Author: Guido Helbling, Avosound - last updated June 20, 2017
An impulse response contains the acoustic information that later becomes a convolution reverb plug-in used to create a reverb. To generate impulse responses, a loud signal is required to create echoes over the entire frequency spectrum. The simplest noise that occurs naturally and covers all frequencies, is a loud bang or gunshot.
A loud sound may be generated in different ways. Naturally, an alarm shot or a starter pistol are better than grandpa's old gun from the attic. Acoustic engineers like to burst balloons to make measurements. Even in large rooms, air balloons can be loud as hell, as I myself have found out.If there are no balloons or guns available, you can also rely on so-called ‘shot flaps’: by banging together two flat boards (similar to a clapperboard), you can produce ridiculously bad gun shots (which are, incidentally, still used today in theaters). And if you want to make it really loud, shoot carpenter nails into a concrete floor with a nail gun.
The 'Bang-Method' surely sounds like fun (nail gun!). It does, however, carry the risk of causing irreperable hearing damage to yourself or others - or of creating a panic when handling firearms in a crowd.
Also, a shot is unpredictable. It can't be reproduced exactly and is sometimes louder, sometimes quieter. Furthermore, bang signals are difficult to record because they are highly dynamic and can only be leveled after several test attempts. A loud bang can not only overdrive the pre-amp, but also the microphone itself. Overdriven or distorted impulse responses quickly become dull and useless
Clever scientists have found out that a signal can be separated into sine waves using mathematical equations. Using mathematical convolution it is possible to represent the bang as a sine signal, giving us an audio signal that we can actually play back via a loudspeaker. To cover the entire frequency spectrum, our signal is a sinusoidal signal that sweeps through the entire frequency range of 20 - 20,000Hz
The mention of the word 'speaker' should generate some shouts of joy, because playback by speaker generally meanss:
But playback by speaker also means:
Hmm...
Since sweeps are not as dynamic as a bang or a shot, they are a lot easier to control in regard to sound level and can be much more precisely set to an optimal level. A sweep can also reproduced as many times as needed - although loud sweeps can quickly become annoying to other members of the public.
Impulse responses with sweeps are hence the alternative to the 'Bang Method'. Unfortunately, it is not done with the inclusion of sweeps. As mentioned above, sweeps need to be converted to impulse responses. Only then will there be an impulse response that can be applied. This conversion of sweeps into impulse responses is a mathematical process and is achieved by deconvolution.
Deconvolution refers to 'expansion', but in a strictly mathematical sense of the word. Folding and unfolding are mathematical transformations, which find use in all kinds of audio and video applications. With the method of deconvolution, it is possible to transform a sine-sweep signal into an impulse response in our application.
See Deconvolution for more information, formulas and examples (if you want to convert your own sweeps with a calculator).
Author: Guido Helbling, Avosound