More on Convolution A simple convolution: a guitar power chord (the impulse response), a vocal excerpt, and the convolution of one with the other. We have sonically turned the singer's head into the guitar chord: he's singing "through" the chord.
Another interesting use of the phase vocoder is to perform convolution. We talked about convolution a lot in our section on reverb, (both in the time domain, and in the frequency domain).
Soundfile .x
Remember that a convolution multiplies every frequency content in one sound by every frequency content in another (sometimes called a cross-multiply). This is different than simply multiplying each value by its corresponding value. In fact, as we mentioned, there is a famous and surprisingly simple relationship between these two concepts: multiplication in the time domain is the same as convolution in the frequency domain (and vice versa). As you probably saw in Chapter 3, the mathematics of convolution can get a little hairy. But the uses of convolution for transformations of sound are pretty straightforward, so we'll explain them below.
Cross Synthesis
Convolution is a type of cross-synthesis: some aspect of one sound is imposed onto another. A simple example of this returns us to the subject of reverb. We described the way that, by recording what is called an impulse response of a room (its resonant characteristics) using a very short, loud sound, we can place another sound in that room (at whatever position we "shot") by convolving the impulse response with that sound. Surprisingly, by convolving any sound with white noise, we can simulate simple reverb.
Using Convolution
Although reverberation is a common application of this technique, convolution can be used creatively to produce unusual sounds. Simple-to-use convolution tools (like that in the program Soundhack, by Tom Erbe) have only recently become available (within the last 10 years) to a large community of musicians, because up until recently, they only ran on quite large computers, and in rather arcane environments. So we are likely to hear some amazing things in the near future using these techniques! In this convolution example the sound file on the left and the middle sound file are convolved with the sound file on the right. The two sound sources are: noise and an inhale.
Noise sound file
Inhale sound file
Noise and Inhale convolved in to a new sound file.
Soundfile .x