Callback Freeverb by Daniel McKemie

Freeverb is one of those algorithms that quietly underpins a huge amount of music software without most people knowing its name. Written by Jezar at Dreampoint and released as open source decades ago, it’s a Schroeder/Moorer-style reverb built from eight comb filters and four allpass filters in parallel — a structure that shows up, in one form or another, inside countless plugins and open-source DAWs. Callback Freeverb by Daniel McKemie takes that exact topology and ships it as a free, no-frills AUv3 plugin for iPhone and iPad. There’s no attempt to dress it up with a tape emulation layer or a modulation section. It’s the algorithm, with simplified controls, available as a drop-in effect in your AUv3 chain.

The honest limitation, stated directly in the App Store listing, is that this reverb runs in mono with no stereo spread. That’s not a bug or an oversight — it’s the classic Freeverb behavior, and McKemie isn’t pretending otherwise. In practice this means Callback Freeverb is best suited to mono sources: a single synth voice, a vocal, a mono drum bus, anything where you’re adding depth and tail rather than building a wide stereo field. If you’re looking for a lush stereo wash with width controls, this isn’t that plugin — but if you want the specific, well-documented character of Freeverb’s comb-and-allpass network on a mono signal, dropped into a chain at zero cost, it does exactly what it says.

Callback Freeverb is one of a growing run of small, single-purpose AUv3 utilities McKemie has been releasing under the Callback Audio name — Bit Crusher, Wavefolder, Multimode Ladder Filter, Raw Byte Audio Machine, and Delay Line form a loose catalog of focused effects, several of which are bundled together as the Callback FX Bundle. The pattern across all of them is the same: pick one classic algorithm, implement it cleanly, keep the UI to a handful of knobs, and price it at free or close to it. For producers building a mobile rig in Loopy Pro or Drambo, that approach has a practical upside: these are light enough on CPU that stacking several of them across a session doesn’t become a tax on your iPhone’s processor the way a heavier multi-effect plugin would. Drop Callback Freeverb on a mono synth voice inside Drambo’s modular environment, or use it as a send effect in Loopy Pro for a quick reverb tail on a looped vocal — either way, it does one thing and gets out of the way.

At 7.8MB and free with no in-app purchases, there’s effectively no reason not to have this in your plugin folder if you ever find yourself reaching for a quick, characterful mono reverb. It won’t replace a dedicated algorithmic or convolution reverb for stereo work, and it doesn’t try to. What it offers is the real Freeverb sound — the same algorithm that’s been quietly doing this job in open-source audio software for years — in a tiny, free AUv3 package for iPhone and iPad.

Key features:

• Classic Freeverb Algorithm: Schroeder/Moorer-style reverb using the original 8-comb-filter + 4-allpass-filter topology by Jezar at Dreampoint
• Simplified Controls: Stripped-down parameter set for fast, no-fuss reverb tails
• Mono Operation: Runs in mono with no stereo spread, matching the behavior of a classic Freeverb implementation
• Lightweight AUv3 Footprint: Low CPU overhead makes it practical to run several instances in a single session
• Standalone and AUv3: Use it as a self-contained app or load it as a plugin in any AUv3-compatible host
• Universal on iPhone and iPad. No accounts, no tracking, no analytics. Requires iOS 17.0.

App price: Free. No in-app purchases.

Original release: June 5, 2026  |  Latest update: June 5, 2026