Filters & Equalisers

For an audio engineer or producer, filters or equalisers (EQs) offer a very powerful tool for shaping the tone and feel of a sound, similar to the tone controls on your stereo. Unlike the tone controls on your stereo, however, filters and equalisers offer you much greater control over the effect the filter  or equaliser has on the sound. To help you get your head around the different concepts presented by filters and equalisers, we have prepared the following guide.

To make things easier to understand, a filter is a single device for affecting a particular band of frequencies in a certain way (depending on the type of filter), while an equaliser is more like a set of two or more filters in one device. We will start by looking at filters individually, so that you can see the different types of filters available.

• Low-pass Filter (LPF): A low-pass filter is designed to remove any frequencies above the cut-off frequency, while allowing frequencies below the cut-off frequency to pass through unaffected. Most low-pass filters also contain a resonance control that, when turned up, accentuates the frequencies around the selected cut-off frequency.
• High-pass Filter (HPF): A high-pass filter is designed to remove any frequencies below the cut-off frequency, while allowing frequencies above the cut-off frequency to pass through unaffected. Most high-pass filters also contain a resonance control that, when turned up, accentuates the frequencies around the selected cut-off frequency.
• Low-shelf Filter: Similar to the bass control found on your stereo, a low shelf filter allows you to either boost, or cut all frequencies that fall below the selected cut-off frequency, while leaving frequencies above the cut-off unaffected. Unlike your stereo's bass control, however, a low-shelf filter typically allows you to adjust the cut-off frequency of the filter.
• High-shelf Filter: Similar to the treble control found on your stereo, a high shelf filter allows you to either boost, or cut all frequencies above the selected cut-off frequency, while leaving frequencies below the cut-off frequency unaffected. Unlike your stereo's treble control, however, a high-shelf filter generally allows you to adjust the cut-off frequency of the filter.
• Band-pass Filter (BPF): Also band-pass filter is designed to allow all frequencies within a specific band to pass through, while removing any frequencies outside of the band. Most band pass filters allow you to control the band-width, or range of frequencies that are allowed to pass through.
• Band-Reject or Notch Filter: The Notch Filter is designed to remove all frequencies within a specific band, while allowing frequencies outside the band to pass through. Most band reject, or notch filters allow you to control the band-width (Q), or range of frequencies that are removed.
• Parametric Filter: A parametric filter is similar to both a band-pass and band-reject filter, in that a parametric filter is able to either boost, or cut frequencies within a specific range. Most parametric filters also allow you to adjust the band-width (Q), or range of frequencies that are affected, as well as allowing you to adjust the amount of boost or cut, and which frequency the band is centred around.
• Multi-mode Filter: A multi-mode filter is more of a combination of filters, rather than a single one. A multi-mode filter typically allows you to select which type of filter to use (LPF, HPF, High or low shelf, etc), although only one type of filter can generally be used at any one time.

It must also be noted that not all filters of the same type sound the same. For example different filters use different roll-off slopes, which determines how the surrounding frequencies are affected, as well as different filters using different methods to actually process the audio, meaning that some filters can sound clean and transparent, while others may colour the sound or have a gritty or slightly distorted feel. It is up to you which filter you use for any particular task, depending on what sort of sound you are after. Some of the controls that you typically find on filters are:

• Frequency or Cut-off Frequency: This control allows you to adjust the centre frequency of the filter, and is commonly found on most filter types.
• Gain: Alternatively known as boost or cut, the gain allows you to adjust how much the affected frequencies are turned up (boosted) or down (cut). Gain controls are typically not found on high-pass or low-pass filters.
• Band-width or Q: This control allows you to adjust the width of the filter, or the range of frequencies that it will affect. Band-width controls are typically only found on band-pass, notch or parametric filters.
• Resonance: The resonance control allows you to adjust how much the frequencies around the cut-off frequency are accentuated. This control is typically only found on high-pass, low-pass, and shelf type filters.

As you can see, there are many different types of filters available, each with different traits that affect how the resulting audio sounds. As mentioned, equalisers are generally a combination of two or more filters. Some of the most common types of equalisers are:

• Parametric Equaliser: A parametric equaliser typically contains two or more bands of parametric equalisation. Unlike a standard parametric filter, however, some or all of the bands on a typical parametric equaliser can be switched between different filter types. For example this can allow you to apply high-pass filtering, low-pass filtering, shelving, and band-pass or notch filtering all with a single equaliser. Parametric equalisers tend to be extremely powerful tone shaping tools, and are one of the main tools in any engineer's arsenal.
• Graphic Equaliser: Graphic equalisers are generally a combination of band-pass/band-reject filters, each with a fairly narrow bandwidth and a fixed centre frequency. Graphic equalisers tend to include between three to thirty one separate bands of equalisation, although some offer even more. Graphic equalisers are most commonly used in live sound situations, where they are used to help shape the sound to suit the acoustics of the listening environment, as well as other tasks such as feedback reduction. Standard analogue graphic equalisers  are not typically found in a studio environment as they tend to impart more noise on the processed sound than a parametric equaliser.

It must be noted that typically you will get better results by cutting the frequencies that you don't want or need, rather than boosting the ones that you do want. This allows more space in the mix for other instruments or sounds to fit in, as well as helping to make the overall sound cleaner.