Whenever I hear somebody say something like “and now I'm going to boost 60 Hz” there's a part of me that thinks “okay but is there much 60 Hz present in the audio that you're mixing”? If you're using a reasonably wide filter then in most cases there's likely to be some useful energy at the point at which you place the filter but this isn't always the case, particular at the bottom end. This aspect of audio, the fact that it's full of ‘gaps’, isn't as well understood as it should be. In this article and accompanying video I demonstrate why gaps in the frequency content exist and the cases where this issue isn’t a problem.

Pitched Versus Non-pitched Sounds

Some sounds have a recognisable pitch, other sounds don't. While many sounds contain elements of both this pitch characteristic happens because the sound repeats itself in a regular way. This is referred to as the sound being ‘periodic’. Many of the sounds found in nature aren't periodic, being essentially chaotic in nature. This is often referred to as ‘noise’. Noise contains all frequencies, though the spectral content can favour particular parts of the spectrum.

Periodic sounds repeat themselves in a more or less regular fashion and, with the exception of a sine wave, contain multiple simultaneous frequencies which are all mathematically related to the lowest frequency - the fundamental. The fundamental is the frequency we recognise as the pitch of the sound and the particular mix of harmonics or overtones gives rise to the character of the sound otherwise known as ‘timbre’

Harmonics And Timbre

In the examples given below we see a bass guitar playing a low G and a piano also playing a low G. While the same overtones are present, the specific relative levels of those overtones differs and this, and the way in which these levels change over time, are what produce the characteristic timbre of a bass guitar as opposed to a grand piano.

When using an equaliser an understanding of this harmonic series which dictates the distribution of energy through a pitched sound becomes relevant because while energy is evenly distributed in a sound with a significant noise element to it, for pitched sounds there are gaps in between the harmonics which contain no energy at all. If you are for example trying to apply a narrow cut to a kick drum it is possible to place that EQ cut in a gap. In the case of a pitched sound which is playing a melody the pitch will change from note to note and the harmonic series will move with these changes in pitch meaning the EQ move will affect different pitches differently.

This harmonic series and its relationship to static equalisers is easier to understand if you see it demonstrated with an example. In the video below I demonstrate the difference between pitched and unpitched sounds and the effect blending harmonics in different proportions has on timbre, as well as demonstrating how an equaliser interacts differently with pitched and non-pitched sounds.