Impulse Response audio files
Fichiers audio à réponse impulsionnelle

What is convolution
Convolution is the effect of multiplying every sample in one wave or impulse by the samples that are contained within another waveform. In a sense, this feature uses one waveform to “model” the sound of another waveform. The result can be that of filtering, echoing, reverberating, phase shifting, or any combination of these effects.

For example, “convolving” someone saying “Hey” with a drum track (short full spectrum sounds such as snares work best) will result in the drums saying “Hey” each time they are hit.
With the proper impulses, any reverberant space can be simulated. For example, if you have an impulse of your favorite cathedral, and convolve it with any audio, then the result would sound as if that audio were played in that cathedral.
You can generate an impulse like this by going to the cathedral in question, standing in the spot where you would like the audio to appear it is coming from, and generating a loud impulsive noise, like a “snap” or loud “click”. You can make a stereo recording of this “click” from any location within the cathedral. If you used this recording as an impulse, then convolution with it will sound as if the listener were in the exact position of the recording equipment, and the audio being convolved were at the location of the “click”.

What is an impulse response file
An “impulse response file” is the data by which every other sample in your waveform will be multiplied. If the impulse is a single sample of a full volume “tick”, then the convolution of that impulse with any audio data will just be that audio data itself. If that “tick” is at half volume, then the original audio data will be reproduced at half volume.

If there are several ticks, descending in amplitude over time, such as one tick every 100 milliseconds, and half as loud as the previous tick (fig. 2), then the result of convolution (fig. 3) with some audio (fig. 1) will be that sound echoed with 100ms between each echo, and each echo at half the volume of the previous echo.

fig. 1	fig. 2
fig. 3

Another interesting use for convolution is to generate an infinite sustained sound of anything. For example, one singing “aaaaaah” for one second could be turned into thousands of people singing “aaaaaah” for any length of time by using some white noise.

To get a feel of convolution, load up and play with some of the sample Impulse files on this page using your favorite hardware or software convolver.
Enjoy !

A bit more about convolution
Imagine the following example :
An audio file containing 8 samples (X0 to X7) convolved with an impulse file containing 4 samples (i0 to i3).
That would give the following table :

This can be represented by the following diagram :

Which correspond to the following expression :

Y(n) = i0 · X(n) + i1 · X(n-1) + i2 · X(n-2) + i3 · X(n-3) + ... + ik · X(n-k)

Or to the following general equation :