EQ is a post-effect. It uses four filters (Low, Mid, Mid, High) to shape the audio of any connected Source blocks. Each filter can boost or reduce (attenuate) an adjustable band of frequencies within the audible frequency range. This process is called equalisation.
This can dramatically alter the tone or character of the sound, having more low frequencies boosted creates more bass, and higher frequencies more treble. Boosting frequencies of audible pitches creates resonance. An EQ can only boost or reduce frequencies already present in the original audio.
The options panel shows a “parametric” graph. It’s a visual representation of the equalisation, with draggable filters. The Low filter is a “shelf” type filter. It sets the level of frequencies below a certain point. The High filter is also a “shelf”, setting the level of frequencies above a certain point. The two Mid filters are “peaking” type filters. They boost or attenuate frequencies at a certain point, and within a set range (known as “bandwidth” or “Q”). The Q width can be set by adjusting the bars at the bottom of the graph.
The EQ block uses Filter components from Tone.js.