154 Lecture 9 
160° 210° 240° 270° 300° 330" 360° 
Fig. 9.24, Cavitation bubbles at different phases infront of a vibrating piston made at 30° intervals. 
limiting frequency is only 30 Mcps) and traveling wave oscilloscopes available 
now are not sensitive enough, 
Therefore, a Fourier analysis of the event is suggested. For this purpose, 
the coaxial line is replaced by a coaxial resonator, which is successively tuned 
to different frequencies as shown in Fig. 9.21. The resonant frequency filtered 
out from the shock wave is magnetically coupled from the resonator and is ap- 
plied to a superheterodyne receiver. Oscillograms ofthe output signal are shown 
in Fig. 9.22 for the frequencies 280 and 950 Mcps. The multiple reflections of 
the shock wave in the quartz can be clearly seen. If such measurements are 
carried out in the frequency range from 200 to 1000 Mcps, the curve displayed 
in Fig. 9.23 results. The time derivative is plotted against the frequency of the 
measured pressure component, that is, the product of "spectral intensity times 
frequency." The small diagram in the upper right-hand corner with a logarithmic 
ordinate scale and square-law frequency scale demonstrates that the measured 
values may be approximated by a Gaussian distribution. From this, it follows 
that, for a shock wave with a peak pressure of 65 atm and a rise time of 
0.95-10~° sec, the shock front thickness is 1.4-10~* cm. It is interesting to note 
that the sensitivity of the quartz microphone used in these measurements was 
-180 db relative to 1 v/ub. It was 20 mm in diameter and 8 mm thick. 
9.3.5. Sonoluminescence 
We will now treat a physical effect which is closely related to the collapse 
of cavities in liquids and, thus, alsowiththe generation of high sound intensities, 
namely, sonoluminescence. This effect is observed when a single cavity, for 
example, the Christmas-tree-decoration sphere mentioned in Section 9.1.2, col- 
330 345° 350 355° 360° 
f = 25kk2 760°S 400 ps 
Fig. 9.25. Cavitation bubbles at different phases in front of a vibrating piston at 5° intervals. 
