Maybe you could use a strobe effect: Have an audio sample that loops every 1/100th of a second, and then put a "fan" spinning at the same rate in front of the e

Maybe you could use a strobe effect: Have an audio sample that loops every 1/100th of a second, and then put a "fan" spinning at the same rate in front of the electron source, which blocks the electrons except for 1/60th or less of the 1/100th of a second. As you vary the phase of the fan, you capture the needle at different positions in the loop.