Review: AudioKit Synth One

The freeware, open-source synth you want on your iPad
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There are a lot of free, iOS music apps available, but quality and usefulness can vary widely. So when I was invited to preview the completely open source Audio-Kit Synth One—conceived by Matthew Fecher and a team of devoted coders—I jumped on it.

Synth One’s main page lets you view two panels of synthesis tools, simultaneously. Shown here are the audio signal path and microtonal section.

Synth One’s main page lets you view two panels of synthesis tools, simultaneously. Shown here are the audio signal path and microtonal section.

The vision behind the app was to create a high-quality, freeware softsynth that sounds great (so that schools and users on a tight budget would have access to a full-featured subtractive synth) and can also be modified by capable programmers. As a professor, I loved the idea, but as a sound designer, I wanted to give it a thorough workout.

The overall architecture will be familiar to most EM readers: Two oscillators, a noise generator, and a sine/square wave sub-oscillator feed a resonant multi-mode filter, with dedicated filter and amplifier ADSR envelopes. Synth One also provides a simple FM section that lets you coax a wider range of timbres out of the oscillators before they hit the filter. Additionally, the dual LFOs can be routed to a comprehensive array of destinations, including the effects, which include reverb, delay, bit-crushing and a phaser.

The smartly designed interface lets you scroll between sections, a bit like a really long front panel that can be switched using simple right/left arrows, and it’s all laid out intuitively. On the downside, with the keyboard on screen, you may have to pass through unneeded areas to get to the parameters you want to edit. However, sacrificing that keyboard view lets you open a second panel of parameters for easy access. A lot of thought went into this instrument’s usability.

In addition to its well-rounded synthesis engine, Synth One includes a beautifully implemented arpeggiator that also doubles as a one-measure, keyboard transposable step-sequencer. It’s familiar territory and definitely adds to the usefulness of the synth.

A more esoteric feature is the microtonal tuning section, which supports over a hundred modes ranging from harmonic-based options to a whopping fifteen different North Indian scales, with numerous pentatonic and tetrad types as well. And that’s just scratching the surface as there are more preconfigured possibilities than I’ve seen on any other iOS synth.

On the performance side, Synth One includes Bluetooth MIDI, a smart MIDI Learn mode for facilitating knob/fader controllers, and a few selections for quickly routing the mod wheel to common destinations. A pair of x/y pads provides control over LFO 1 and filter cutoff/resonance.

The version I tested included Inter-app Audio and Audiobus integration. AUv3 compatibility is on the roadmap for a future update. I tested Synth One in AUM and, despite being an ongoing work-in-progress (open source is like that), the performance was solid. Here’s hoping an intrepid developer adds Ableton Link to the LFOs and arpeggiator, as that would complete its compatibility list.

All in all, Synth One is a stunning endeavor that will inevitably bring the essentials of subtractive synthesis to a huge audience. It combines timeless design with a few modern twists that differentiate it nicely. Whether you’re a music teacher looking to enhance your curriculum, a student on a tight budget, or just an iOS fanatic who craves more toys, don’t miss this app.

Impressive dual-oscillator synth. Basic FM synthesis. Micro-tuning scales. Customizable arpeggiator. Open source.

Envelope modulation for filter and amp only. Not Ableton Link compatible.


Francis Preve has been designing synthesizer presets professionally since 2000. More info at