test for the destruction of coli bacteria are described also.