What Is Semi-Parametric Equalization/EQ In Audio?

Equalization is easily one of the most important audio processes/effects. Among the different types of EQ, there are a significant number of semi-parametric EQ units to choose from.

In this article, we'll discuss semi-parametric EQ in detail, comparing it to [fully] parametric EQ and taking a look at some different examples along with how to use semi-parametric EQ to the best of its capabilities.

A Short Primer On EQ

Before we get to semi-parametric EQ in greater detail (skip ahead by clicking here), let’s quickly define EQ in more general terms.

EQ or equalization is a process that balances the frequency content of a signal by adjusting the relative amplitude between frequency bands within an audio signal. Equalization works by increasing or decreasing the amplitude of some defined bands relative to other bands.

The range of frequencies affected by a certain filter of an EQ is typically referred to as a “band”.

EQ cutting and boosting refer to the decreasing and increasing of the relative amplitude of defined frequency bands, respectively.

Filters are typically thought of as processes that eliminate frequency content below, above or between two set frequency points. However, in the context of EQ, a filter can also be used to define the aforementioned boosting and cutting.

EQ is used in mixing, tone shaping, crossovers, feedback control and more. It is one of the most important tools for working with audio.

What Is Semi-Parametric Equalization?

Semi-parametric EQs are essentially parametric EQs with fewer options. Most often, this missing functionality means no Q control. However, other parameters may also be lacking, and it's some “semi-parametric” EQs will have Q controls.

Parametric EQs ideally have continuous control (within defined ranges) over all the important parameters of equalization. We'll get to these parameters shortly.

Semi-parametric EQs, then, will have some but not all of these controls. They generally are missing one or more key parameter controls.

That being said, the adjustable parameters that semi-parametric EQ units do offer make them go-to choices for more involved EQ applications, including, but not limited to, tone-shaping and surgical EQ.

The parameters of a semi-parametric EQ differ depending on the filter type (which is often selectable). The number of bands in a semi-parametric EQ is often less than in fully parametric EQs but will typically be between 3 and 7. Plugins tend to have many more bands than this.

Semi-parametric EQs can be designed to processes mono, stereo, mid-side and other multi-channel audio formats.

In the following section, we'll consider the potential semi-parametric EQ parameters.

Potential Semi-Parametric EQ Parameters

Semi-parametric EQs will have some, but certainly not all, of the following parameters. Note that there are no hard-set rules here, and a semi-parametric EQ can essentially be designed with any combination of the following:

• Centre frequency (of mid bands)
• Gain (boost/cut)
• Quality factor (Q)
• Cutoff frequency (of low-pass and high-pass filters)
• Slope (of the low-pass and high-pass filters)
• Filter selection (per band)

Centre Frequency

The centre frequency parameter will allow users to continuously sweep the centre frequency value of a bell/peak or notch filter in the semi-parametric EQ.

Gain (Boost/Cut)

The gain control parameter, measured in decibels (dB), refers to the relative amplitude (boost or cut) applied to a specific band of a semi-parametric EQ.

Applying negative gain will cause an EQ cut, while positive gain will cause an EQ boost.

When dealing with bell/peak filters, the amount of gain will be realized at the centre frequency of the band in question. Adjacent frequencies will be affected by a lesser degree within the band’s bandwidth/Q.

When dealing with shelving filters, the amount of gain will be realized below a cutoff point (in a low shelf filter) or above a certain cutoff point (in a high shelf filter). There will be some sort of transition period with a slope around the defined cutoff frequency.

Note that low-pass, band-pass, high-pass, and band-stop/notch filters will not have gain controls as they’re designed to eliminate frequencies above, above and below, below, and between cutoff frequencies, respectively.

Quality Factor (Q)

The quality factor (known simply as “Q”) is a dimensionless value equal to the ratio of the centre/resonant frequency to the filter's bandwidth.

Q=\frac{f_C}{BW}=\sqrt{\frac{f_Hf_L}{f_H-f_L}}

where:
• f_C: centre/resonant frequency
• f_H: high cutoff frequency (where the signal drops off by 3 dB)
• f_L: low cutoff frequency (where the signal drops off by 3 dB)
• BW: bandwidth

Put another way, Q is a measurement of how narrow/steep or wide/gentle the boost or cut of the given filter/band is. Higher Q values produce a narrower band in which a smaller range of frequencies is affected.

With boosts and cuts below 3 dB, the bandwidth may be registered between the points where the boost or cut is half its maximum (at the centre frequency). In this case, we have “half-gain Q” or “Robert Bristow-Johnson’s (RBJ) Q”.

The Q of semi-parametric EQs (when available, which is rare) will often be technically incorrect in the true definition of Q. What remains the same, regardless of calculation, is that by increasing the Q, we narrow the bandwidth (whatever it happens to be). By decreasing the Q, we widen the bandwidth of the filter.

Cutoff Frequency

The cutoff frequency parameter will allow users to continuously sweep the centre frequency value of a high-pass, low-pass, shelving filter or perhaps even a band-pass filter in the semi-parametric EQ.

The cutoff frequency of a filter is defined as the -3 dB attenuation point.

Here’s an illustration of a band-pass filter with both low (f_L) and high cutoff frequencies (f_H) marked to get a better idea:

Slope

Some equalizer bands will have slope controls in order to adjust the roll-off rate of their filters. The slope is largely determined by the order of the filter (the number of reactive components in the circuit), though digital EQ and EQ plugins can achieve roll-off rates/slopes via other methods.

Below is a table of filter orders (from 1st to 5th) and their respective slopes:

Filter Selection

Some bands in a semi-parametric EQ will have options when it comes to choosing the filter type.

A semi-parametric EQ’s filter options may include:

• Low-pass filter
• Low shelf filter
• Bell curve filter
• Notch/band-stop filter
• High-pass filter
• High shelf filter
• Band-Pass filter

Low-Pass Filter

What is a low-pass filter in audio? A low-pass filter (LPF) “passes” the low-frequencies below their cutoff frequency while progressively attenuating frequencies above their cutoff. In other words, low-pass filters remove high-frequency content from an audio signal above a defined cut-off point.

Low Shelf Filter

What is a low-shelf filter in audio? A low shelf filter is a filter that either boosts (increases amplitude) or cuts (decreases amplitude) frequencies below a certain cutoff frequency. These filters generally have a well-defined transition band and a levelling-off of amplitude in the lower end.

Bell Curve Filter

What is a bell curve filter in audio? A bell curve filter is a filter capable of producing resonance (boost in EQ) or anti-resonance (cut in EQ) around a specified centre frequency. These filters are defined by a central frequency, Q factor (width of the boost/cut) and relative gain.

Notch/Band-Stop Filter

What is a band-reject filter in audio? A band-stop filter (aka band-reject or notch filter) works by removing frequencies in a specified band within the overall frequency spectrum. It allows frequencies below the low cutoff frequency to pass along with frequencies above the high cutoff frequency.

High-Pass Filter

What is a high-pass filter in audio? A high-pass filter (HPF) “passes” the high-frequencies above their cutoff frequency while progressively attenuating frequencies below the cut-off frequency. In other words, high-pass filters remove low-frequency content from an audio signal below a defined cut-off point.

High Shelf Filter

What is a high-shelf filter in audio? A high shelf filter is a filter that either boosts (increases amplitude) or cuts (decreases amplitude) frequencies above a certain cutoff frequency. These filters generally have a well-defined transition band and a levelling-off of amplitude in the upper end.

Band-Pass Filter

What is a band-pass filter in audio? A band-pass filter “passes” a band of frequencies (a defined range above a low cutoff and below a high cutoff) while progressively attenuating frequencies below the low cutoff and above the high cutoff.

Using A Semi-Parametric EQ

Semi-parametric EQs are powerful audio processors and are often a bit more affordable than their fully parametric counterparts.

The continuous control of certain parameters makes semi-parametric equalization versatile and flexible. It can be used in a wide variety of EQ applications.

These applications include, but are not limited to, the following:

Tone Shaping With Semi-Parametric EQ

Shaping the tone/character of an audio signal/track by EQing it in a specific way.

Eliminating Problem Frequencies With Semi-Parametric EQ

Noise, resonances and other nasty frequencies can be sought and destroyed with the continuous frequency controls of the parametric EQ.

Feedback Elimination With Semi-Parametric EQ

Similar to problem frequencies. Can be used effectively in live sound situations.

Tuning Monitors With Semi-Parametric EQ

Monitors and speakers are imperfect electro-acoustic transducers that may benefit from adjustable parametric EQ.

Examples Of Semi-Parametric Equalizers

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 5 different semi-parametric equalizers to help solidify our understanding of this EQ type.

In this section, we’ll discuss:

• 500 Series semi-parametric EQ unit: Fredenstein Artistic EQ
• 19″ rack mount semi-parametric EQ unit: Maag Audio EQ4M
• Semi-parametric EQ effect pedal: Aguilar Tone Hammer Bass
• Semi-parametric EQ Eurorack module: ALM Busy Circuits ALM016 – PE-1
• Semi-parametric EQ plugin: Waves V-EQ4

Fredenstein Artistic EQ

The Fredenstein Artistic EQ is a single-channel 3-band semi-parametric EQ in the 500 Series format.

This EQ is relatively straightforward. There are 3 bands (low, mid and high). Each band has a knob to select the centre or cutoff frequency within a defined range and a gain knob to dial in the EQ cut or boost within ±14 dB.

The low and high bands have a toggle switch to change between shelving and bell-type filters, and the unit has a Normal-Color switch that, when set to Color, imparts a vintage-style tubelike flavouring to the output signal.

Because the bandwidth/Q is not adjustable, this unit would be considered “semi-parametric”.

Maag Audio EQ4M

The Maag Audio EQ4M is a rack-mountable dual-channel 6-band mastering EQ.

This superb unit acts largely as a graphic EQ with 5 fixed bands at SUB, 40 Hz, 160 Hz, 650 Hz and 2.5 kHz. Each of these bands can be boosted/cut by ±5 dB.

The highest band, labelled the “Air Band,” is a semi-parametric high shelf boost filter. It offers up to +20 dB of clean gain at a selectable corner frequency of 2.5, 5, 10, 15, 20 or 40 kHz.

This mastering EQ barely makes the cut as a “semi-parametric” but is worth mentioning here as the Air Band makes it so.

Aguilar Tone Hammer Bass

The Aguilar Tone Hammer Bass is a great semi-parametric EQ (and preamp/DI) pedal designed specifically for bass guitar.

This pedal is well laid out with 3 bands of EQ. The EQ portion is controlled via the 4 central knobs:

• Treble: ±18 dB @ 4 kHz
• Bass: ±18 dB @ 40 Hz
• Mid Freq: sweepable from 180 Hz to 1 kHz
• Mid: ±17 dB

Note that the pedal is fixed in the low and high-frequency bands and that the Q is not adjustable in any bands. However, because the mid-band frequency can be swept, this EQ pedal could be considered “semi-parametric”.

The AGS (Adaptive Gain Shaping) circuitry, when engaged, produces a “vintage-voiced” EQ that adapts as you adjust the gain knob.

ALM Busy Circuits ALM016 PE-1

The ALM Busy Circuits ALM016 – PE-1 is described as a dual-band parametric EQ and simple two-channel mixer for Eurorack modular synths. That being said, the design is actually more “semi-parametric”.

This module has two inputs that are both affected by the EQ equally. There is a single attenuation control for the upper input signal.

As for the EQ, each band has a cut/boost control of ±12 dB. One band's centre frequency can be set between 62 and 1.5k Hertz, while the other ranges between 1k and 8k Hertz.

There's no option for Q adjustments, so it could be argued that this EQ is only “semi-parametric”.

Waves V-EQ4

The Waves V-EQ4 is modelled after the 1081 console module. It's a 4-band EQ plugin with selectable low-pass and high-pass filters.

In addition to the LPF and HPF, which each have adjustable cutoff frequencies and a roll-off of -18 dB/octave, this EQ has 4 bands.

The low and high bands can be set to either bell or shelving filters and have discrete centre/cutoff frequency points along with continuous gain control between ±18 dB.

The mid bands (LMF and HMF) are bell filters with discrete centre frequency points and continuous gain control between ±18 dB. These bands also have a switch between “normal” Q and Hi-Q settings (wide and narrow Q).

So this EQ plugin is pretty parametric. However, because the frequency points are not fully continuous nor are the Q values, it could be considered “semi-parametric”.

Related Questions

What are the different types of EQ? When it comes to audio equalization, there are several types of EQ to be aware of. They are as follows:

• Graphic EQ
• Parametric EQ
• Semi-Parametric EQ
• Dynamic EQ
• Linear Phase EQ
• Passive EQ
• Shelving EQ
• Stereo EQ
• Mid-Side EQ

What is shelving EQ? Shelving EQ utilizes high and/or low shelf filters to affect all frequencies above or below a certain cutoff frequency, respectively. Shelving can be used to either boost/amplify or cut/attenuate and, ideally, affects all frequencies equally beyond (below for low shelf and above for high shelf) a certain point.