Equalizers

Did you ever try to record yourself with just a microphone and recording device, only to find that no matter how good a musician you are, and how good the microphone is, the quality just isn’t anywhere near that of a studio recorded song? Recording studios have equipment that is designed specifically for getting great sounding recordings. Some of this equipment is too expensive for the average musician to purchase. 

 

There are several types of processing devices to enhance recordings. Among those devices are equalizers. They are designed to equalize the balance of frequencies in your recording. They are filters that allow you to boost or cut a select range of frequencies. Equalizers are used both in the studio and on live performances. However in live performances they have an additional function….to minimize or eliminate feedback.

 

 Equalizers come in many different types. There are standard equalizers on your radio. They are called bass and treble controls. Many of the mixing boards used in live performances have an additional control called mid range. There are graphic equalizers with as many as 62 different frequency controls. Parametric equalizers will allow you to select a center frequency and widen or narrow the bandwidth to affect neighboring frequencies. Notch filters are used in live sound reinforcement rather than in the studio

 

because they target feedback.

 

Standard Bass & Treble

The human ear can only hear frequencies as low as 20 Hz (hertz, which means vibrations per second), and as high as 20 kHz (kilohertz, or thousand vibrations per second). Standard bass and treble controls usually divide the frequency range. The bass control will then boost or cut all of the frequencies from 20 Hz to about 500 Hz (this will vary with different equipment.) The treble control then boosts or cuts everything from 500 Hz to 20 kHz.

 

Mixing boards

Some mixing boards will have mid range controls as well as bass and treble. Some will even have two mid range controls. One mid range control will boost or cut the entire mid range of frequencies, which are generally (but not always) 250 Hz to 7 kHz. Two mid range controls divide the mid frequencies into low mids and upper mids. Generally, the low mids are from 250 Hz to around 800 Hz and the upper mids are 800 Hz to 7 kHz.

 

 Mixers with a low, mid and high control are called 3 band equalizers because you are dealing with 3 different bands or ranges of frequencies. If they have two mid controls, they are called “4 band”.

 

 Graphic equalizers

Graphic equalizers divide the frequency range into 5 or more bands. These bands are equal divisions of the entire frequency spectrum. Each band is controlled by a slider instead of a dial for boost and cut. The frequency listed below the slider is the “center” frequency. When this frequency is boosted or cut, the neighboring frequencies on both sides of it are affected. For instance, if you have a 2 octave graphic equalizer, you may have these three neighboring frequencies: 500 Hz, 2 Khz, 8 Khz. If you boost or cut 500 Hz, you will also affect 250 Hz (1 octave below 500 Hz), and 1 Khz (1 octave above 500). If you boost or cut 2 Khz, which is 5 octaves above 500 Hz, you will also affect 1 Khz (1 octave below 2 Khz) and 4 Khz (One octave above 2 Khz). Boosting or cutting the 8 Khz will also affect 4 Khz (one octave below 8 Khz) and 16 Khz (One octave above 8 Khz).

 

 5 band graphic equalizers are generally 2 octave type and usually have the following center frequencies: 40, 160, 640, 25.6 Khz, 10.2 Khz. This type of equalizer is found mostly on high performance car stereos and some of the more expensive acoustic-electric guitars.

 

 Some of the inexpensive mixing boards usually have 10 band graphic equalizers at their master output section in addition to their 3 or 4 band individual channel equalizers. The 10 band divides the audio spectrum into 10 equal octaves. These are designed to help to control feedback, but don’t do a very good job of it, because each feedback frequency is only one frequency. When you boost or cut a frequency on a 10 band, you are affecting an entire octave of the music and it will change the sound of what is coming out of the mixing board. The 10 band is better suited for adjusting tuning the speaker system to the room.

 

 The 31 band (one third octave) equalizer is a little better suited for controlling feedback because it affects less of the music frequencies. However, it is better suited for tuning a speaker system to a room and will give you a more accurate tuning.

 

 The 62 band graphic equalizer is more commonly used for controlling feedback and for tuning speaker systems to rooms. Because of it’s narrow bandwidth, pulling out a feedback frequency will only affect 2 scale notes on an instrument. For instance, if you have a feedback frequency at 110 Hz and cut that frequency to stop the feedback, the only amount of your music that will be affected will be a low A to B notes on the instrument.

 

 Parametric Equalizers

The word “Parametric” is taken from the word “Parameter”. A Parametric equalizer is an equalizer with several parameters. Most parametric equalizers have only 4 bands. However, their parameters give them more flexibility than other types of equalizers. These parameters are: Sweep, Bandwidth, Boost and cut.

 

 

Sweep

A frequency sweep is a dial that allows you to select the center frequency. A 4 band parametric will be divided into your 4 basic frequency ranges, but the sweep control will allow you to select the desired center frequency within that range. Most of the bands overlap, so you can sweep the same frequency from more than one band. 

 

Bandwidth

The bandwidth is how wide or narrow you want your frequency band to be. For instance, you may want to select a 2 octave bandwidth for controlling bass or treble output or you may want to select a 12^th octave or narrower to control one note that stands out too loud or is too soft. You may want to narrow the bandwidth down to as much as a 20^th of an octave to control a feedback frequency without affecting any of the music at all. This feature makes the parametric the most flexible of equalizers and ideal for controlling feedback as well as other studio applications.

 

Boost and cut

These are the same boost and cut controls that you have on most equalizers. Parametric equalizers usually give you anywhere from 12 to 18 db (decibels…units of measure in loudness) boost or cut.

 

 Notch Filters

A notch filter is designed specifically for controlling feedback, and has very narrow bandwidths that can pull out a feedback frequency without affecting any of the sound of the music. Notch filters are limited to cut only and have no frequency boost capability. Some of the newer notch filters have a feedback detection device that locates the feedback frequency and then automatically attenuates it (cuts it). 

 

Other filters

Every equalizer has filters in it that are responsible for their frequency selection and boost/cut capabilities. Different types of equalizers have different types of filters: Peak, shelf, Lo-pass, hi-pass, and band pass. 

 

Peak

A peak filter is one that selects a center frequency. When you boost or cut that frequency, the center frequency is boosted or cut the most and neighboring frequencies are affected somewhat. The following example shows a center frequency boost of 1 Khz. Notice how the neighboring frequencies of 500 Hz and 2 Khz are also boosted, but not by as much as the center frequency.

 

Peak filters are common in the mid range of mixer input equalizers, all graphic equalizers, parametric and notch filters (notch filters will be cut instead of boost or cut). 

Shelving Filters

Shelving filters do not use a center frequency. They use a starting frequency and boost everything from that frequency. For instance, a lo-shelf filter with a selected frequency of 500 Hz will boost or cut all of the frequencies from 500 and below, in equal amounts. A high shelf filter at 500 Hz will boost or cut everything from 500 and above in equal amounts. 

 

The following example shows a low shelf filter with a 12 db boost at 500 Hz:

 

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Most mixing boards’ channel input equalizers have peak filters for the mids and shelf filters for the highs and lows. 

 

Lo-pass filter

The lo-pass filter gets it name because it allows only the low frequencies below the selected frequency to pass through, while the frequencies above the selected frequency are cut. Some engineers refer to this type of filter as a “High Cut Filter”. The amount of cut is designated in “db per octave”, selectable in increments of 3, 6, 12, 18 and 24 db. The selected frequency is the point at which the first db increment cut occurs. For instance, if you select a 500 Hz lo-pass filter with a 6 db per octave slope, 500 Hz would be cut by 6 db. 1 Khz would be cut by 12 db. 2 Khz would be cut by 18 db. 4 Khz would be cut by 24 db, etc.

 

Hi-pass filter

The hi-pass filter gets its name because it only allows highs to pass above the selected frequency and cuts everything below the selected frequency. It is sometime called a “Lo-cut” filter. The hi-pass filter is the opposite of the lo-pass filter.

The bass roll off switch found on one of the more expensive microphones is a good example of a Hi-pass filter. You may hear this referred to as a “bass roll off”, “Lo-cut” or “hi-pass” filter. 

 

Band Pass Filter

The band pass filter passes a selected band of frequencies with a center frequency. In some cases, the bandwidth is fixed and in other cases, it is selectable. A center frequency is then selected and the bandwidth with that center frequency are unaffected while everything above and below the range of the bandwidth are cut by the selected db per octave as in the Hi and Lo pass filters. 

 

For example, if you select a center frequency of 500 Hz, with a 2 octave bandwidth, and a 12 db per octave slope, you will pass frequencies from 250 Hz to 1 Khz. 250 Hz is one octave below 500, and 1 Khz is one octave above 500, giving you’re 2 octave bandwidth. On the low side, 125 Hz (one octave below 250) will be cut by 12 db and 62.5 Hz (one octave below 125) will be cut by 24 db. On the high side, 2 Khz (one octave above 1 Khz) will be cut by 12 db and 4 Khz (1 octave above 2 Khz) will be cut by 24 db, as shown in the following example:

 

 

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