Mixing

Most chapters in this guide have dealt with one sound at a time. Music usually has several, and parts that play at the same time in the same frequency range or stereo position can make each other harder to hear. A mixer gives each part a channel, and each channel has controls for its level, its position between the speakers, and usually a filter or equaliser and sends to effects. The channels are then added together, in a mixing desk, a modular mixer module or the mixer of a digital audio workstation, into a stereo pair. Balancing the channels against each other is called mixing.

The tool below is a four-channel mixer for a short loop of drums, bass, a chord and a lead. Drag the faders up and down and the pan controls left and right, and click M to mute a channel or S to solo it. Each channel also has a highpass filter (HPF) of the kind described in the filters chapter, with a slope of 24 dB per octave. Click HPF to switch it on, and drag across its response curve to move the cutoff between 20 Hz and 1 kHz. The meters show each channel's level after its fader, and the master meter shows the sum.

Level and balance

Faders are marked in decibels because loudness is heard on a roughly logarithmic scale, so equal movements of the fader give equal changes in loudness. Balancing usually starts from the part the piece is built around, often the drums or the vocal, and brings the others up around it until each can be followed without straining. Solo is useful for listening closely to one channel, but the balance has to be set with all the channels playing, because a part that is clear on its own can be masked by other parts in the same frequency range.

Panning

The pan control sets how much of a channel goes to each speaker, which places the part somewhere between left and right. Parts that are central to the piece, such as the kick, the bass and a lead vocal, are usually kept in the middle, where both speakers carry them equally. Two instruments in the same frequency range can be panned to opposite sides, which makes each easier to hear.

Frequencies and headroom

When two parts have energy in the same frequency range, the louder one masks the quieter, making it harder to hear or inaudible. Masking is common at low frequencies, because the kick, the bass and the lowest notes of chords all have energy there. A highpass filter (HPF) on the parts that do not need low frequencies removes them, so those parts no longer mask the kick and bass. Mixing desks often label this filter low cut. Its cutoff is set just below the lowest frequency the part needs, which for a chord or a lead is often between 100 Hz and 300 Hz. The sum of all the channels has to fit below the maximum level the output can carry, 0 dBFS in a digital system, or the peaks are clipped and distorted. Leaving a few decibels of headroom on the master, and keeping each stage at a sensible level on the way, is called gain staging. The finished mix then goes to mastering.