Quick Guide To EQ and Filters

Quick Guide To EQ and Filters

Confused about shelves, notches and band passes filters? We roll down the slopes and sweep through frequency-charted waters…

If you’ve never tweaked an EQ control, you’ve never mixed! So goes some old studio engineer’s saying. Without a doubt, EQ is the most used – and abused! – of all studio effects and it’s an essential part of virtually all types of synthesis. That’s because it works on the most essential element of sound – the tone. In fact, at its simplest, all EQ does is cut and boost certain frequencies of sound. Think of a hi fi’s treble and bass controls – they’re basic EQ controls. But, of course, technology has made EQ rather more sophisticated than that…

The good old days

EQ is short for ‘equalisation’. In the good old days of recording – we’re probably going back as far as 78s here – the recording process was significantly less sophisticated than it is today and it would typically lose the higher frequencies.

EQ was devised as a corrective process to boost the higher frequencies to compensate for the loss to make the recorded sound ‘equal’ to the original. That’s no longer the case with modern recording equipment and EQ is now used more as a creative effect.

No limit

The human range of hearing is typically quoted as running from 15Hz to 20kHz although this varies considerably with the individual. In practice, it’s generally not that wide and we lose sensitivity to the upper frequencies with age and by listening to loud rock and dance music so many people’s upper limit may be closer to 15kHz.

You might expect this to be the range of frequencies you’d be interested in EQing. However, experiments have shown that frequencies above our normal range can affect our perception of the sound in subtle ways so don’t ignore them.

The combination of frequencies is what gives a sound its distinctive tone (see Oscillators for more about this) and when you EQ a sound you boost and cut specific frequencies which alters the tone.

In control

There are several controls typically used in filters and EQ effects:

  •  Cutoff frequency/Cutoff point. The frequency at which a filter comes into play and starts to affect the sound. This determines what part of the frequency spectrum the filter works on.
  •  Attenuation. This is the opposite of amplification – in other words it reduces the target frequencies.
  • Level/Gain. A level or gain control allows you to cut (attenuate) or boost (amplify) a set of frequencies.
  • Bandwidth/Q/Resonance/Emphasis/Peak. The range of frequencies that will be affected by the filter either side of the cutoff point. It is usually measured in Hertz so it relates to frequency although it would be more useful if it was measured in octaves.The lower graph shows a frequency boost at around 3kHz with a wide bandwidth. The upper graph shows what happens if the bandwidth is narrowed.

The terms ‘bandwidth’ and ‘Q’ are most commonly used in recording circles. ‘Bandwidth’ is the most useful because it tells you exactly what the control does. Q is an abbreviation for ‘Quality’ or, more exactly, ‘figure of Quality’ although these fuller descriptions are never used.

‘Emphasis’ and ‘Peak’ are rarely used in modern parlance.

EQ - Resonance: In a low pass filter in an analogue synthesiser, as the resonance is increased, it boosts the frequencies around the cutoff point.Resonance is particularly common in synthesis. It boosts the frequencies around the cutoff point and can be used to put the filter into self-oscillation – not something you’d want to do with a recording.

Rolling around

There’s one more factor to consider before looking at EQ types and that’s the roll-off curve or slope. This essentially describes how ‘strong’ or severe a filter is. The steeper the slope, the more severe it is and the greater the filtering effect as you move away from the cutoff point.

The roll-off is usually measured in dB and the distance from the cutoff point is measured in octaves. A gentle filter, therefore, might have a roll-off of 6dB per octave which means that for every octave away from the cutoff point, the signal is attenuated by 6dB. A filter with a 12dB/octave roll-off would be twice as strong and attenuate the signal by 12dB every octave.

Poles apart

EQ - The steeper a filter's slope or roll-off curve, the more quickly it attenuates the frequencies further away from the cutoff point. The steepest slope here is 24dB/octave.In analogue synthesisers there are components within the filter circuits called poles that apply an attenuation of around 6dB per octave.

Some synthesisers use several poles and you may hear references to a 1-pole, 2-pole, 3-pole or 4-pole filter. These would have a roll-off of 6dB/octave, 12dB/octave, 18dB/octave and 24dB/octave respectively. The more poles a filter has, the steeper the roll-off and the fewer frequencies will pass beyond the cutoff point. The famous Moog synthesisers were among the first to use 4-pole filters which contributed to their ‘fat’ sound.

Not all synths tell you how many poles their filters have (or, in the case of digital synths and filters, how many poles they emulate) or let you change them which ought to be easy in software. Lacking any precise information, you might assume a typical filter roll-off of 12dB/octave.

EQ type

There are a handful of common types of EQ and filters:

  • Fixed EQ. This is typically a single control such as Treble or Bass. A mixer might have Low (bass) and High (treble) EQ controls. The cutoff frequency here is fixed and you control the amount of cut or boost.
  • Graphic EQ. This divides the frequency range into a series of bands that you can cut and boost individually. They are easily recognised  A software graphic EQ with ten bands, each twice the frequency (and, therefore, one octave up) from the previous band.by a row of sliders, each controlling a specific frequency band. They are common on hi-fi systems, as stand-alone studio devices and can be found in software. Although you can technically divide the spectrum into any number of bands, typical divisions on hardware units are 15 and 31 covering two-thirds and one-third of an octave range respectively. Software developers are a law unto themselves and any number goes.
  • A highly-graphic parametric EQ with three adjustable frequency bands, gain controls and adjustable Q (bandwidth).Parametric EQ. Whereas with a graphic EQ the frequency bands are fixed, parametric EQ lets you change the frequency of the bands. Parametric EQs typically have three or four bands, each with three controls – cutoff frequency, bandwidth and level, allowing you to home in on specific frequencies within a very tight range.
  • Paragraphic EQ. This is a relatively recent development born of the software revolution and it blurs the line between graphic and parametric. It typically offers several bands like a graphic but with user-definable frequency bands like a parametric; it’s a sort of super-parametric. However, as this can become quite complex, they also tend to have highly-graphic interfaces so the user can see exactly what frequencies are being affected and by how much.
  • Sweeping/Semi-parametric EQ. This is a half-way house between a fixed EQ and a full parametric EQ – while the bandwidth is fixed, you can control the centre frequency (the cutoff point). It’s common to many mixers. Typically you may find fixed Low and High EQ with a sweepable mid range control.

Filter tips

Somewhere within the gamut of filters hovering between paras and graphics, there reside several popular filter types. They are easy to understand as they perform only one filtering function. The first four are most often, but not exclusively, found in synthesis while the shelving filters are more commonly used in recording.

  • Low Pass. This passes frequencies below the cutoff frequency and attenuates the higher ones. It is the most natural-sounding filter as higher frequencies are typically the first to be lost in natural environments.EQ - A low pass filter passes frequencies below the cutoff point and attenuates the ones above it.
  • High Pass. This is the opposite of the low pass filter – it passes frequencies above the cutoff frequency and attenuates lower ones. It is useful for removing unwanted bass frequencies and heavy use can remove the fundamental of a tone resulting in a very thin sound.EQ - A high pass filter passes the frequencies above the cutoff point and attenuates the ones below it.
  • Band Pass/Peak/Bell. As its name suggests, this passes a band of frequencies around the cutoff frequency and attenuates those either side. It’s highly selective and particularly useful for homing in on specific frequencies such as tones produced by solo instruments or for tackling problem areas in a mix such as hum and noise. It is, in effect, a parametric EQ.EQ - A band pass filter passes frequencies either side of the cutoff point and attenuates the others.
  • Band Reject/Notch. This is the opposite of the band pass filter. It attenuates frequencies either side of the cutoff frequency and passes the others, effectively notching out a frequency band.EQ - A band reject filter rejects or attenuates the frequencies either side of the cutoff point while passing the others, allowing you to home in on and remove troublesome frequency spots.
  • High Shelf. This usually boosts frequencies above the cutoff frequency. It’s used as a tone control to shape the upper section of the frequency spectrum.
  • Low Shelf. This is the opposite of the high shelf filter and boosts frequencies below the cutoff frequency.

On the shelf

The terms high shelf and high pass are often used synonymously, as are low shelf and low pass. Technically, however, a high shelf filter boosts frequencies above the cutoff point whereas a high pass filter simply passes them and attenuates the frequencies below. The nett result may appear to be the same but the resulting frequencies within the filtered sound will not be the same.

Tech terms
Frequency response
The ear is more sensitive to some frequencies than other and this is also dependent upon loudness. Lower frequencies need to be louder than higher ones for us to perceive them as being the same volume.

Growl and wha
If you apply a LFO (see Oscillators) to a filter it will vary the tone colour of the sound resulting in an effect called Growl. Controlled variations of the tone can produce a wha effect.

Generally the lowest (technically the first harmonic) and strongest frequency in a sound and the one that gives the sound its pitch.

Having said that, modern shelving filters, particularly in software, can cut as well as boost so cutting with a high shelf filter is effectively the same as using a low pass filter (got that!). It’s just one of the ways in which modern technology blurs the lines.

Passive and active filtering

One final filter thing – you may occasionally hear some filter devices called active or passive. Passive simply means that they can only attenuate frequencies, not boost them, much like low and high pass filters. You’ll find typical passive filters in guitar tone controls and the like.

There are so many EQ variants that it’s important to know which ones do what. Armed with this information, you will be better equipped to use them in recording and synthesis.

For more info…

Audio software and software synthesisers all use EQ and filters so check their documentation for relevant information. Most books on recording contain a chapter on EQ, too.