Audio Units (AU) are Apple's native plugin architecture for macOS, and for the experimental musician, they are a foundation for deep sonic exploration. Because AU is tightly integrated into the Core Audio framework, it offers lower latency and more stable real-time performance than bridged VST or AAX formats. This stability is essential for the unpredictable, modulation-heavy workflows that define experimental music. Rather than fighting driver conflicts or buffer underruns, you can push a system to its limits, routing audio through complex feedback loops and extreme automation that would cripple a less optimized setup. Approaching AU plugins not just as tools but as active instruments in their own right opens a new dimension of creative potential.

Understanding AU Plugins in the Experimental Music Context

AU plugins are categorized into three core types: effect plugins (reverberation, delay, modulation, distortion), instrument plugins (synthesizers, samplers), and MIDI-controlled processors (arpeggiators, chord generators, note repeaters). In a conventional production setting, these categories stay separated. An instrument plays notes, an effect processes sound, and MIDI tools generate sequences. Experimental music begins when you blur these lines. The moment you route an audio track through a synthesizer's audio input or feed a drum loop into a spectral filter, you are using the AU format as a modular environment. The native integration inside Logic Pro, GarageBand, or MainStage means that every AU parameter is instantly available for automation and sidechain routing without additional wrapper software, making complex inter-plugin communication seamless.

Creative Modulation That Goes Beyond LFOs

Standard low-frequency oscillators mapped to filter cutoff are a starting point, but experimental music relies on modulation that feels organic, chaotic, or generative. The AU environment, with its robust automation and sidechain capabilities, is ideally suited for these advanced techniques.

Sidechain and Envelope Following

Sidechain routing allows one audio track to control the parameters of a plugin on another track. Instead of a simple pumping compressor effect, route an unpredictable audio source—like a field recording of rain or a heavily distorted percussion loop—into the sidechain input of a gate or envelope follower. This creates a rhythmic modulation that is intrinsically linked to the audio material. The density of the rain triggers faster modulation, while the distortion creates complex, jagged control voltages. You can use a dedicated AU envelope follower like SideChain Compressor or Envelope Modulator to map the amplitude swings of any audio track to parameters like wavetable position, reverb decay, or filter resonance. This transforms static synthesis into a responsive, evolving texture.

Audio-Rate and FM/AM Synthesis

When an LFO speed exceeds 20 Hz, it enters the audio range. Instead of a slow filter sweep, you are now applying amplitude modulation (AM) or frequency modulation (FM) to the signal. Many AU synthesizers, such as Kilohearts Phase Plant or u-he Hive, allow modulators to run at audio rates. Pushing an LFO to 200 Hz and mapping it to the amplitude of a sine wave creates a ring-modulation-like timbre, rich with sidebands and inharmonic content. By automating the rate of this modulator, you can sweep from a tremolo effect to a metallic buzz, blending the line between pitched and unpitched sound. This is a direct route to creating percussive textures, clock-like noises, and robotic voice effects without leaving the stability of the AU environment.

Chaotic and Sample-and-Hold Modulation

Move beyond predictable sine and triangle waves. Sample-and-hold (S&H) modulation creates stepped, random voltages. Mapping a S&H generator to the pitch input of an oscillator or the cutoff of a filter produces glitchy, staccato patterns. Many AU synths include "chaos" generators—Lorenz attractors or multi-dimensional random walks—that provide modulation never repeating the same path twice. Pair a chaotic modulator with an AU reverb like Space Designer, mapping the chaos to the reverb size and diffusion. The result is a wash of sound that constantly shifts in depth and density, creating a living, breathing atmospheric bed.

Advanced Signal Chain Architectures

The way you route plugins is as important as the plugins themselves. Moving beyond the linear insert chain allows for complex interactions between processors, creating behaviors that no single plugin can achieve alone.

Parallel Processing for Coexistence

In a series chain, every plugin modifies the entire signal. This can quickly lead to a muddy or overly processed sound. Parallel processing, using AUX sends or output routing, allows you to blend a heavily processed signal with the dry original. This is essential for maintaining the attack and clarity of a sound while adding extreme texture. For instance, create a parallel bus with a granular AU plugin set to 100% wet and a reverb with a long tail. Send only a fraction of the original dry signal to this bus. The result is a transparent, ghostly layer that floats around the source, never obscuring its core character. You can also compress the parallel bus heavily to bring up the noise floor, adding a layer of analog-style grit.

Feedback Loops as a Compositional Engine

Creating a feedback loop is one of the most powerful techniques for generating unexpected material. In your DAW, route the output of a bus back into its own input. Insert an AU delay or reverb with a high feedback setting. The sound will cascade into infinite, self-oscillating chaos. To control this, place an AU compressor or limiter inside the loop. A limiter will clamp down on runaway peaks, turning the feedback into a controlled drone. You can then automate the feedback percentage and the filter on the delay to shape the evolution of the sound. A subtle change in the filter frequency can cause the entire loop to morph from a low rumble into a high-pitched whine, providing a dramatic arc to an otherwise static piece.

Routing Audio Through Instrument Plugins

Many AU synthesizers feature an audio input. Instead of using the synth's internal oscillators, route an external audio track—like a vocal or a guitar—through its filter bank, distortion circuitry, and modulation matrix. This treats the synthesizer as a powerful multi-effects processor. For example, route a simple piano chord through an AU synth like Alchemy or Massive X. Use the synthesizer's envelope follower to open the filter in response to the audio amplitude, then apply the synth's internal LFOs to the filter cutoff. The piano chord is now animated by the synthesizer's entire modulation engine, creating a hybrid sound that is part acoustic, part electronic.

Automation as a Compositional Gesture

Automation is often treated as a mixing afterthought—a way to fix levels or add a sweep at the end of a bar. In experimental music, automation is the performance itself. It is the act of shaping sound over time.

Generative Automation with LFO Tools

Dedicated AU modulation plugins like Kilohearts LFO or Shapes allow you to draw complex, multi-stage modulation curves. You can create rhythmic patterns that are tempo-synced but evolve over time. Draw a curve that speeds up and slows down, then map it to the panning and filter cutoff of a drone. The sound appears to walk around the stereo field while changing its tonal character. By using multiple LFO tools running at different rates, you create a polymetric modulation network. This is the essence of generative music within the DAW: setting initial conditions and letting the modulation create evolving patterns that you can record, edit, or simply listen to.

Recording Real-Time Automation

Nothing beats the nuance of a human hand moving a fader. Connect a physical controller, hit record in your automation lane, and perform the modulation. The tiny imperfections and hesitations in the automation data give the sound a human quality that is difficult to draw with a mouse. Map a single knob to control the wet/dry mix of a reverb and the feedback of a delay simultaneously. As you turn the knob, the sound shifts from a dry, present source to a distant, cascading echo. Recording this live performance of the automation captures a specific moment in time, a gesture that becomes a permanent part of the composition.

Diving Into Granular and Spectral Processing

These two categories of AU plugins manipulate sound on a microscopic level, deconstructing audio into tiny grains or individual frequencies. They are the tools of choice for producers looking to transform recognizable sounds into abstract textures.

Granular Synthesis for Time Manipulation

Granular AU plugins like The Mangle or Borderlands chop audio into tiny grains (typically 10–100 ms) and rearrange them. Load a vocal phrase or a field recording, set the grain size to 40 ms, and scatter the playback position. The result is a shimmering, stuttering cloud of sound. To create a denser texture, increase the grain density and spread the pitch across a wide range. The voice becomes a choir of alien insects. Automate the grain size parameter from large to small: the sound will shift from recognizable, granular chunks into a smooth, continuous tone. This is a powerful way to create transitions or build tension in an experimental piece.

Spectral Processing for Frequency Isolation

Spectral AU plugins, such as Spektral Delay or Fracture, break an audio signal into hundreds of frequency bands (bins) and allow independent processing of each band. You can delay the high frequencies while leaving the lows untouched, or reverse only the middle range of a sound. The most powerful technique in spectral processing is freezing. Capture a slice of a sound—a single piano chord—and freeze it. The plugin sustains this frequency snapshot indefinitely, creating an evolving, ethereal pad. The sound's harmonics continue to interact in the spectral domain, creating new micro-tonal variations that never appear in the original source. This is an excellent way to create backgrounds for ambient experimental music.

Integrating MIDI Control and External Controllers

Tactile control is the bridge between the physical world and the digital domain. In experimental music, where improvisation and spontaneity are key, a physical controller is not a luxury but an essential performance tool.

Macro Mapping for Complex Gestures

Most DAWs allow you to create "Macro" controls—a single knob that can control multiple parameters across several plugins. Map a Macro to the filter cutoff on a synth, the feedback on a delay, and the wet/dry on a reverb. Now, one physical knob on your controller simultaneously alters the timbre, the echo density, and the space around the sound. This creates a unified control that changes the entire character of a mix with a single gesture. You can map a joystick or XY touchpad to two Macros (e.g., "Density" on the X-axis and "Brightness" on the Y-axis), allowing for fluid, two-dimensional morphing of the sound.

MPE and Polyphonic Expression

If you have an MPE-compatible controller (like a Roli Seaboard or LinnStrument), you can take advantage of AU instruments that support Polyphonic Aftertouch. Instead of modulating the entire sound, MPE allows you to apply pitch bends, slides, and pressure changes to individual notes. This brings a level of expression to synthesized sound that mimics acoustic instruments. Playing a chord, you can add vibrato to the top note while applying a filter sweep to the bottom note. This polyphonic control is deeply expressive and ideal for atmospheric, textural music where every note is a distinct voice.

Practical Workflow for Experimental Sessions

Creative flow is easily broken by technical issues. A solid workflow free you to focus on sound design and composition rather than troubleshooting routing errors.

Building a "Sandbox" Session Template

Create a standard session template with a few predefined tracks and buses. Include one track with a granular AU plugin loaded, one with a spectral AU plugin, one with a feedback loop bus (with a limiter inside the loop), and one with a sidechain routing setup. Save this as your default template. Every time you start a new experimental project, you are instantly ready to create complex patches. Over time, you can add your favorite specific AU plugins as presets on these tracks, saving hours of setup time.

Resampling and Destructive Processing

Resampling is a cornerstone of experimental electronic music. Once you have created an interesting texture—a feedback loop, a granular cloud, a spectral freeze—bounce it to audio. This commits the sound and frees up the CPU. Then, load the bounced audio into a new track and process it again through a different chain. Push a vocal through a granular processor, bounce it, load it into a sampler, play it as an instrument, bounce that, and put it through a feedback loop. Each iteration is a new layer of abstraction, moving the sound further from its source. This iterative, destructive process is how unique timbres are born.

Cataloging the Unexpected

When you stumble upon a configuration that produces a stunning sound, save it immediately. Use descriptive names that capture the feeling of the sound, such as "Crackling Ice Cave" or "Broken Satellite." Save the entire channel strip preset (including the plugin chain, routing, and automation lanes). If your DAW supports it, save the modulation routing as well. Building a personal library of "happy accidents" means you never have to recreate that moment of inspiration from scratch. You have a starting point for a new composition or a texture to drop into an existing track.

External Resources for Further Exploration

  • Apple's official developer documentation provides an in-depth look at the Audio Unit architecture, useful for understanding the technical capabilities and limitations of the format.
  • MusicRadar offers a comprehensive roundup of essential AU plugins, including a wide range of free instruments and effects that are perfect for experimental workflows.
  • Sound On Sound has an excellent guide on creative modulation techniques in the DAW, offering advanced tips that apply directly to AU plugins and sidechain routing.

Conclusion

AU plugins provide a stable, low-latency environment for deep sonic experimentation. By moving beyond basic presets and conventional routing, you can transform these simple tools into complex instruments. Exploring advanced modulation, building intricate parallel chains, harnessing the power of feedback loops, and manipulating sound on a granular or spectral level allows you to create music that is constantly evolving and unpredictable. The technical integration of AU within macOS means you can push these ideas to their limits, trusting the system to handle the load. The most important tool is your curiosity. Route an unlikely signal, twist a parameter you have never used, and save the chaos. In experimental music, the sound that sounds like a mistake is often the one that defines the entire track.