Free MPE-capable synths keep arriving, but few take such a distinctive approach to sound design as Overtonium, a new additive synthesizer released on October 3, 2026 by developer Benjamin Dehli. It is completely free, open source, and ready for expressive controllers from day one.

A Synth Built Like a Mixing Desk

Additive synthesis builds sounds by stacking sine waves, each one a partial of the played note. Overtonium gives you 32 sine-wave partials locked to the harmonics of the note you play, and presents them as a channel-strip interface, much like a mixer. Each strip has its own tuning, an ADS envelope with strike and delay controls, and dedicated modulation.

That layout matters. Traditional additive synths can feel like spreadsheets of numbers; here, every harmonic is a channel you can see, level, and shape. Two LFOs per channel handle pitch and amplitude modulation, and a separate noise channel offers the same shaping tools for breath, attack transients, and texture. A built-in multi-effects section adds character, wobble, echo, and reverb.

Where MPE Comes In

Overtonium offers full MPE support, and the headline detail is how pressure is handled: every one of the 33 channels (32 partials plus noise) has its own pressure amount. As the developer puts it, this makes the instrument "super versatile."

In practice, this means pressing harder on a key, pad, or surface doesn't just make the note louder. You can decide which partials respond to pressure and by how much. Push a few upper harmonics to bloom with pressure while the fundamental stays stable, and you get a bright, vocal swell. Route pressure to the noise channel and your attack gets grittier the harder you play. Because MPE delivers pressure independently for each note, every finger in a chord can bloom differently.

This kind of per-partial response is exactly what additive synthesis has long promised and MPE is finally able to deliver physically. If you play on a ROLI Seaboard, an Expressive E Osmose, a LinnStrument, or even a mobile MPE controller, the instrument rewards experimentation.

Formats, Platforms, and Price

Overtonium ships as a VST3, AU, and LV2 plugin, plus a standalone app. It runs on macOS (native on both Apple Silicon and Intel), Windows, and Linux. The source code is available on GitHub under the AGPLv3 license, so developers and curious tinkerers can inspect how the MPE handling works or contribute improvements. You can grab it from the official Overtonium page.

As always with free software, it's worth testing in your own DAW. MPE behavior varies between hosts, so check that your DAW is set to pass MPE data to the plugin correctly (our DAW MPE support overview is a good starting point).

Why It Matters

Additive synthesis has had a quiet momentum in 2026, with several new releases exploring harmonic-level control. Overtonium stands out because it is free, cross-platform, and designed with expression in mind rather than adding MPE as an afterthought. For students, hobbyists, and sound designers on a budget, it's a low-risk way to explore what per-note, per-partial expression feels like.

It also demonstrates a broader trend: the MPE ecosystem is no longer only about expensive hardware and big-name plugins. Independent developers are building focused, creative instruments and sharing them openly, which lowers the barrier for anyone with an expressive controller to start making music that responds to touch.

Getting Started

Start with a pad-style patch: raise a handful of mid-range partials, lengthen their envelopes, and assign pressure so the upper harmonics respond more than the lower ones. Then try the reverse. The difference between the two is the heart of what Overtonium can teach you about harmonic expression.

If you are new to the underlying technology, our introduction to what MPE is explains how per-note pitch bend, pressure, and slide work together.