Deconvolution in Impulse Response Utility
In Logic Pro, Space Designer combines, or convolves, an impulse response of an acoustic space with an audio signal. The end result is that your audio is placed “inside” the acoustic space represented by that impulse response. Think of an impulse response as the total echoes (reflections) in a given physical space, following an initial signal spike. The impulse response file is an audio recording made in a space that contains these echoes.
There are two practical approaches to impulse response recording: the transients method and the sine sweep method.
The transients method involves recording an impulse in a space, using a device such as a starter pistol. The gunshot is the impulse, and the audio file you record captures both that impulse and the room response.
The advantage of this method is that the recorded audio file can be used in Space Designer without any further processing. You can use Impulse Response Utility to record your starter pistol impulses and then combine them into a Space Designer Impulse Response (.sdir) file and a Space Designer setting file.
The disadvantage of this method is that it is difficult to make a perfect, undistorted recording of a starter pistol shot. This is due to the extremely loud initial transient of the shot. A further issue is that starter pistol shots contain very little high or bass frequency information—which, in turn, limits the usable frequency range of the convolved reverb.
Sine sweep method
Use of a sine sweep is the preferred method for creating impulse responses. You play a broadband audio sine wave sweep into a space and record the sweep (and space) at optimal levels. A sine sweep that covers the entire audible frequency range is used, resulting in a broad-range, and often higher-quality, impulse response.
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 transients method, where the entire response is contained at the beginning of the file in a very short impulse (the gunshot).
When you use a sine sweep, Impulse Response Utility uses a process called deconvolution to align the times and levels of 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. Impulse Response Utility can then generate an .sdir setting from the impulse response.
Download the guide:
Impulse Response Utility User Guide: Apple Books