SmoothIR

SmoothIR is a tool for creating impulse responses (IRs) through spectral matching of two audio files – with a focus on musically useful results rather than purely technical accuracy.

The idea is simple:

The spectral difference between a Reference and a Source is transformed into an IR – then deliberately smoothed and band-limited.

The result is an IR that works very well for creative applications (e.g. guitars, reamping, sound design).

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Features

• Spectral matching (Reference vs. Source)
• Minimum-phase IR generation with adjustable IR length
• Controllable low-/high-end rolloff
• “Smooth” control for musical smoothing
• Stable, deterministic results (no heuristic surprises)

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IR Length

The IR length defines the duration of the generated impulse response and directly affects both frequency resolution and low-frequency accuracy.

• Longer IRs provide better resolution at low frequencies and allow for more accurate spectral matching, especially in the bass range.
• Shorter IRs reduce latency and CPU usage, but may introduce artifacts or instability at very low frequencies.

As a rule of thumb, the lowest reliably representable frequency is approximately:

f_min ≈ sampleRate / IR_length

For stable results, the effective low-frequency processing range should stay above this limit.

In practice:

• 2048 samples → suitable down to ~40–50 Hz
• 4096 samples → suitable down to ~25–30 Hz
• 8192+ samples → recommended for deep low-end processing

Frequencies below ~20 Hz are generally ignored, as they provide little practical value and can introduce numerical instability.

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Usage

smoothir -r <reference.wav> -s <source.wav>

Parameters

• -r, --ref
  Path to the reference file (target sound)

• -s, --src
  Path to the source file (input sound)

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Sound Shaping

After matching, the IR is further shaped:

Low / High Cut

• Removes unwanted spectral regions
• Prevents low-end mud and high-end fizz
• Uses clean, monotonic rolloff (Butterworth-like behavior)

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Smooth

A continuous control for spectral smoothing:

• 0.0 → maximum detail transfer
• ~0.2 – 0.4 → musical sweet spot
• 1.0 → heavily smoothed, very soft result

Internally, the original spectrum is blended with a smoothed version.

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Typical Workflow

1. Choose Reference and Source
2. Generate IR
3. Set Low/High Cut
4. Adjust Smooth to taste
5. Apply IR to target signal

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Notes

• Low and high frequency extremes often contain little usable information
  → controlled rolloff significantly improves stability and sound quality

• Too much smoothing can remove important details
  → use moderately

• The generated IR is minimum-phase
  → efficient and practical for real-world audio use

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Example

An interesting use case:

• Reference: Piano
• Source: Harp
• Application: Guitar

→ results in an IR with an unusual but musical character

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Dependencies

SmoothIR relies on a small set of widely available libraries:

• X11 – windowing and basic system interaction (Linux)
• cairo – 2D graphics rendering for the UI
• libsndfile – reading and writing audio files
• FFTW3 – fast Fourier transforms for spectral processing

Install (Debian/Ubuntu)

sudo apt install libx11-dev libcairo2-dev libsndfile1-dev libfftw3-dev

Build

Make sure all dependencies are installed, then compile using your preferred build system or compiler.

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Build

 git clone https://github.com/brummer10/SmoothIR.git
 cd SmoothIR
 git submodule init
 git submodule update
 make
 sudo make install

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Concept

SmoothIR intentionally avoids:

• post-processing the IR
• heuristic “fixes”
• unstable spectral fitting tricks

Instead:

Clear separation between analysis, shaping, and synthesis

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License

BSD-3-Clause

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If it sounds good, it is right.