How To Successfully Steal Your Friend’s Cello (With IRs)

A Guide To Making Your Own Acoustic Impulse Responses

Introduction

Impulse Responses (IRs for short) are quickly becoming a popular tool for musicians to use in their creative workflow and on stage. They can be thought of as a powerful EQ, although instead of being controlled by a collection of filters they are shaped by the tonal balance of a recording.

Guitarists and sound designers have made use of IRs to capture the tonal balance of speaker cabinets and the acoustics of rooms, respectively, with the processing power needed to use an IR in real-time becoming more commonplace.

The way an IR has generally been recorded is almost identical to normal recordings, by setting up microphones in their favoured positions and hitting record. The difference is that instead of recording, say, a guitarist’s loose interpretation of Jimi Hendrix, a short full-range click is played through the guitar amp. If the speaker cabinet and mic were made to be a perfect representation of the input, then the click will sound no different in the recording and produce a flat and equal tonal balance just like a sample of white noise.

The usefulness of IRs comes from the imperfection of these systems, where the speaker cabinet imparts its own timbre on the input signal and blasts the sound about the room (with its own set of reflections and resonances) into the microphone (all of which have a unique sound). The result is a mangled and scattered version of the click, which has an equally mangled and scattered tonal balance far more intricate (and realistic) than your average EQ can manage.

Enter strings players and their expensive wooden cabinets (affectionately called ‘instruments’).

An acoustic stringed instrument can be thought of as an over-engineered speaker cabinet, fitted with a mechanical input through the bridge rather than an electrical input through a wire. Recording an IR of an acoustic instrument is then just a matter of imparting a short full-range click through the bridge and recording the resulting resonances and open strings.

The benefits for these poor folk are in situations where using a nice sounding room and microphone would be great, but not possible. Playing on stage in a band springs to mind immediately, where pickups have been a necessary evil to capture a healthy amount of sound from an instrument while rejecting feedback from monitors on stage. I say necessary evil because the sound of a stringed instrument through a piezo pickup is hot garbage and should only be used as a last resort.

Thankfully, what makes a piezo pickup such hot garbage is the vaguely flat-sounding representation of the bowed string, making it a prime candidate for applying the complex filtering of an IR and bringing it back into tolerable territory.

Guide

Okay so here is what I’ve tried so far in making my own recordings of acoustic impulses. The internet didn’t really have as many answers as I was expecting, mostly because a few companies sell their own IRs (mostly of guitar cabinets) and don’t want to give idiots like me a detailed guide on how to replicate their work. I did happen across a thesis describing fun ways to whack acoustic violins with a hammer and record their screams, but the mathematics of finite impulse responses and transfer functions sailed right over my head. 

I am basing most of my experimentation on a low resolution image I found of a violin in a dead room (acoustically and emotionally) being tapped with a small impulse hammer on a pendulum (I think?).

  1. Find something small and rigid to use as an impulse hammer.

I tried mostly metal utensils, including a little screwdriver and the base of a pair of tweezers. It needs to have just enough mass to make a clicking sound when it hits the bridge and not a dull thud.

Things to try: teaspoon, 50c piece, small screwdriver, use your imagination

Things to avoid: an actual hammer

  1. Set up one or more microphones in your preferred positions.

This will be the same spot you usually like to have a microphone while recording. Take care in avoiding overly resonant or bass-heavy spots like the f-holes or sound hole on a classical guitar. I used a small clip-on mic behind the bridge just from personal preference and because my room is far too reflective.

Things to try: experiment by recording a short piece from different mic positions

Things to avoid: the sound of your room (unless that’s the goal), boomy recordings (cardioid mics get very bassy when they’re close)

  1. Record an impulse.

My best results were from hitting the bridge of my violin from the side, on the treble side (next to the sound post) higher up close to the strings. The harder you hit the more high frequencies will be kept, but avoid knocking the bridge out of alignment or leaving strike marks if it matters to you.

Things to try: set the gain on your interface to cope with loud peaks, let the strings ring out, record multiple strikes in one take with different amounts of gusto

Things to avoid: hitting the mic (yes, I am uncoordinated), hitting the wing slots on the bridge

  1. Edit the recordings.

I picked out the recordings with the loudest peaks as the quieter ones sounded dull. If you used multiple microphones keep them on seperate tracks to make comparison easier. Take a look at the recordings in an FFT or spectrum analyser to see your instrument’s unique sound.

Things to try: EQ the impulse if you think there is a bit too much resonance, adjust the gain of each clip to -1dBU or close enough (for consistent volume)

Things to avoid: hearing damage with loud playback, clipping on the master bus, playing back all recordings at the same time (trust me)

  1. Export your new IRs.

For a quicker export I dragged each clip onto a separate track and exported as stems, although I couldn’t get the length to be less than a full bar. The length that you use will depend on whether you are going to be using a pedal or a plugin, but generally acoustic impulses can be from 500ms to 50ms or less, at standard bit rates of 44.1khz/24bit (common in pedals) or 48khz/24bit as a WAV file.

Things to try: mono or stereo exporting (pedals use mono IRs mostly), give each track a descriptive name (Amati+condenser, Strad+ribbon, etc.)

Things to avoid: realtime export (again, trust me)

Examples

Here are two examples of IRs, one of a violin and the other of a grand piano (thanks to Reuben Leng). They are at 48khz 24bit, with the piano strings close to 5 seconds long (too long for cab sims but fine for reverbs).

Here’s a quick demo of my electric violin before and after using an IR.

Using the damn things.

Using an IR requires software or hardware that can load them and has enough processing power to be useable. Short mono IRs need the least amount of processing, although if your computer is capable of a modern DAW then power won’t be an issue. Most of the plugins listed will use stereo IRs at the session rate of your choosing.

IR – A free and open source convolution reverb plugin for Linux

Plugins you can use:

  • Logic Pro X Space Designer
  • Pro Tools Space
  • Lancaster Audio Pulse
  • Waves IR-1
  • Ignite Amps NadIR
  • 3 Sigma Impulsive
  • LePou LeCab
  • Redwirez MixIR 2
  • TSE X50 V2
  • IR-LV2
  • Guitarix
  • Native Instruments Reflektor

Note that any plugin that can load IRs can be used, even if it is intended as a reverb or cab simulator plugin, they all do the same process called convolution.

Hardware you can use:

  • Hotone Omni / Omni AC / Binary
  • NuX Mini Studio / Solid Studio / Cerberus / Optima Air
  • Fishman Aura Spectrum
  • Mooer Radar / GE200 / GE300 / Preamp Live
  • Logidy Epsi
  • Fractal Audio Axe-Fx
  • Kemper Profiling Amplifier
  • Boss GT-1000 / GT-1000 Core
  • Line 6 Helix / HX Stomp
  • Strymon Iridium
  • Joyo Cab Box
  • Poly Effects Beebo / Hector

These aren’t exhaustive lists, but hopefully they give a good idea of what’s out there.

Caveats and Creative Uses

Applying an IR to the same instrument that was captured will result in the resonances of that instrument being over-emphasised and difficult to control. This can be managed by cutting out the most resonant parts of the instrument out with an EQ or wood saw, lending solid-body electric instruments an advantage in that they do not have any of their own body resonance.

IRs can also be used as reverbs, as long as the recording is sufficiently long. A space’s reverberation can be recorded by using a loud impulse in the room, such as clapping your hands, using a slate, or using a loudspeaker with a short click of white noise. This allows the capturing of the room acoustics to be added in later, for example with ADR in films or instruments recorded in a dead room.

IRs have also been used for processing electric-acoustic guitars, capturing an ideal guitar and microphone combination in a professional studio and using it in a home studio to make a direct pickup recording sound like they were in the studio with an entirely different setup and instrument.

As far as creative uses go, the processing of IRs (convolution) has a few interesting uses. The IR of an old radio would be interesting as a lo-fi processor, and playing a drum mix through a cello IR can only lead to terrible, terrible things that should never be tried.

Special thanks to Alex Olijnyk at Hyperdynamic for helping me out with testing impulse responses on her cello 🙂


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