21 - Spatialization





Our ears and brains are sophisticated frequency analysis machines with a lot of interesting capabilities. The field that deals with all of this is called psychoacoustics, and we will only touch on some of the most basic principles here. We already looked a bit into how we perceive loudness and pitch. Let’s now look a bit into the way we perceive space.

In real life sound is always a spatial phenomenon. We can estimate pretty accurately where a sound is coming from, especially if it is in front of us. Low-frequency sounds and sounds coming from behind us are more difficult to pinpoint. In sound production the most common way to create an illusion of space is by using two speakers, one on the left, one on the right. Various forms of surround sound are also being used, but the vast majority of sound production happens in stereo.

We tend to place higher-frequency sounds higher in the space, and loud and dry sounds closer to us. Taken together with a sound’s position along the left-right axis we get a perceptual model that looks something like this:

The ‘Amplitude and effect’ part might need a bit of clarification. If a sound is very ‘wet’, which means that the pure signal is heavily mixed in with for example a reverb, we will interpret it as being far away from us. We can use this to create a sense of something moving away from or towards us by automating the amount of reverb we give to a signal and synchronizing this with a change in amplitude. If the amplitude goes down while the reverb mix goes up we perceive the sound as moving away from us. Conversely, if the reverb mix goes down and the amplitude goes up, we will perceive the sound as moving towards us.

If we change the frequency content of a sound it will sound as if it is rising or falling down. This can be done with different filters, and, even if it has become a bit of a techno-cliché, it remains an extremely useful technique.

We can also play around with the stereo field to create a dynamic sound space. A classic effect is the ping-pong delay, where the sound is bouncing back and forth between the left and the right speakers.

In general you want to make sure that each sound you use has its own place in the sound space. If two sounds are very similar in terms of frequency content, you probably want to place them on the opposite ends of the stereo field. Alternatively, you can sculpt the sounds by filtering out different frequencies and create niches for them to live in. We will get back to some of the ways to deal with frequency content later on in the workshop.