Sound Design Resonators

In our latest foray into sound design techniques, we explore the link between comb filters and delays

It may not be obvious, but comb filters and very short delays are essentially the same technology. That is, setting a delay to a value below 16 milliseconds will generate a perceivable pitch when its feedback levels are sufficiently high. This is due to the fact that tiny delay times with extreme feedback create a very short audio loop.

The maths for this loop is quite straightforward. A millisecond is

1000th of a second, so when you set a delay to 1 millisecond and increase its feedback to 99% (100% will create an endless tone) you’ll generate a sound that repeats 1000 times a second: 1000Hz. With a little division, if you set the delay to 2ms, the pitch drops to 500Hz (1000 divided by 2). 4 milliseconds yields 250Hz, and so on.

This process serves as the basis for the Karplus-Strong string

synthesis algorithm of the early ’80s, which uses a noise burst to excite a digital delay, resulting in a tone that sounds quite like a plucked string. It’s also the source of flanging effects, which introduce LFO modulation to the delay time, thus changing its pitch as it’s processed for that familiar ‘jet engine’ whooshing effect.

This is also one reason why synthesists sometimes compare swept comb filters to flangers and choruses: the underlying principles for both of these are based on these delay times – and the effect is often known as a resonator.

This month, we’ll examine two ways to create resonators. In these tutorials, we’ll use Phase Plant’s dedicated Resonator effect and Serum’s array of comb filters, which also include resonant phasers for added timbral range.