E. Meyer 145 
is present, light is diffracted by the density gradients produced by the sound 
wave, so it passes the sides of the Schlieren diaphragm and is measured with 
a photomultiplier. The density gradient can be determined, if the width of the 
beam of light is very small. Ifthe beam is wide, the arrangement integrates over 
the respective area and the compression is measured directly. This microphone 
seems to be rather attractive, but it is very difficult to obtain frequency reso- 
lution up to 1 Mcps. This is caused by insufficient parallel adjustment of the 
diaphragm gap and by discontinuities at the gap edges. On the other hand, the 
above described method is perfectly suitable for photographing the spatial 
structure of the entire incident wavefront as indicated in Fig. 9.19. 
For time or frequency analysis of a shock front, a special microphone is 
required. The frequency response curve should exceed, by far, the fundamental 
frequencies of the smallest quartz microphones, since these fundamental fre- 
quencies are only of the order of magnitude of several Mcps. Eisenmenger [21] 
proposed the design of a microphone similartothe hypersonic quartz transducer 
reported by Bommel and Dransfeld, and Jakobsen. Inhis design, the water-borne 
sound wave hits the front surface of a longitudinal expander quartz rod shown 
in Fig. 9.7. The front surface is, for this purpose, adjusted exactly parallel to 
the wavefront. The dilatational wave excited in the quartz rod is propagated 
through the quartz until it is reflectedatthe free rear surface, which is polished 
plane and parallel to the front surface. Simultaneously with the reflection, the 
electric space-charge wave traveling with the acoustic wave is coupled out by 
means of a low-resistance coaxial line circuit. Apart from the attenuation in 
the quartz, the frequency response curve is almost entirely dependent upon the 
parameters of the electrical circuit. The lower limiting frequency is given by 
the length of the quartz rod. Towards higher frequencies, the frequency range 
extends into the kMc range. 
Finally, still another microphone must be mentioned, which has the ad- 
vantage that measurements may be carried out even at hard-to-reach points 
by proper shaping of the sensitive element. In principle, this microphone is 
closely related to the quartz coaxial line microphone, but utilizes the magneto- 
strictive effect. According to Koppelmann [11], the sound pressure of a sound 
wave incident on the tip of a thin nickel wire immersed in water generates an 
extensional wave which propagates along the wire. Acoil wound around the nickel 
shock wave quartz capacitive probe 
Fig. 9.7. Quartz hydrophone for 
shock wave measurement. 
oscilloscope 
