156 Lecture 9 
emission of the luminescence pulse which, in these experiments, is generally 
composed of a great number of closely succeeding individual pulses. During one 
half-period, that is, in the phase interval 0° to 180°, there are no bubbles at all. 
They occur quite suddenly between 180° and 240°. Just before the phase reaches 
360°, their number and volume rapidly decrease as shown in Fig. 9.25. It can 
be inferred from such experiments, together with measurements of the spectral 
distribution of the luminescence light [19], that luminescence is caused by 
thermal excitation of the highly compressed gas in the bubble in the last stage 
of collapse. 
Luminescence occurs just at the end of the collapse and simultaneously the 
shock wave mentioned in Section 9.1.2 is radiated. This coincidence is especially 
emphasized in a photograph made by Kuttruff displaying the luminescence pulse 
and the shock wave shown in Fig. 9.26. The luminescence pulses were recorded 
with an oscilloscope, and simultaneously the luminescence pulse triggers an 
illumination spark for photographing the shock wave with Schlieren optics. If 
the time delay between these events is known, both effects (luminescence pulse 
emission and shock wave generation) are easily correlated. A great number of 
such measurements have been made andthe results are plotted in Fig. 9.27. They 
show the close relation between shock wave generation and sonoluminescence. 
DISCUSSION 
PROFESSOR D. SETTE questioned whether, in the measurements ofacoustic 
absorption at different sound intensities using light diffraction, the effect of 
nonplanarity in the waves had been taken into account. He and his colleagues, in 
1948, had noted at low intensities a distortion of the diffraction pattern with in- 
creasing distance from the source, which they had attributed to the divergence 
of the beam. 
PROFESSOR MEYER: Experimental data on sound absorption in liquids ob- 
tained at various sound intensities with the light diffraction method were taken 
from the literature. I believe that the effect of nonplanarity of the sound waves 
has been properly considered in the respective studies. 
DR. H. MEDWIN commented upon the uncertainty of the magnitude of the 
acoustic pressure in the front and the tail of the shock disturbance in a liquid. 
When the shock impinges on the face of the quartz detector, we can neither as- 
sume that the quartz is a rigid boundary when immersed in a liquid nor that the 
pressure is doubled, as we are no longer dealing with linear acoustics. When 
using a quartz detector for shock waves in a gas, the assumption of its behavior 
as a rigid boundary for reflection is justified, and, for shock reflection, the re- 
sultant acoustic pressure is somewhat greater than twice the value for the in- 
cident disturbance. For shock reflection in liquids, the amplification factor would 
require experimental determination. The use of the reciprocity technique for 
the calibration of microphones assumes that operations are in the field of linear 
acoustics, where it is possible to calculate the attenuation caused by divergence 
and energy absorption (by the medium) between the source and receiver. 
PROFESSOR MEYER: With respect to the very interesting remark of Dr. 
Medwin, I would like to confirm that there are considerable differences in the 
