A high amplitude sound pulse is generated using the piezoelectric transducer. This sound pulse travels through the liquid and explodes every electron bubble in its path. The sound takes about 40 microseconds to travel through 1 cm of liquid. During this time the liquid is illuminated by a pulse of light from a flash lamp. Light scattered from the exploded electron bubble is recorded using a video camera. In the first experiment this was a home camcorder but now this has been replaced by a more sensitive camera.
To make a movie we repeat this process several times per second with the sound and the light synchronized. Each time that sound and light are applied, energy is deposited into the helium which increases the temperature. To prevent this temperature increase from becoming too large, we have to keep the number of sound and light pulses introduced per second below a critical value. In our first experiments we could put in only four pulses per second and so the electron bubble moved several millimeters in between the times at which it is photographed. You can see the very first movie by clicking here. In more recent experiments we have been able to increase the rate at which the sound can be applied. Some frames from more recent movies are available here.
These experiments are difficult to perform because it is necessary to have a sound transducer which can produce a pressure swing large enough to cause the bubble to explode. The transducer has to have a large area so that it can produce a sound wave of large area which will explode all of the electron bubbles within a large volume. The transducer is a slab of piezoelectric material to which is applied an alternating voltage to make it oscillate. The transducer often breaks before the amplitude is high enough to explode the electron bubbles.