6. Frequency processing: Equaliser and Filter

Besides dynamic processing, controlling the frequency spectrum of the material is the second major component of mastering. This includes using small, broad-band adjustments to match different tracks or sections. But that's not everything! For example, if the snare drum appears too loud in the mix, you might try to use an EQ to attenuate the corresponding frequencies. While doing this, you need to pay attention on how this affects the other elements of the mix. A small, narrow-band adjustment of 1-2 dB may already be sufficient.

Manley Massive Passive Mastering EQ: Passive filter elements in combination with make-up gain amplifiers lead to a special sound

EQing can drastically alter the tonal characteristics of the music. If a ballad doesn't sound as intimate as you'd like, try a moderate cut of the entire treble range using a shelving EQ. If a song sounds dull and indirect, boosting the high midrange and presence can be the cure. A muddy mix might benefit from a low shelf that keeps the low frequencies in check. Some equalisers can also be used to „sweeten“ and refine the sound. High-grade inductor filters can give the high end a silky, creamy charakter and help to iron out sonic rough edges.

In addition to the creative, musical side it is often necessary to perform technical corrections of the frequency spectrum. Narrow EQ bands can be used to eliminate troublesome frequencies like a squeaky bass drum pedal, an unwanted resonance in a guitar or a drum, or the like. These corrections should ideally have been taken care of during mixdown, working with the individual signals. In practice, however, problems like these often go undetected until the mastering stage, which can in part be attributed to better monitoring facilities at the mastering studio.

Filters are passive processors: They can only attenuate signals, unlike an equaliser, which can also boost. During mixdown and mastering, low cut / high pass and high cut / low pass filters are the most commonly used types of filters. For sound synthesis, band pass and band rejection filters are also used. The low cut filter is used in mastering to eliminate low frequencies that do not contain any musically relevant information. This reduces the strain on playback systems (amplifiers, speakers) and increases the power available for the musically relevant signals.

Equalisers are active—in addition to attenuating, they can also boost portions of the signal and are very flexible. Special units like the Manley Massive Passive EQ or the SPL PassEQ combine passive filters with active make-up gain amplifiers, meaning that they combine the sonic advantages of passive filters (neutrality, low harmonic distortion) with the capability of boosting the signal.

  • Low pass and high pass filters usually only have a control for the cutoff frequency. At the cutoff, the signal is already attenuated by about 3 dB. Signals above or below the cutoff aren't eliminated completely, but attenuated gradually with increasing distance from the cutoff. This line of attenuation is called filter slope and is measured in decibels per octave. Common values are 12 and 24 dB/oct.

  • Equalisers come in the forms of shelving and bell (peak) EQs. Typically, the highest and lowest bands can be switched between shelving and bell, while the center bands work as bells exclusively. A shelving band typically has an adjustable cutoff frequency like a filter. However, unlike a filter, which cuts out the signal, the shelving band attenuates or boosts the signal by an adjustable gain value. If positive values are used (boost), a high shelf EQ can for example be used to boost all frequencies above 12 kHz by 3 dB.

  • A bell or peak band is a completely different animal. Here, the frequency control determines the center frequency of the band. The gain setting controls how much the signal is boosted or attenuated at this frequency. The Q factor lets you set the width of the band: Higher numbers mean a more narrow band, while small numbers lead to a wider band. If the Q factor can be adjusted, the band is referred to as „fully parametric“. If the Q factor is fixed, it is „semi-parametric“.

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