Comparison

Undersong vs Bent Spool

Price, ratings, monetisation and update history for both apps, side by side — with what reviewers say about each.

Head to head
About

Undersong

This audio effect acts like an instrument, generating synthesized notes in key based on incoming audio. Explore a vast array of sound combinations with various materials, modes, and patterns for unique ambient performances.

Highlights
  • Generates in-key synthesized notes from audio input
  • 12 material types and 10 performance modes
  • 10 pattern options for sound shaping
  • Self-generating ambient pad mode
  • Gravity feature for melodic anchoring
  • Customizable scales and root notes
  • Integrated reverb effect
Currently charting
Read full description

Undersong is an AUv3 audio effect that plays along with whatever audio you send it. It's technically an audio effect, but it acts more like an instrument. It's based on an idea I had for a live performance that ended up taking on a life of its own. The short version is that everything Undersong plays comes from the audio coming in. It listens to your track for "action", like a pluck, a drum hit or a swell, and each of those can spawn a particle. A particle is one small synthesized note, often a pluck but not always, and it's always in key. Soft playing gets soft and lower responses, and digging in gets you brighter ones. I feel like Undersong is something that wishes to be the main focus of a composition, but how you use it is totally up to you. I really enjoy using it as the centerpiece of ambient jams with a bit of percussion or a slow moving bassline alongside. If you're not sure where to start, send in a drum loop (even just a series of blips or clicks will work...), turn up the Mix to 100% and just experiment for a while. Twist some knobs and see what you find. You can't break it. The particles can be one of 12 materials, mostly inspired by modular synthesis techniques. Each material can then be performed and shaped by one of 10 modes and 10 patterns. That makes for 1200 combinations before you turn a single dial. One of the modes, Aurora, stops making particles altogether and turns into a self-generating ambient pad that your playing strikes. It's one of my favorite things in the device. I tried to build this like a playground for ambient performances. It's maybe a bit like sitting down at someone else's modular rig... it takes some exploration. I've set up an environment that I think is inspiring and I hope you can find your way around and enjoy it as well. There's various ideas and features hidden throughout (in plain sight) that work nicely together or on their own. Gravity, an idea I brought over from my Augur device, pulls the notes toward your root so the melodies sound like they're actually in your key. With Ear on, Undersong listens for the pitch you're playing and answers in unison, grain plays tiny pieces of your own audio back as notes, and Input lets little glimpses of your audio through, so you hear the thwack of your pick or your snare, but ringing at the particle's note. Step puts everything on a steady rhythm, one note per step, with Density deciding how busy it is. Step plus the Loop pattern is a fun little corner to jam in. Freeze holds whatever's currently ringing out so you can play over it. There's a Drone dial that builds a bed under everything, and 35 scales plus your own custom ones. Plus a quick one-knob reverb, because I really like reverb and I think almost every one of my devices has received at least a little reverb, just in case you need it quickly. To sum up, I'm proud of this and I have been consistently getting lost in it while building and testing. I feel it's the kind of device that you have to get to know a bit. It's pretty dense. It's maybe not for everyone, but for the people who it resonates with, I'm glad you're here and I hope it brings you some inspiration. If you run into any issues or have some feature requests for a later update, don't hesitate to reach out at ian@ijoaudio.com. I appreciate you very much. Cheers! ian - ijo audio

About

Bent Spool

This music app offers creative audio effects for live performance or use within other music apps. Explore rhythmic tremolo, pitched repeats, and looping fragments of your audio. It includes built-in overdrive and tape effects for unique sound shaping.

Highlights
  • Rhythmic tremolo effects
  • Pitched repeat sequences
  • Overlapping audio loops
  • Built-in overdrive and fuzz
  • Vintage tape effects
  • Standalone and AUv3 host support
Features
Read full description

Bent Spool is a creative effect that can transform simple musical parts into unexpected rhythms and melodies, played live or as an effect in your favorite host app. Shape a tremolo pulse, turn a note into a run of pitched repeats, or build a moving layer from fragments of what you just played. Drive the input with built-in overdrive or fuzz, add a core effect, then finally layer in vintage analog tape effects. THREE MODES TREM: a pulsing volume effect, from a rounded sine through triangle and sawtooth to a hard square gate. Syncs to your host's tempo or runs free. REPEAT: up to 64 pitched copies with their own decay and feedback, across five pitch stations from two octaves down to two octaves up. LOOPS: up to six overlapping fragments of your recent audio, spread across pitch, direction, read position and the stereo field. CHARACTER CHAOS adds movement that suits each mode: octave jumps in REPEAT, held or stalled pulses in TREM, wandering reversals and pitch drift in LOOPS. WEAR adds tape wear, magnetic saturation and digital crush after the active mode. At the input, OD is a softer clip; FUZZ is a harder distortion that sputters as it decays. PRESETS 48 factory presets: 10 TREM, 15 REPEAT, 15 LOOPS and an 8-preset Fuzz Tremolo pack. Unlock to save your own. FREE TO TRY TREM plays in full on the OD voice with ten presets. No time limit, no interruptions. A single $9.99 in-app purchase unlocks REPEAT, LOOPS, FUZZ, the rest of the presets and preset saving. No subscription. Runs as a standalone app or as an AUv3 effect in AUM, Loopy Pro and other AUv3 hosts, on iPhone and iPad.

Screenshots

Undersong4 screens
Bent Spool4 screens

Verdict

The call

The clearest difference is price: Bent Spool at Free against Undersong's $9.99. Undersong's advantage is in-app purchases (none vs 1) and chart position (#112 vs unranked). On rating, ratings volume, update cadence and ads there is nothing between them.

Scored on Price · Rating · Positive reviews · Number of ratings · Update frequency · Ads · In-app purchases · Monetization · Best chart rank · Devices · Requires iOS

CostPrice · In-app purchases · Ads · Monetization

Bent Spool is free to download; Undersong costs $9.99 up front. Bent Spool sells 1 one-off in-app purchase. Undersong asks for nothing beyond the download.

ReceptionRating · Positive reviews · Number of ratings

On the App Store both sit at effectively the same score — 5.0 for Undersong against 5.0 for Bent Spool. Both are backed by a comparable volume of ratings — 11 and 5 respectively.

UpkeepUpdate frequency

Both ship on a similar rhythm, roughly daily. Both last shipped on October 31, 2026.

Scorecard4 real differences · 6 level
Undersong versus Bent Spool: the parameters behind the verdict, then further details
ParameterUndersongBent Spool
Price$9.99Free — better
Rating5.0 (11 ratings)5.0 (5 ratings)
Number of ratings11 — better5
Update frequencyDailyDaily
AdsNoNo
In-app purchasesNo — betterYes
MonetizationPaidFree with in-app purchases — better
Best chart rank#112 — better—
DevicesiPhone, iPadiPhone, iPad
Requires iOS17.017.0
Further details — not scored
Size19 MB23 MB
Age rating4+4+
Developerian johnstonPlastic Factory LLC

In-app purchases

None

Undersong

No in-app purchases

1 purchases

Bent Spool

One-time1
  • Bent Spool Unlock$9.99

Questions

Is Undersong free?
Undersong costs $9.99, with no in-app purchases.
Is Bent Spool free?
Bent Spool is free to download, with in-app purchases available.
Which has better reviews, Undersong or Bent Spool?
Undersong and Bent Spool are both rated 5.0 out of 5 on the App Store, from 11 and 5 ratings respectively.
Do Undersong or Bent Spool have ads?
Neither Undersong nor Bent Spool shows ads.
Which is updated more often, Undersong or Bent Spool?
Undersong ships an update daily, and Bent Spool daily. Most recently, Undersong was updated on October 31, 2026 and Bent Spool on October 31, 2026.

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