What is a Convolution reverb?
Convolution Reverb digitally simulates the reverberation characteristics of an acoustic space using a piece of software. For instance, it can recreate a realistic simulation of any acoustic space like cathedrals, symphony halls, your bedroom etc.
The development of this sort of reverb is as recent as two decades. Since the dawn of digital reverb in 1976, people were using digital reverb based on multiple feedback delay lines. These provided workable sounds but they were not as realistic as a convolution reverb. In 1999 Sony came up with DRE S777, which was the first real time convolution Processor. It was a game changer for that time.
The earlier processors used to take a day to process the Impulse responses. Since then we have come a long way to be using DSP based Convolution processors, which is pretty amazing.
How does it work?
Audio samples like a gunshot, snare hit or a sine sweep tone are played in a real space. In addition, a microphone captures the impulse response of the place. This audio sample is consequently fed into the convolution processor. After that, the calculations treat any incoming signal to replicate the characteristic sound of that space. In simple words, an acoustic space is excited with a sound. So, the resulting impulse response of that space is recorded and used to react to an entirely different signal.
In terms of processing the software processes two forms of reflection:
- Early Reflections:
As the name suggests these are the reflections, which are captured first. Therefore these reflections are discrete and occur as peaks.
- Diffused Reflections:
The sound strikes various surfaces, creating the first reflections. In turn, these reflections are reflecting on other surfaces and creating other reflections, due to the presence of many surfaces and objects. These reflections are continuous and appear as continuous noise.
What is Impulse response?
A sample recorded by excitation of an acoustic space is called an Impulse Response (IR). Additionally, it contains all the information regarding the reverberation of the space. In the context of acoustical analysis, you might also think of an impulse response as the acoustical “signature” of a system. The IR contains a wealth of information about an acoustical system including arrival times and frequency content of direct sound and discrete reflections, reverberant decay characteristics, signal-to-noise ratio and clues to its ability to reproduce intelligible human speech, even its overall frequency response.
Most Common methods to create Impulse Responses:
- Sine Sweep method:
A long sine wave tone encompasses all the frequencies.
Specifically, this means that it generates reverberation for all the frequencies, creating high-quality simulation of the space.
- Short Impulse method:
Sound engineers create and record a very short impulse likes a clap, balloon burst, gunshot etc. in an acoustic space and record the impulse response. Moreover, this method is very common and is used by companies making such reverb.
Generally speaking, unless you’re recording IRs inside a concert hall, it’s more rewarding to seek something that sounds good than something that’s ‘correct’. You can also push this principle further put mics near walls, under sheets, under seats, in sinks. Experimentation is the whole point of making IRs yourself.
Commonly used Convolution Reverbs:
- Space Designer – Logic Pro
- Altiverb – Audio Ease
- TL Space Native Edition – Trillium Lane Labs
- IR 1 – Wave
Additional Resources & Source Texts