E. Meyer 141 
ducers; the fundamental vibration of the system is excited by a single current 
or voltage pulse producing a damped wave train. This method of excitation is 
a transition from stationary sinusoidal excitation to a single compression or 
rarefaction pulse, which has rather interesting qualities. The three methods of 
exciting such single pulses will be discussed: (1) the collapse of a cavity in 
liquid, (2) an underwater spark, and (3) an electrodynamic pulse transducer. 
As is well known, collapsing bubbles play an important part in cavitation. 
Cavitation is observed, for example, as "flow cavitation" at the tip of ships' 
propeller blades or when a stream of liquid impinges on a sharp edge. It can 
also be generated in the underpressure phase of a sound radiator as "sonic or 
ultrasonic cavitation." The collapse of the cavity produces a shock wave, the 
intensity of which increases as the gas pressure in the cavity just before the 
collapse becomes smaller. Therefore, the collapsing of gas-free or almost gas- 
free cavities in degassed water produces rather intense shock waves. This is 
often easily observed by just listening to the event. 
It is not necessary for the cavity to be formed by underpressure inflow or 
by the underpressure phase of an oscillation. It is also possible to introduce 
"ready-made" cavities into the liquid by blowing hot steam from a nozzle into 
cold water, where the hot steam bubbles collapse. A very elegant method is to 
completely evacuate a thin-walled glass sphere, such as a Christmas-tree 
decoration, or to fill it with a certain gas at a given low pressure. This sphere 
is then immersed in water or another liquid and destroyed by mechanical shock. 
The liquid streaming into the cavity is stopped abruptly, radiating intense shock 
waves. Such experiments were made atthe III Physikalisches Institut at Gottingen 
[20] as a simple means of studying sonoluminescence described in section 9.3.5. 
Shock waves can also be generated by the use of an underwater spark. 
Frungel and Bailites [8] studied this method using the very simple circuit shown 
in Fig. 9.1. A condenser of 0.01 to 0.05yuf is charged to 7 to 9 kv and then dis- 
charged through a series circuit containing the underwater spark gap, an air 
spark gap, and the circuit inductance of about lph. In order to increase the 
durability of the system, one electrode of the sparr gap consists of a resilient 
strip of metal and the other of a brass or copper block. The sudden evaporation 
Fig. 9.1. Sound generation by underwater 
sparks. 
