It's a broadband bang that can be heard across the whole spectrum. It becomes audible when listening to radio broadcasts.

Regular radio transmissions are comparatively narrow band, allowing lots of simultaneous transmissions in the same airspace, each on its own frequency. The spark gap transistor is very wide band, so it basically sounds as if you are sending a bang sound across all radio frequencies at the same time.

It wouldn't destroy radio equipment, but the radio transmissions. It's basically as if you'd use a radio jammer as a morse code transmitter.