Standard Filter Types
Filters And Their Uses
You may well have encountered visual filters in Photoshop or social media platforms like Instagram. Such filters manipulate the composition of an image removing colors, saturation and shades. They can also increase such content
Just as a water filter, separates unwanted fine particles from a purified result, audio passes through an audio filter and unwanted content is removed.
Just like image filters, but unlike most other types of filter, audio filters can also amplify elements of what is present in the source audio, to produce a new desired audio result.
In reality filters are frequency dependent circuits, with audio filters working within the audio frequency range, 0 Hz to 20 kHz. There are many different filter types.
Audio filters are used to amplify (boost), pass or attenuate (cut) source signals at selected frequency ranges.
Filters are used for many different audio applications. These include instrument amplifiers, effects units, musical synthesizers, recording studios, sound reinforcement, hi-fi systems, computer audio and virtual reality set ups.
In practical terms filters in an EQ are used to treat issues, like enhancing a vocal sound, or treating drum recordings to reduce spill or bringing clarity to a muddy mix by reducing unwanted bass and low mid frequencies and avoiding instruments cancelling each other out.
The filters used in synthesizers are related, though controls and circuitry may differ. Many of the goals remain the same. In this article we will focus on the EQ applications of filters.
Table of Contents
High and Low Pass Filters
High Pass Filters and Low Pass Filters are the most type of audio filter. These filters begin to cut the amplitude of an input signal at a selected threshold frequency or cutoff frequency.
In the case of a high pass filter, all frequencies below a selected frequency will be progressively cut. Similarly, for a low pass filter, the frequencies above a selected threshold frequency will be progressively cut. These filters are sometimes called Cut-off Filters.
High and Low Pass filters commonly have a gentle cut of -6 dB per octave, -12 dB per octave or a more aggressive -18 dB per octave.
The circuits used to implement filters are not ideal. This means that the gain does not instantly change from 0 dB to -18 dB as the frequency crosses the threshold. The gain change is progressive. The quoted cut off frequency is the frequency at which the gain reduction is already 3dB below the input amplitude. This point is also called the 3dB Down Point.
High Pass Filter
Parametric EQ’s generally provide a High Pass Filter (HPF).
An HPF removes frequencies below a frequency set by a frequency control. Frequency is cut from below the set frequency at a steep curve. Frequencies above that frequency remain unaffected. In other words it passes the high frequencies and cuts the low frequencies, hence the two names for this filter.
High Pass Filters are ideal for removing any unwanted low-end frequencies, such as mains hum and other low frequency noise. They are also commonly used to clear space for the bass and kick drum in a mix.
Low Pass Filter
Parametric EQ’s generally also provide a Low Pass Filter (LPF).
An LPF removes frequencies above a frequency set by a frequency control. Frequency is cut from above the set frequency at a steep curve. Frequencies below that frequency remain unaffected. In other words it passes the low frequencies and cuts the high frequencies, hence the two names for this filter.
Low Pass Filters are ideal for removing any unwanted high-end frequencies, such as fan noise, hiss and other high frequency noise. They are also commonly used to clear space for cymbals in a mix.
Bell Filters or Peak Filters
Bell curve filters are also known as peak filters. A bell curve attenuates or boosts frequencies around a specified center frequency. The bandwidth Q control sets the width of the bell curve, or the breadth of the frequencies affected.
Bell curves are very versatile filters. They can be used to boost or reduce a range of frequencies. They can be very accurate.
For example, Set a low Q for a broad curve, and gentle gain able to subtly boost a wide range of musical frequencies. Set a high Q for precise narrow cuts to correct any troublesome signal.
This kind of filter applies an equal level of boost or cut to all frequencies beyond a user defined threshold frequency. Yet again the filter response is not ‘ideal’
High Shelf Filter
A high shelf filter will attenuate or boost all frequencies above a specified frequency, adjusted by the frequency control, to the level adjusted by the gain control. Unlike low pass filters/ high cut filters, high shelf filters don’t cut frequencies out completely. Instead, they reduce or boost high frequencies to a plateau gain level.
Shelf filters make broad tonal changes by boosting or reducing high-end frequencies.
Low Shelf Filter
A low shelf filter will attenuate or boost all frequencies below a specified frequency, adjusted by the frequency control, to the level adjusted by the gain control. Unlike high pass filters/ low cut filters, low shelf filters don’t cut frequencies out completely. Instead, they reduce or boost bass frequencies to a plateau gain level.
Shelf filters make broad tonal changes by boosting or reducing low-end frequencies.
A Band-Pass Filter passes frequencies, in a chosen passband around a specified frequency. Frequencies beyond the band width are not allowed through the filter. Essentially beyond the Q (bypass and) all frequencies are cut (attenuated).
A high-Q filter has a narrow passband while a low-Q filter has a wide passband. A high-Q filter is also known as a narrow-band band-pass filter. A low-Q filter is also known as a wide-band band-pass filter.
A Band-Stop Filter or Band-Rejection Filter, is the opposite of a Band-Pass Filter. Stop-Band Filters cut (attenuate) all frequencies, within a chosen Q (stopband), around a specified frequency.
As with a band-pass passband, a high-Q filter has a narrow stopband while a low-Q filter has a wide passband. A high-Q filter is also known as a narrow-band notch filter. A low-Q filter is also known as a wide-band notch filter.
A Notch Filter is a narrow-band Band-Stop Filter. They are also called a “T-Notch Filter”, “Band Limit Filter”, “Band-Elimination Filter”, and “Band-Reject Filter”.
The stopband is typically 1 to 2 decades wide i.e. the highest frequency cut is 10 to 100 times the lowest frequency cut, when used outside of audio applications. For audio applications, notch filters have high and low frequencies that can be just semitones apart.
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