Abstract
It is wellknown that the spectrum of the noise generated by ultransonic cavitation contains not only numerous harmonics of the ultrasound, but also subharmonics and their harmonics. But the reason why the subharmonics are generated has not yet been clarified. On the basis of a series of experiments it has been thought that they are caused by the radiated pressure waves from either the pulsating bubbles or the cavitation bubbles which are generated and collapse. When the intensity of the ultrasound is weak, cavitation bubbles are generated and collapse in each cycle of the ultrasound. With the increase of the intensity of the ultrasound, there appears an excitation range in which cavitation noise shows the so-called "two-cycle phenomena" or "two-cycleness". In the excitation range we have examined in detail the growth and collapse of cavitation bubbles. Cavitaion bubbles were produced by a nickel transducer of frequency 5kHz and were photographed under the illumination by a micro-flashlight of arbitrary time lag from the standard pulses. The standard pulses with frequency 1/2 of the ultrasound were produced from the sound pressure in the tested liquids (Fig. 2). In our experiments we selected the stable region of the two-cycleness of cavitation noise, so that the phase fluctuation of the pulses was about 1/40 of the period of ultrasound. From our experiments it was verified that in the excitation range in which cavitation noise shows two-cycleness, cavitation bubbles collapse completely in every two cycles. Moreover we found that the so-called "incompletely collapsing cavitation" is only produced in the middle part of the driving surface and that the bubbles produced on the circumference of the surface collapse in each cycle (Fig. 3). From this we may conclude that the component of the first subharmonic contained in the cavitation noise generated under intense ultrasound is due to the cavitation bubbles which are generated and collapse in every two cycles and not to the bubbles pulsating in several or more cycles. It is generally believed that the larger the intensity of the ultrasound, the brighter the sonoluminescence is. But the increase of the brightness of the sonoluminescence is suppressed in the excitation range in which cavitation bubbles show two-cycleness. Analysis of the light intensity scattered from cavitation bubbles by using a frequency analyser showed that the first subharmonic at this moment is very large (Fig. 8). Such phenomena are independent of the kind of liquid tested, so that the two-cycleness of cavitation begins to appear with roughly the same excitation range and has no relation to the kind of liquid tested. From these facts we may conclude that the two-cycle phenomena of cavitation take place in the excitation range determined by the experimental equipment. According to the theory of isolated cavity, cavitation bubbles repeat their growth and collapse in each cycle under the weak ultrasound. With the increase of the intensity of ultrasound, it becomes impossible for the bubbles to collapse within the first cycle and it is in the second cycle of ultrasound that they collapse completely. Thus the initial conditions for the growth of cavitaiton bubbles are different with the cycle, so that their growth and collapse begin to present two-cycleness. With a further increase of the ultrasound, cavitation bubbles become such that they cannot collapse in one cycle and the so called "incompletely collapsing cavitation" is produced (Fig. 10〜Fig. 12). The larger the initial radius of a cavity, the more easily two-cycle phenomena of cavitation are produced (Fig. 13). The above consideration may provide a satisfactory explanation of our experimental results.
