With the never-ending development of computer processing, convolution plugins have started to become popular for recording impulse responses. Each of the plugins mentioned requires different procedures for making a custom library of impulse responses, however we would like to cover the general ideas behind recording good impulse responses, that apply to any convolution/deconvolution tool.
Some common ones include:
- Audio Ease Altiverb
- Logic’s Space Designer
- Avid TL Space
- Waves IR-1
- McDsp Revolver
These plugins are normally bundled with extensive libraries of impulse responses, but what makes them even more valuable is how simple it is to record and utilize your own impulses as well. This customizes your mixes, and makes it helpful in post-production and in the design of new sounds.
Convolution is where a single example of a sound is duplicated by each example of another sound. It is unique in relation to the plain duplication of two sounds where a solitary example of the main sound is increased by the comparing single example of a second sound. In other words, Convolution of two audio signals is equivalent to filtering the spectrum of one sound by the spectrum of another sound. Convolution of spectra means that each point in the discrete frequency spectrum of input A is convolved with every point in the spectrum B.
Over recent years, convolution has been used to reproduce the reverberation of spaces, the sound of equipment units (reverb units, equalizers, tape machines, guitar amps, and so on) and in creative sound design. Impulse responses enable us to catch and store the acoustic characteristics of a space, or bit of sound changing hardware. It could be depicted as an acoustic 'photo', where, rather than catching a space outwardly, it is captured aurally.
The impulse responses are captured by recording how a space reacts to a full range of frequencies (commonly 20Hz to 20,000Hz). This is accomplished by playing back an impulse of a full range of frequencies inside the space and recording it. For more exact outcomes, usually to utilize a sine-sweep over audible frequency range.
The recorded impulse response is an audio file that can be imported into a convolution program/software to reproduce the captured space in a digital manner. Some convolution modules like Altiverb and Waves IR-1 utilize their own formats, but they can still import regular WAV or AIFF files also.
The prompt utilization of impulse responses is to reproduce real spaces. It is not hard to find impulse responses of famous venues from around the globe or even vintage gear. It is extremely useful in post-production where it is important to re-make realistic spaces, coordinate ADR to areas and locations and in making convincing sounding results in the mix.
The convolution procedure is also valuable for designing interesting sounds by using impulse responses of objects such as tin cans, plastic pails, bottles or any sound altering device. An impulse response can be utilized to make a new library of sounds out of an existing recording. The impulse responses themselves can be manipulated in a DAW (before being imported into the convolution plugin/software) to make a much more prominent library of fascinating sounds. Additionally, its common to import non-impulse response records into convolution software. Eg: convolving the sound of a synth with a drum kit to make a rhythmic synth.
What to record?
- Gear / Hardware
- Software / plugins
- Anything that changes sound and is excitable!
Recording Impulse Responses
To record an impulse response, you require a sound (source) to energize the space and some recording equipment.
- Sine sweep: A sine sweep is the most favored technique in recording an impulse response. Depending upon the length of the sweep, it usually provides the best signal-to-noise ratio. The longer the length, the greater the signal-to-noise ratio but also greater the chance of recording resonances/noisy vibrations in a space. The sweep is played off a speaker to excite the space.
- Transient method: The second most common method and perhaps the easiest is to use a starter pistol, balloon or clapper board (especially on location for a shoot). Anything that is loud and creates a broadband burst of noise will do. The advantage of using a transient sound (starter pistol, clapper board, balloon) is that there is no need for any post-processing. The recording of the impulse can directly be used in a convolution software. As with most recording sessions, the quality depends on the location, microphone technique, and ambient noise level. It might not always be possible or convenient to carry a speaker and playback a sine-sweep, although sine-sweeps are the most preferred (and closer to accurate) method.
It is best to try and maintain a minimal and clean signal chain, as every bit of gear affects the impulse response
- Speaker(s): Depending on the source and recording technique used, there might be a need for one or more speakers. The speaker (and its capability to reproduce the sine-sweep) will have an obvious impact on the recorded impulse response.
- Microphone(s): Anything from mono all the way up to multichannel formats is possible (as long as the convolution software supports the configuration). It is common to record using multiple configurations from different perspectives. Just like the speakers, the microphones and microphone technique will affect the impulse response. There are no hard and fast rules as far as polar patterns or microphone types are concerned (although condensers are the most common for obvious reasons), be creative and experiment!
- Playback/Recording device: The playback and recording devices could be separate or the same (a laptop). The choice depends on the location and is a question of convenience.
- Documentation: It is always a good idea to document your recordings with at least a camera and a notepad.
Some of the most common formats:
- Ambisonic – Ambisonic recordings are made with Ambisonic microphones that output four channels of the ambisonic format (B-Format: W,X,Y,Z). It is possible to derive any of the multi-channel formats mentioned above or any custom format from these four signals. Some convolution plugins like Logic’s Space Designer can import and process ambisonic impulse responses. There are also varieties of tools available to convert ambisonic recordings into any of the standardized formats.
- Other 3D Sound Systems: Guido Helbling recently posted his thoughts on recording impulse responses for Auro 3D and Dolby Atmos.
- Always record at 24 bit
- If you can, record at higher sample rates to give you the option of processing the impulse response in post-processing (pitching down and/or time stretching can create very interesting effects).
- Try to maintain the cleanest signal chain when recording.
- The position of the speaker and microphones are obviously important. Do a couple of test recordings, if possible.
- Noise: are there any broadband noises you can get rid off? Ventilation or electric hums?
- Try to maintain a MINIMUM of 4-5dB of headroom.
- Options! Record different perspectives to give you choices.
To learn about post-processing of recording impulse responses, check out Part 2 of this series!