Recording Impulse Responses: Post-Processing
In Part 1 of the Recording Impulose Responses blog series, we covered the process of recording an impulse response including plugins, sources, recording gear, recording formats, and provided a few recording tips. Part 2 of this series covers the post-processing of impulse responses.
Recorded Sine Sweep
A recorded sine-sweep cannot be directly used in a convolution software. It needs to be de-convolved into a usable impulse response. The manual for Logic’s Impulse Response Utility describes the deconvolution 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.
There are varieties of tools available for sine-sweep generation and/or the deconvolution 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 Wine to run it.
Logic’s Space Designer: Hidden within Space Designer’s menu is a de-convolution utility, where it can create an impulse response 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 impulse responses. 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 impulse response it generates can be used in any other convolution software (rename it from .SDIR to .WAV).
AudioEase Altiverb: Altiverb 7 makes the whole process of recording custom impulse responses 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 impulse responses 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.
A deconvolved 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!
Most standalone deconvolving utilities, like Voxengo’s Deconvolver, output a regular .WAV file. Impulse Response Utility creates a .SDIR file. This is a usable audio file that could 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 deconvolving impulse responses, although they are capable of importing WAV files.
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 deconvolution process, it is great fun to affect the impulse response with a variety of DSP before importing it into a convolution plugin (process it with a flanger, delay, or distortion or pitch it down or…).
Using Logic’s Impulse Response Utility
Out of all the available tools, the Impulse Response Utility makes the process of recording impulse responses the easiest without converting the impulse response into an unusable format.
Remember to document your recordings. Your notes and descriptions will be useful for future reference. Here are a few examples:
IR source (sweep/transient)
In case you missed it, feel free to jump back to Part 1 of this Recording Impulse Responses article series
Author: Fayez Saidawi