These are simply rooms that are designed with specific acoustics in mind and, in some cases; the room dimensions can be altered. The concept is quite simple: design a room where you have control over the reflections and adjust the room’s dimensions to attain different reverb values; record within this room by using multi microphones positioned in different places to attain different textures. There are still studios that have dedicated chambers that have afforded a specific colour to a recording and are therefore still in demand by recording artistes and producers.
Some reverb chambers were very specific, an example being plate reverb chambers that were designed to house large plate reverbs and the room was used to further shape the reverb information. In fact, it didn’t end there; some rooms were built as trapezoidal with the speakers being placed against one wall and the microphones were placed against an opposing wall. This allowed for some very creative reverb responses and the shape was designed to abate standing waves.
Nowadays we emulate these spaces by recording in rooms with specific microphones so as to capture the room’s ambiance. In other words, the room acts as the environment for the natural ambiance/reverb captured by the microphones. This is a basic and simple version of a designed chamber. I have worked in studios in the past whereby the chambers had adjustable screens that acted as portable walls. These screens were designed with control of reflections in mind and could be moved to create new reflective results thus allowing for an almost infinite number of room designs and modes. Of course, not everyone can have a dedicated reverb chamber at their disposal so multi miking (using more than one microphone) and recording in specific rooms and locations is the best and most affordable way to capture a room’s natural response, and this has now become common practise and to some degree preferred over using conventional reverbs. But this in itself can cause problems for those on a budget with limited access to different rooms and multi microphones of different designs and quality. And for this reason alone the best way to capture a room’s response, or to have access to different reverb types and designs, is with the use of ‘impulse responses (IR)’.
Other relevant terms used to describe IRs are convolution or ‘sampling’ reverbs. The idea is to capture the response of an acoustic space and to playback the response using a convolution playback device, be it software or hardware, although nowadays the software playback devices are more commonly used. The basic principle is to record a test signal, a swept sine wave signal is the most commonly used test tone, and to convert the captured signal into an impulse response which is then loaded into the playback device and played back as an ‘effect’.
You are not limited to just reverb effects but IRs can be created of mic pres, compressors, amps, tape and so on. This not only allows you to have access to different types of effects but also the different characteristics of varying much sought after hardware/software units. I have IRs of some notable and sought after units that I could not afford to buy or source. It is an extremely cheap and easy way to access and use just about any piece of equipment that can be used to act as a medium for the test signal. Play the signal through a nice boutique piece of gear, record the output and then convert to an IR to use in a playback device: simple and yet so effective. I find using IRs are a much better option for creating ‘real spaces’ than standard reverb plug-ins although nowadays some reverb plug-ins have made a lot of headway and are very usable. You can get as creative as you want with IRs.
I have created manic IRs by running a test signal through a hi hat triggered gate, or through a plastic pipe and so on. If it can act as a medium for a test signal then it can be recorded and used as an IR.
There are many convolution playback devices available and one or two are free. There are also a number of sources, some free, for IRs available on the internet. Basically, this allows you to have access to many IRs of varying sources at no or very little cost.
I prefer to use IRs for real spaces than standard reverb plug-ins simple because the real space captures sound more natural than algorithmically generated responses. There are, of course, certain limitations with using IRs; IRs are static responses, in other words they are a capture of an instance of a recording and are therefore simply the equivalent of a single frame of film. Additionally, some playback devices afford no editing of the IR which limits it to be used ‘as is’. Finally, your computer’s cpu overhead is directly related to the length of the IR as the computer has to calculate the incoming audio in real-time. However, nowadays we have very powerful computers that can handle many simultaneous instances of IRs and playback devices have advanced tremendously to include all sorts of editing features that allow the IRs to be manipulated extensively.