Sounds improvised by electrons...
Whilst playing with a 25kV flyback supply and some capacitors to make some nice fat sparks, I wondered what the effect would be if they were combined with a resonant tube. A quick experiment with some plastic tubing looked very promising, and so was born the Spark-O-Phone...!
The caps are charged via 100 megohm high-voltage resistors, producing a
regular spark discharge at intervals in the range 0.2 to 5 seconds, depending on the
applied voltage and gap width.
As the number of tubes increases, the interaction effect tends to avarage out, leading to a more regular beat for each of the tubes, but the larger number of different notes tends to disguise the regular beat of each tube and produce a more random-sounding 'tune' (jazz mode - nice). Small adjustments to the gaps have a significant effect on the sound patterns. Adjusting the supply voltage slowly controls the avarage tempo of the generated sounds, and other interesting effects can be produced by setting some gaps wider so they only start firing at the higher input voltage settings..
Watch & hear the Spark-O-Phone in action : MPEG version (2.6MB) AVI version (4.6MB) Sound only : MP3 (375K) (Note that the audio compression artefacts on the MP3 version, tend to turn the very sharp clickiness of the sparks into a softer 'fuzzy' sort of noise, the sound on the AVI and Mpeg versions is more realistic).
Things get more interesting if you start modulating the power supply voltage over time. Simply switching it on and off at regular intervals imposes a rythmic beat to the sound, as sparks will only occur when power is on. Adjusting the on/off ratio controls how strictly the spark timing follows the beat - with a short on-time, the sparks happen pretty much together, usually several at a time. As the 'on' time gets longer, the spark timing spreads out.
Hear the result : pulsed.mp3 (1.4MB) In this sample, the only manual intervention was occasional adjustment of the pulse rate and on/of ratio.
Other interesting effects can be acheived by slowly ramping the voltage up and down, producing patterns which get gradually more and less 'frantic'.
Acrylic tube was used so the sparks were nicely visible, and a set of
tubes were cut to resonate at musical notes A (440hz) upwards. The number of tubes was
dicated by the number of capacitors I had - these were 2.2n 25KV parts left over from my Marx Three 1MV marxgen.
Detail of one tube assembly - resistor and cap mounted on PCB, with fuse clips to attatch assembly to supply rails. Thumbwheel allows gap adjustment
Alternative arrangement using rings instead of vertical supply rods - this could easily be hung like a chandelier to keep the high voltage out of reach of people.
To help investigate the effects of controlling the supply voltage, I designed a purpose-built power supply, based on a PIC microcontroller, which allows software control of the drive to the flyback transformer, and hence output power. It currently provides modes for simple continuous adjustable output, square wave with adjustable duty cycle and min/max values, plus ramp & triangle modes. It is planned to add to this over time to provide things like external inputs (e.g. remote trigger, light, sound or PIR sensors to make it react to external influences), as well as sequencing and save/recall of preset settings. The controller board also provides a second PWM channel to allow dual supplies to be independently controlled.
This smaller unit is a lot less impressive however, mostly due to the much lower volume as the gap firing voltage needed to be kept below 8KV to avoid killing the caps.