About Filters and EQ

Filters and EQ

A filter is a device that attenuates or removes a user-defined range of frequencies from an audio waveform, while passing other frequencies. Typical filters are low pass, high pass, and band pass.

An equalizer (EQ) is a type of filter that corrects for losses in the transmission of audio signals, making the output equal to the input, or making an otherwise inconsistent frequency response "flat," giving all frequencies equal energy.

See the Filter Operation section for more information.

Parametric EQ

A parametric EQ is a variable equalizer that offers control of all parameters, including amplitude (boost or cut), center frequency (frequency), and bandwidth (Q). This allows you to control the amplitude of each band, shift the center of frequency, and widen or narrow the affected area.

High pass filter

A high pass filter passes a band of frequencies extending from a user-defined cutoff frequency (greater than zero) up toward the high end of the frequency spectrum. All frequencies above the specified cutoff frequency are allowed to pass, attenuating all frequencies below.

Low pass filter

A low pass filter passes a band of frequencies extending from a user-defined cutoff frequency (less than infinite) down toward the low end of the frequency spectrum. All frequencies below the specified cutoff frequency are allowed to pass, attenuating all frequencies above.

Band pass filter

A band pass filter is a combination of high pass and low pass filters, allowing the band of frequencies between the low and high cutoff points to pass.

Bass and treble tone filters

A tone filter, also known as a shelving filter, gives the ability to cut or boost gain evenly above or below a given frequency. The end-band shape gives the visual appearance of a shelf. Tone filters are generally applied to program material and are expressed as bass and treble control.

Filter Defaults and Ranges

The table below shows each filter type with the relevant parameters and default settings. The second table, immediately following, shows parameters for each filter type and a typical setting range for each parameter.

Type	Frequency	Parameter 1	Parameter 2
Parametric	1000.0 Hz	Boost/Cut: 0.0 dB	Q (frequency divided by bandwidth): 1.000
High pass	100.0 Hz	Slope: 6 dB per Octave	N/A
Low pass	10000.0 Hz	Slope: 6 dB per Octave	N/A
Bass	100.0 Hz	Boost/Cut: 0.0 dB	Slope: 6 dB per Octave
Treble	8000.0 Hz	Boost/Cut: 0.0 dB	Slope: 6 dB per Octave

Filter Parameter	Settings Range
Frequency	20 Hz to 20 kHz
Boost/Cut	-24.0 dB to +24 dB
Q (EQ only)	0.707 to 15.000
Slope (Filters only)	1st Order (6 dB) to 2nd Order (12 dB)

Definitions of the filter parameters are as follows:

Frequency - This is the point in the Frequency spectrum at which a filter or EQ is applied. For parametric EQ, this point is referred to as the center frequency. For all other filters, this is the cutoff frequency (band pass filters contain high and low cutoff frequencies), and cutoff or "corner" frequencies for tone controls. Cutoff frequencies begin at the half-power point or "3 dB down" point.
Boost/Cut - The amount of gain (boost) or gain reduction (cut) that is applied at a given frequency. This applies only to parametric and shelving filters (tone filters).
Slope - In a high pass or low pass filter, this is the amount of attenuation in dB per octave. Slope values are in 6 dB increments, corresponding to the number of circuits in the filter. For example, a single circuit filter (also known as "1st order") will have a 6 dB slope, while a 2nd order filter (two circuits) will have a 12 dB slope. Multiple filters can be added together in series to increase the slope.
Q - The ratio of the center frequency divided by the bandwidth. Q stands for "Quality Factor."