What is Impulse Response?

The concepts behind impulse response processing and convolution reverberation are too deep to cover here in any meaningful way. But I will try, nonetheless….

Normally, an impulse-response (IR) is considered to be a recording or model of the acoustics of a room or reverberant space. To capture the IR, a short burst of noise, or a swept sine wave is played into the room, and captured at some distance with a measurement microphone. The original burst or sweep may then be processed along with the recorded sound to produce a mathematical model of the room, which may then be applied to other sounds. They should now sound as though they are being played in that space.

Although it might seem at first that this wouldn't be a particularly useful method for producing models of microphones, the impulse-response contains time, frequency and phase information, all of which would be altered in some way by a 'real' (i.e. imperfect) microphone. The mic can be considered as a filter or sound processor.

Many vintage microphones are a long way from being flat in response, and have definite and recognisable sonic signatures. It would be nice to think that if we processed a sound with the microphone's impulse response then it would sound as though it had been recorded with that mic and we could recreate, convincingly, the sound of a vintage U47 or RCA 44BX. However, it is not possible to completely decouple the microphone from the space in which it is used, short of using a perfect, infinite, anechoic chamber. Ours is somewhat smaller!

To some extent a measurement mic can be used to account for the room response, but most (or all) measurement mics are omnidirectional, and see all sides of the room equally. A cardioid large diaphragm condenser or a figure-of-8 ribbon mic will hear only parts of the room, making calibration more challenging. So, we must remember that the response contains information from both the mic and the environment.

Some useful links below describe the process in (far) more detail:
Designing Sound has a VERY good description - much better than mine!
Wikipedia on IR
Impulse response library at the University of York

Software for creating reverbs and processing sound using IR files include….

AudioThing Fog Convolver
AVID / Digidesign TL Space
Waves IR1
McDSP Revolver
Apple's Logic includes Space Designer

I have personally used Audiothing's Fog Convolver and TL Space, and can't comment directly on the others.

Thanks to Henry Olonga at Nebula Presets for his enthusiasm and advice.