The arc is the speaker
When I got my first Tesla coil, I did what everyone does. Paired it over Bluetooth, hit play on Spotify, and waited for something cool to happen.
It sounded crispy and most songs weren't even recognisable. Took me a while to understand why, and the answer turns out to be pretty interesting.
A Tesla coil is a resonant transformer. It takes low voltage and steps it up to tens or hundreds of thousands of volts. That voltage ionises the air and you get an electrical arc. Every time that arc fires, it heats the air around it and creates a pressure wave. One arc is a crack. Thousands of arcs per second at a steady rate... that's a tone.
Change the rate, change the pitch. That's it. That's the whole trick. The arc itself is the speaker.
Why Spotify sounds bad on a coil
Here's the thing most people don't realise when they buy a Bluetooth Tesla coil. The coil's interrupter (the circuit that switches it on and off) is listening to the audio signal you feed it. But it can only do one thing: fire or not fire. On or off. There's no in-between.
So when you send it a Spotify track with vocals, drums, bass, synths, and reverb all layered on top of each other... the interrupter doesn't know what to do with that. It tries to follow all of it at once and the result is a mess. Buzzy, indistinct, nothing like what you expected.
What a coil actually wants is simple. One note at a time. A clean frequency. A square wave.
How interruption creates pitch
The interrupter is the key bit of hardware. It switches the coil on and off at whatever rate you tell it to.
Interrupt at 440 times per second and you hear an A4, concert pitch. 261 Hz gives you middle C. 880 Hz is A5, one octave higher.
The coil doesn't care what frequency you pick. It'll reproduce any tone in its operating range, usually somewhere around 20 Hz up to about 2,000 Hz depending on the hardware. The sweet spot for most coils is roughly 100 Hz to 2,500 Hz, which covers the majority of musical content.
Square waves and that chiptune sound
Because the arc is either on or off with no gradual transition, the output is basically a square wave. Square waves are full of odd harmonics (3rd, 5th, 7th and so on), which gives them that buzzy, aggressive tone.
If you've ever played an NES or Game Boy, you already know what a Tesla coil sounds like. The timbres are really similar. Both are working with simple on/off waveforms, both have that same raw, 8-bit quality. It's why video game soundtracks from that era tend to sound great on a coil.
From tones to melodies
One tone is easy. Melodies need you to sequence through different frequencies in time with the music.
Most musical Tesla coils use a digital interrupter that takes MIDI or audio input. Each MIDI note maps to a frequency:
| MIDI Note | Name | Frequency |
|---|---|---|
| 60 | C4 | 261.6 Hz |
| 69 | A4 | 440.0 Hz |
| 72 | C5 | 523.3 Hz |
Note on, coil fires at that frequency. Note off, it stops. String enough of those together and you've got a song playing through lightning.
Duty cycle
Beyond frequency there's duty cycle, the percentage of each cycle where the coil is actually firing.
50% duty cycle means on half the time, off half the time. 10% means short pulses with gaps between them. Higher duty cycles are louder but dump more heat into the coil. Most people run somewhere between 5% and 20% for sustained playing. Push it too high and you'll cook your hardware.
What I built to fix the Spotify problem
After the disappointing Spotify experience I built teslacoil.app. It's a browser-based set of tools for driving musical Tesla coils.
The sound library has over 150 tracks converted from MIDI by the team at teslacoil.shop (Wardenclyffe), specifically for coil playback. Monophonic, in the right frequency range, no full-spectrum audio confusing the interrupter.
Then there are tools for making your own stuff:
- Keyboard for playing notes live
- Sequencer for programming step patterns
- Tone generator for continuous tones at specific frequencies
- Theremin for touch-controlled pitch Connect your phone or laptop to your coil over Bluetooth or a 3.5mm cable, and the app outputs the audio signals your interrupter needs. The coil hardware does the actual high-voltage work.
Getting started
If you've got a musical Tesla coil with an audio input, try teslacoil.app. No install needed, runs in any browser.
Start with the keyboard to hear what your coil sounds like on individual notes. Then try the sequencer to build something. The sound library is there when you just want to hit play and hear Thunderstruck come out of an electrical arc.