An IR (impulse response) is data that records how a space or a piece of equipment reacts to a very short sound. In reverb it takes the form of an audio file representing the response of a room, hall, plate, amplifier, and so on. Loading it into a convolution reverb applies those characteristics to your input signal.
What it records
Between a sound being made and its reflections dying away, you get the initial direct sound, the early reflections returning from walls and other surfaces, and the dense reverb tail. An IR holds all of these changes over time and across frequency together. Even though it looks as if nothing more than a short click was captured, it contains a great deal of the character of the space.
Not just for spaces
IRs are used to recreate not only rooms but also reverb units, guitar cabinets, filters, and more. What you expect from an IR depends on the job. Choosing a room IR in place of a cabinet can sound unnatural, so check what the data was made to reproduce.
Length and how it sounds
Longer is not automatically better. Short IRs tend to preserve definition, while long ones create more decay and width. If a sound gets buried in the track, try a shorter IR, shortening the tail, or tidying up the low end of the reverb.
Things to watch when loading
Check that the IR’s sample rate, channel count, and file format are within what your plugin supports. Stereo IRs make width easy to achieve but also call for attention to mono compatibility. With loud IRs, watch that the input and output levels do not climb too high.
How to choose one
First decide whether what the dry sound needs is distance, width, or coloration. Judge the amount of reverb in the full mix, not just from the soloed impression. How processing with IRs works is covered in the reverb article.
