A Level Music Technology

contact me

11 EQ and filtering

This page is an overview of the content for this topic; enough for revision or to get you started. For more in depth explanation and diagrams, you can purchase the full course here. The small fee for the full courses supports me keeping the website running.

EQ increases and / or decreases the volume of different frequency ranges; an EQ unit normally gives a combination of various frequency changes and / or filters that can be used by themselves or together. EQ can be used creatively or correctively; examples of corrective EQ are to remove a ring on the snare drum, an unwanted resonance in a room, bass guitar spill on a vocal track through the wall or rumble / hum. Whilst EQ units can often make many changes to a number of different frequency ranges at once, a filter is responsible for one specific change; it lets some signal through but cuts the level of a specific frequency range.

EQ and filter parameters

  • The cutoff frequency determines which frequencies will pass through the filter unaffected and which frequencies will be removed; on a LPF this will be frequencies above the cutoff, and below on an HPF
  • We measure the cutoff frequency where there has been 3db of gain reduction
  • The centre frequency is the frequency around which a band pass filter allows frequencies to pass through or a band EQ boosts / cuts
  • Resonance is the characteristic narrow boost of frequencies around the cut off; this makes the sound ‘whistly’ or if over-used, introduces self-oscillation, where the boost is so narrow, a sine wave is created at that frequency
  • Resonance, along with bandwidth, is known as ‘Q’; a high value means that the filter is narrow, low = wide
  • The slope of the filter determines how sharply a pass filter will act at its cutoff frequency, measured in dB / Oct
  • Filters are sometimes labelled with two letters and a number (e.g. LP12 or LP24); the LP tells us the filter type (in this case low pass), and the number tells us how severe the filters slope will be.

Types of EQ and filter

  • Low pass filters reduce the level of all frequencies above the cutoff frequency, but let the frequencies below this value pass by unaltered; they can be used to reduce hiss on a recording
  • A high pass filter lets the frequencies above the cutoff frequency pass unaffected but cuts those below; they are used as rumble filters, where the cutoff is set between 80 and 150Hz to eliminate unwanted low frequencies
  • A band pass filter is essentially a combination of a LPF and an HPF; they are used in wah wah pedals, where the centre frequency is swept to give the characteristic effect – they can also be used to give a ‘telephone effect’
  • Band EQ is also known as parametric EQ because you can alter the gain (boost / cut), centre frequency & Q
  • A semi-parametric EQ has a fixed Q and thus you can only cut or boost around a variable centre frequency
  • Shelving EQ cuts or boosts the signal beyond a specific cutoff frequency; this cut or boost is then evenly applied until the end of the audio spectrum, leaving the rest of the signal unaffected – their graphs look a bit like shelves
  • A low shelf boost can be used to increase the low frequencies in a kick drum or bass guitar part
  • A high shelf boost can bring something forward in the mix or to add clarity to drum overheads.

The impact of EQ and filtering

  • EQ was initially corrective to compensate for mic issues but their usage gradually became more creative
  • Frequency response tells our brain about proximity – brighter things seem closer and muffled further away
  • In the 1950s, engineers were able to use EQ for greater creative control over a recording’s frequency content
  • In the 1960s, use of equalisation to enhance low frequencies became more common with use of the electric bass
  • The use of consoles by SSL and Neve in the 70s with built-in EQ allowed for greater control over EQ
  • Filtering was used on synthesisers such as the Moog to create timbres and manipulate frequency in real time
  • Early examples of filtering in studios involved tapping into / ‘misusing’ the filter section of an analogue synth
  • While rarely used as a studio effect at the time, guitarists were using filters in the 1970s in wah wah pedals
  • When recordings are made digitally, this improves the HF response – EQ can be used to increase brightness on the mix or on individual instruments without having a negative effect on the amount of hiss
  • In the 1990s, musicians embraced the TB-303, using it to create filtered resonant basslines in acid house
  • DJs use filters to add build / breakdown on the fly; in the 70s using turntables, 00s with CDs and now on laptops
  • CDs meant that listeners could experience much improved high frequency playback in the 1990s
  • The DAW and EQ plugins made editing more convenient; the tendency to ‘over-do’ EQ can result in restricted or over-emphasised parts and the abundance / simplicity of EQ plugins can prompt a lazy ‘fix it in the mix’ attitude
  • Modern masters have accentuated low-end and a bright and sculpted high frequency response; engineers often compare the EQ / frequency response of their final master with a reference track
  • Plugins such as auto-filters and dynamic EQ / resonance suppressors have added to the tools available.