142 Lecture 9 
9 5-15 kV 
hydrophone 
] flat solenoid insulating foil copper foil 
\ 
water filled tube 
7 
Fig. 9.2. Electromagnetic generation of pressure pulses. 
of the liquid wall of the spark discharge duct generates a high-pressure impact 
in the surrounding water, which propagates as a shock wave. 
Even more efficient is a method known from the electrolytic interrupter. 
The two electrodes in the liquid are separated from one another by means of 
an insulating plastic plate. If there are oneor more holes in this plate, the liquid 
in the holes is evaporated after connecting the condenser with the electrodes 
and the discharge then passes through the vapor. The authors, mentioned above, 
report that a mean acoustic power of 200 w with peak powers of up to 300 kw 
was obtained with a plate with 10 holes. The reproducibility of the cavity shock 
transmitter is low due to the nonspherical collapse of the bubbles, even if the 
initial sizes and gas contents are equal. 
These difficulties are overcome in an electromagnetic shock transducer 
developed by W. Eisenmenger [9], shown in Fig. 9.2. Opposite to a flat solenoid 
and separated from it by thin plastic foilis a copper membrane, which terminates 
the water column into which a shock wave is to be radiated. A current discharge 
through the flat solenoid induces strong eddy currents in the copper membrane. 
The resulting magnetic fields produce very strong repulsive forces between the 
flat solenoid and the copper membrane. Typical data are: A current of 4000 A 
flows through a solenoid consisting of 50 turns, whena condenser with a capacity 
of 0.8uf charged to 20 kv is discharged through it. The duration of the current 
pulse is only 2usec and pressures of up to 200 atm are produced. A condenser 
with 8yuf capacity and a charging voltage of 12 kv produces shock waves with 
peak pressures of up to 700 atm. 
9.2, MEASUREMENT OF HIGH SOUND INTENSITIES 
In the last decade, water-borne sound measuring technique has reached the 
same high standard as has existed for a long time in the measurement of air- 
borne sound fields. There is a great variety of hydrophones available. They are 
usually calibrated like air-borne sound microphones by the reciprocity method. 
We will discuss only those types of microphones for the measurement of high 
intensities which are sufficiently insensitive, overload-resistant, and either very 
small or very large compared to the wavelength to be measured. They should 
have a very high upper limiting frequency and should, furthermore, be of a 
very rugged design. Many conventional types of microphones fail to meet these 
requirements. 
