Recording Impulse Responses

With growing computing power over the last decade, convolution plugins have become commonplace. Some of the most common ones include Audio Ease Altiverb, Logic’s Space Designer, Avid TL Space, Waves IR-1 and McDsp Revolver. They are usually packaged with large and useful libraries of impulse responses (more on what all this means below), but what makes them really powerful is the fact that it is quite easy to record and use your own impulse responses. This not only helps ‘personalise’ your mixes, but is extremely useful in post-production and in the design of new sounds.

Each of the above mentioned plugins need slightly different techniques for creating a custom library of impulse responses. This article is a description of the general concepts behind recording good impulse responses and should be easily adaptable to any convolution/de-convolution tool.

What is convolution?

Convolution is the process where a single sample of a sound is multiplied by every sample of another sound. It is different from the plain multiplication of two sounds where a single sample of the first sound is multiplied by the corresponding single sample of a second sound.

Curtis Roads (The Computer Music Tutorial) describes convolution as:

Convolution of two audio signals is equivalent to filtering the spectrum of one sound by the spectrum of another sound. Convolution of spectra means that each point in the discrete frequency spectrum of input a is convolved with every point in the spectrum b.

Impulse Responses (IR)?

Over the recent years, convolution has been widely used to recreate the reverberation of spaces, the sound of hardware units (reverb units, equalisers, tape machines, guitar amps, etc) and in creative sound design.

Impulse responses allow us to capture and store the acoustic characteristics of a space (or piece of sound altering equipment). It could be described as an acoustic ‘photograph’, where, instead of capturing a space visually, it is captured aurally.

The impulse responses or IRs are captured by recording how a space responds to a full range of frequencies (typically 20Hz to 20,000Hz). This is achieved by playing back a burst (impulse) of a full range of frequencies within the space and recording it. For more accurate results it is common to use a sine-sweep across the audible frequency range.

The recorded IR is nothing more an audio file that can be imported into a convolution software to recreate the sampled space digitally. Some convolution plugins like Altiverb and Waves IR-1 use proprietary formats, although they are capable of importing regular WAV or AIFF files as well.

How are they useful?

The immediate use of IRs is to recreate real world spaces. It is common to find IRs of famous venues from around the world or even vintage gear. It has proved to be extremely useful in post-production where it is important to re-create believable spaces, match ADR to location sound and in creating believable perspectives in the mix.

The convolution process is also useful for designing interesting sounds by using IRs of objects like tin cans, plastic buckets, bottles or any sound altering device. A single IR can be used to create a fresh library of sounds out of existing recordings. The IRs themselves can be manipulated in a DAW (before being imported into the convolution plugin/software) to create an even greater library of interesting sounds. It is also quite common to import non-IR files into convolution software. Eg: convolving the sound of a synth with a drumkit to create a ‘rhythmic-synth’.

What can you record?

• Spaces (including different perspectives)
• Gear
• Software/plugins
• Objects
• Anything that alters sound and is excitable!

Recording IRs

To record an IR, you need a sound (source) to excite the space and some recording equipment (obviously!).

• Source
  • Sine sweep: A sine sweep is the most preferred method to record an impulse response. Depending on the length of the sweep, it usually provides the best signal-to-noise ratio. The longer the length, the greater the signal-to-noise ratio but also greater the chance of recording resonances/noisy vibrations in a space. The sweep is played off a speaker to excite the space (here’s more information about the sweep on acoustics.net).
  • Transient method: The second most common method and perhaps the easiest is to use a starter pistol, balloon or clapper board (especially on location for a shoot). Anything that is loud and creates a broadband burst of noise will do. The advantage of using a transient sound (starter pistol, clapper board, balloon) is that there is no need for any post processing. The recording of the impulse can directly be used in a convolution software.  As with most recording sessions, the quality depends on the location, microphone technique and ambient noise level. It might not always be possible or convenient to carry a speaker and playback a sine-sweep, although sine-sweeps are the most preferred (and closer to accurate) method.

• Recording Equipment
  • It is best to try and maintain a minimal and clean signal chain, as every bit of gear affects the impulse response
  • Speaker(s): Depending on the source and recording technique used, there might be a need for one or more speakers. The speaker (and its capability to reproduce the sine-sweep) will have an obvious impact on the recorded impulse response.
  • Microphone(s): Anything from mono all the way up to multichannel formats is possible (as long as the convolution software supports the configuration). It is common to record using multiple configurations from different perspectives. Just like the speakers, the microphones and microphone technique will affect the IR. There are no hard and fast rules as far as polar patterns or microphone types are concerned (although condensers are the most common for obvious reasons), be creative and experiment!
  • Playback/Recording device: The playback and recording devices could be separate or the same (a laptop). The choice depends on the location and is a question of convenience.
  • Documentation: It is always a GOOD idea to document your recordings with at least a camera and a notepad

• Recording Format- some of the most common formats:
  • Mono
  • Stereo
  • 4.0
  • 5.0
  • 6.0
  • 7.0
  • Ambisonic – Ambisonic recordings are made with Ambisonic microphones that output four channels of the ambisonic format (B-Format: W,X,Y,Z). It is possible to derive any of the multi-channel formats mentioned above or any custom format from these four signals. Some convolution plugins like Logic’s Space Designer can import and process ambisonic IRs. There are also a variety of tools available to convert ambisonic recordings into any of the standardised formats.
  • Other 3D Sound Systems: Guido Helbling recently posted his thoughts on recording IRs for Auro 3D and Dolby Atmos.

• Recording Tips
  • Always record at 24 bit!
  • If you can, record at higher sample rates to give you the option of processing the IR in post (pitching down and/or time stretching can create very interesting effects).
  • Try and maintain the cleanest signal chain when recording.
  • The position of the speaker and microphones are obviously important. Do a couple of test recordings, if possible.
  • Noise: are there any broadband noises you can get rid off? Ventilation or electric hums?
  • Try and maintain a MINIMUM of 4-5dB of headroom.
  • Options! Record different perspectives to give you choices.

Post-Processing

What is post-processing?

A recorded sine-sweep cannot be directly used in a convolution software. It need to be de-convolved into a usable IR. The manual for Logic’s Impulse Response Utility describes the de-convolution process:

The recorded sine sweep audio file cannot directly be used as an impulse response. The recorded file contains all the echoes and reflections—in other words, the response—of the space, stretched out over the length of the sine sweep. This is very different from the starter pistol approach, where the response is contained at the beginning of the file in an impulse.

When you use a sine sweep, Impulse Response Utility uses a process called deconvolution to time align and level align all recorded reflections-that are present over the entire recorded sine sweep—into the very beginning of the file. This results in an impulse response that Space Designer can use to combine, or convolve, with your audio signal.

Tools

There are a variety of tools available for sine-sweep generation and/or the de-convolution process:

• Voxengo Deconvolver: Deconvolver is a Windows application that generates sine-sweeps and de-convolves them into IRs. It has no recording capabilities. It includes a lot of options and the freely downloadable version comes with a few restrictions. If you are on OSX you can use a utility like Wine to run it.
• Logic’s Space Designer: Hidden within Space Designer’s menu is a de-convolution utility, where it can create an IR when fed the dry sine-sweep and the recorded sine-sweep.
• Apple/Logic’s Impulse Response Utility: Impulse Response Utility is included with Logic (Applications > Utilities) and is a complete solution for recording IRs. It is capable of generating and recording the sweeps, editing, de-convolution and creating Space Designer presets across a range of multichannel formats. Another advantage with Impulse Response Utility is that the IR it generates can be used in any other convolution software (rename it from .SDIR to .WAV and you are good to go).
• AudioEase Altiverb: Altiverb 7 makes the whole process of recording custom IRs very easy, although it encodes them into a proprietary format.
• Waves IR-1: IR-1 is capable of deconvolving sweeps – but only if you use the ones supplied by Waves. Like Altiverb, it encodes the IRs into a proprietary format.
• HISSTools Impulse Response Toolbox: Recently released, Impulse Response Toolbox is a collection of Max/MSP objects to record, de-convolve and convolve impulse responses.

Editing

A de-convolved IR will need some trimming and a fade out to get rid of noise (room tone/hiss). At this stage it is best to avoid any other sort of DSP (it is okay to use some amount of filtering only if required). NO noise-reduction, if you can help it!

Exporting

Most standalone de-convolving utilities, like Voxengo’s Deconvolver, output a regular .WAV file. Impulse Response Utility creates a .SDIR file. This is a usable audio file that be imported into any DAW or convolution software by changing the file extension to .WAV. AudioEase Altiverb, Waves IR-1 and McDSP Revolver use proprietary formats when de-convolving IRs, although they are capable of importing WAV files.

Being Creative

In addition to getting a good representation of the sound of a space, it might be rewarding to get a bit creative – use alternate speakers (gel speakers or any other surface transducers), broken microphones, odd objects, musical instruments….

Post the de-convolution process, it is great fun to affect the IR with a variety of DSP before importing it into a convolution plugin (process it with a flanger or delay or distortion or pitch it down or…..).

This blog post discusses the process of (and fun in) mangling IRs to create a new library of interesting sounds.

Using Apple/Logic’s Impulse Response Utility

Out of all the available tools, the Impulse Response Utility makes the process of recording IRs the easiest without converting the IR into an unusable format. Chapter 3 of the Impulse Response Utility guide describes a step-by-step process in creating IRs with the utility (available here). Most software include similar documentation.

Documentation

Remember to document your recordings! Click photographs and make notes of:

• Location/object/gear
• IR source (sweep/transient)
• Microphones
• Microphone technique
• Preamps
• Convertors
• Software used

Your notes and descriptions will be useful for future reference.

Online Resources

A great video from Audio Ease on recording IRs with Altiverb:

[youtube]http://www.youtube.com/watch?v=UAKuMLu5Dus[/youtube]

Text and tutorials:

Recording & Mangling IRs
IR Reverb Comparisons
Impulse Response Utility Guide
Capturing manipulation and reproduction of sampled acoustic impulse responses
Implementation of Impulse Response Measurement Techniques
Simultaneous measurement of impulse response and distortion with a swept-sine technique
Recording Impulse Responses for 3D Sound Systems

Photo Credits: Chris Precott / Peter Terner