As described on Wired’s Underwire blog:
The band/science experiment ArcAttack channels music through large Tesla coils or DRSSTCs (Dual-Resonant Solid State Tesla Coils). The metallic whine generated by the synchronized coils creates a synthesizer sound the band presents as an audio-visual instrument.
Via Underwire
See also:
New ‘Doctor Who’ companion introduced
Sound decision: Revisiting the lost adventures of ‘Doctor Who’