Impulse Responses and the sound of possibility.
High Fidelity Impulse Responses
Impulse-response-based convolution is a common technique used in sound design, mixing and audiovisual postproduction to simulate reverberant spaces and non-linear processes such as distortion (such as guitar amplifier emulation).
Most of the discourse and research around this technique is focused on creating realistic emulations of real spaces. The democratisation of audio engineering tools means that it is now easier and cheaper than ever to “place” any desired sound source in a virtual simulation of your favourite opera house or professional recording studio. Premium versions of these tools boast high channel counts (for immersive audio applications) and highly customizable and clean responses, but good results can already be achieved using freeware like Reaper’s ReaVerb and IRs available on archives like “freesound.org”
By recording a sine sweep or a balloon popping in a space, anyone can make their own IRs with just a phone and some editing software. The quality of these may not be so precise as professionally recorded responses, but in a dense music or film mix, they can still be used to convincing effect. Often these DIY files require some editing, like EQ and Volume automation, to sound their best. But what happens when we use these tools not to aim for a convincing emulation of a real space, but to create imagined or altered spaces?
Bending Reality
On a practical level, an impulse response is just any other audio file. Any process we would typically apply in a creative context to a sound source can also be applied to these files. Probably the most common modification made to IRs is that of changing the speed of the file, which artificially increases or reduces the perceived “size” of the reverberant space. If one records impulse responses at high sample rates, they can be slowed down significantly without much perceived loss of quality - turning your bathroom IR into something akin to the sound of the inside of an oil tank truck.
EQ, Distortion, spatial effects, pitch-shifting, and more can all be used to subtly bend the “reality” of Impulse Responses, enhancing the qualities of the recording one finds desirable and reducing the perception of undesirable characteristics in the response. Common use cases are removing resonances for more frequency-linear reverbs, repairing clicks and volume jumps or smoothing out reverbs with spatial effects.
These same processors, when applied more intensely, can completely transform a response from a somewhat “realistic” representation of a space to an imagined space, not the recording of what is perceived, but an illustration of an imagined or possible space.
The interaction between an instrument and a space is an incredibly complex phenomenon, and even the most sophisticated Convolution processors cannot perfectly emulate the interactions between real instruments and real spaces. The abstraction of the computer in music-making has the prejudice of distancing sound from “reality”, but in exchange, the musician or engineer gains an infinite canvas of sonic possibilities that go beyond auditory photorealism.
The sound of possibility
Electronics and synthesised sound are at the forefront of music culture in the early XXI century, by their nature, they do not exist “acoustically”, any attempt to position them in a space is, in a way, an abstraction and more of a creative than a practical decision, as microphone placement for acoustic instruments is, for example.
Synthesis gives the musician complete control over every possible parameter of a sound, allowing for the creation of music that is free from the bounds of what is possible acoustically. Electronics enable musicians to create instruments on the fly that best represent their compositional needs, allowing music subcultures to quickly develop and iterate on sounds created from scratch to resonate with a particular crowd, demographic, or zeitgeist.
What the synthesiser is to the instrument, the impulse response can be for the space by harnessing synthesis to create a response file. Synthesis enables artists to create IRs from scratch, eliminating the need to follow how a “real” space responds to an impulse. Noise bursts, resonating notes, complex modulations, wavetables and harmonic processors can be used to generate bursts or sweeps with reverberation characteristics that don’t follow any existing or physically possible space. Electronics may be to some extent less “real” than acoustics, but their almost infinite versatility allows sound to be closer to the idea a musician has in their head and their compositional intention.
From Carlos to Drexcya, synthesis has a storied history of giving musicians the agency to imagine the sound of the future, or rather, the sound of a possible future. Impulse response synthesis has the potential to give musicians and engineers the opportunity to imagine and showcase the sound of possible spaces, whether these are entirely fictional or representative of what a musical space could sound like in the future.
A practical exercise for IR Synthesis
To start synthesising your own impulse responses, I recommend the following process:
Familiarise yourself with impulse responses. How do they sound? Can you predict how the convolution reverb sounds from the sound of the response itself?
Post-process existing impulse responses with various tools in your DAW and familiarise yourself with how the edits you make to the response affect the convolution reverb sound
Record and postprocess your own impulse responses. Ideally, using both the “burst” and “sine sweep” methods to understand the limitations of each technique and how to deconvolve a sweep.
Use already existing non-impulse-response sound files as impulse responses. What happens when you take a bit of Foley, a piano or a synth hit and use it inside a convolution reverb?
Using a synthesiser, create some bursts of noise and tones of various qualities. How do these compare to your previous experiences with impulse responses?
Finally, if you want more control over your synthesised response, create a Logarithmic sweep in your synthesiser, design a hypothetical response and deconvolve it in a tool like ReaVerb to produce highly detailed and complex imagined responses.
This is not a comprehensive tutorial; my advice is to familiarise yourself with the theoretical foundations of how impulse responses work and then go on your own journey to discover what kinds of synthesised or processed responses speak to you and your artistic vision.