Recording Impulse Responses:

With the never-ending development of computer processing, convolution plugins have turned out to be common in recording impulse responses. Some of popular ones are:

They are normally bundled with extensive and valuable libraries of impulse responses, yet what makes them extremely great is the way that it is very simple to record and utilize your own impulses. This customizes your mixes, as well as is to a great degree helpful in n post-production and in the design of new sounds. Each one of the previously mentioned plugins require different procedures for making a custom library of impulse responses. This article is a portrayal of the general ideas behind recording good impulse responses and ought to be effortlessly versatile to any convolution/de-convolution tool.

Convolution:

Convolution is where a single example of a sound is duplicated by each example of another sound. It is unique in relation to the plain duplication of two sounds where a solitary example of the main sound is increased by the comparing single example of a second sound. In other words, 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.

A convolution reverb is a plugin for sound editors like Digital Performer, ProTools, Logic and Cubase VST. It is a room simulator and it works with samples from real acoustic spaces. These spaces are 'captured' using a special sweep tone played by a speaker and recorded by microphones in the actual space, or by firing an alarm pistol and recording that with microphones. A convolution reverb adds the same sound to any sound that it is used on as what the room did to the gunshot or sweep. This sample is called an impulse response, in short: IR.

Impulse responses:

Over the recent years, convolution has been generally used to reproduce the reverberation of spaces, the sound of equipment units (reverb units, equalizers, tape machines, guitar amps, and so on) and in creative sound design. Impulse responses enable us to catch and store the acoustic characteristics of a space (or bit of sound changing hardware). It could be depicted as an acoustic 'photo', where, rather than catching a space outwardly, it is captured aurally.

The IRs are captured by recording how a space reacts to a full range of frequencies (commonly 20Hz to 20,000Hz). This is accomplished by playing back an impulse of a full range of frequencies inside the space and recording it. For more exact outcomes usually to utilize a sine-sweep over audible frequency range.

The recorded IR is an audio file that can be imported into a convolution program / software to reproduce the captured space in a digital manner. Some convolution modules like Altiverb and Waves IR-1 utilize their own formats, but they can still import regular WAV or AIFF files also.

Usage:

The prompt utilization of IRs is to reproduce real spaces. It is not hard to find IRs of famous venues from around the globe or even vintage gear. It is extremely useful in post-production where it is important to re-make realistic spaces, coordinate ADR to areas and locations and in making convincing sounding results in the mix.

The convolution procedure is also valuable for designing interesting sounds by using IRs of objects such as tin cans, plastic pails, bottles or any sound altering device. An IR can be utilized to make a new library of sounds out of existing recording. The IRs themselves can be manipulated in a DAW (before being imported into the convolution plugin/software) to make a much more prominent library of fascinating sounds. Additionally, it s common to import non-IR records into convolution software. Eg: convolving the sound of a synth with a drum kit to make a rhythmic synth.

What to record?

  • Spaces
  • Gear / Hardware
  • Software / plugins
  • Objects
  • Anything that changes sound and is excitable!

Recording IRs:

To record an IR, you require a sound (source) to energize the space and some recording equipment.

  1. Source

  • Sine sweep: A sine sweep is the most favored technique in recording an impulse response. Depending upon 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.
  • 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.

  1. Recording Gear

  • 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

  1. 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 varieties 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.

  1. 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 to 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 to 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.

  1. Tools

There are varieties 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 many options and the freely downloadable version comes with a few restrictions. If you are on OSX you can use a utility like Wineto 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.

  1. 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!

  1. 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.

  1. 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 etc. 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, delay, or distortion or pitch it down etc.)

  1. 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.

  1. Documentation

Remember to document your recordings, 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:

 

Another great video by Rodrigo Constanzo for Max/MSP users using the HISStools impulse response toolbox for creating IRs:

 

A list of free online IR samples that might be useful for many:

Text and tutorials:

 

Fayez Saidawi

Courtesy of RAC