ADVANCED RECORDING PRIMER

CHAPTER 3 - PART 3,  BY ROBERT DENNIS

SIGNAL PROCESSING WITH EQUALIZATION

Types Of Equalizers:

Equalization is the increase or decrease of signal strength for a portion of (a band of) audio frequencies.  The audio we record (the sound made by instruments or voices) is complex.  By this we mean that it is composed of energy at different audio frequencies.  If we take a bass control (a simple equalizer) and turn the knob clockwise, we will get an increase in strength of the signal (or the signal component) that has lower frequencies (usually any component below about 500 Hz).  Thus equalization effects the tone because it changes the level relationship of the fundamental and harmonic frequencies.
Shelf EQ
A Shelf Equalizer similarly boosts (or reduces) energy at the set frequency (called the "turnover frequency") and all audio frequencies above it (a high-frequency self control) or all audio frequencies below it (a low-frequency shelf control).   The frequency that will have maximum boost will be the most extreme frequency (20 kHz for a high-shelf and 20 Hz for a low-shelf.  The turnover frequency will usually have 3dB less boost than the extreme frequency if the equalizer is set to maximum boost. 
Peak EQ
A Peak Equalizer boosts (or reduces) at the set frequency (called the "center frequency") and a boosts band of frequencies around the center frequency by a similar amount.  The frequencies in the band will be boosted by an amount that is up to 3 dB less than the center frequency when the equalizer is set to maximum boost. 

Figure 1 - Peak Vs. Shelf Equalization

The "Q" control sets the width of the band of frequencies that will be boosted or reduced; in other words it affects the amount of frequencies around the center frequency which will have a similar amount of boost or reduction. The width of the band is given in octaves.  Q affects this width, but high Q numbers mean a narrower band of frequencies will be affected.  To start, you will need to be able to identify the "Q" settings that go as narrow as 1/2 octave to as wide as 2 octaves.  The conversion chart below relates Q settings to bandwidth.

Q Setting

Bandwidth

0.7

2 Octaves

1.0

1 1/3 Octaves

1.4

1 Octave

2.8

1/2 Octave

Filters
Filters reject (don't pass) certain frequencies and pass the remaining frequencies.  High Pass Filters begin to reject signals with frequencies below the filter's cutoff frequency.  Low Pass Filters begin to reject signals with frequencies higher than the cutoff frequency.  The illustration below compares the HPF response with the shelf equalizer.  The level of the cutoff frequency is -3 dB compared with frequencies well within the pass band. 

Figure 2 - Peak Equalization Vs. Filter

As you can see by the frequency response graph of the HPF filter, all frequencies below the cutoff frequency are not totally rejected.  Instead the filter attenuates the signal more and more as the frequency drops lower and lower.  How fast attenuation is as the frequency lowers is called the filter's slope.  The slope specifies how many additional attenuation (in dB) that there will be as the frequency shifts beyond the cutoff frequency.  Figure 3 shows the response difference for a filter with a 6 dB per octave slope compared to a filter with a 12 dB per octave slope.

Figure 16a - Effect Of Compression & Expansion

Most filter controls in large-format studio consoles have a 12 dB per octave slope.

Guidelines On Using Equalization

The name of the equalizer comes from the concept that adjusting the energy of the audio signal at different frequency bands could make the sound equal to...  I believe the originators of the circuits had "equal to a real sound" in mind but the modern use extends farther than this as I have discussed in  the introduction to the signal processing chapter. 
But no matter how far left field you go with equalization, probably the first thing you will need to be able to do is to "repair" the instrument's tone if any of it has been lost in the recording process.  In multitrack recording we have things such as noise and leakage which distracts from the instrument's true sound and the equalizer is one of the main tools that we combat the "blurring" effect of noise and leakage.   The frequency response of the microphones and phase cancellation due to reflections (and leakage) can cause energy put out by the instrument to be lost at certain frequencies - a situation that can be corrected with equalization.   When we place the microphones for minimum leakage, we can often accent certain frequencies that would sound accented if the instrument was "heard" from a normal distance - again a good job for the equalizer.
After repairing the instrument's sound the job may extend to enhancing or accenting an instrument's sound  - something again that the equalizer can do.  
The chart below gives commonly used settings in the repair and the enhancement of instruments' sounds. 
Reducing Leakage/Noise Outside Of The Instrument's Frequency Range
Generally use shelf EQ to reduce leakage from other instruments.  Set the turnover frequency one two two octaves away from the extreme (highest or lowest) frequency of the instrument   Example: To remove cymbals from a kick drum, try a turnover frequency of 10 kHz with a high-shelf equalizer reducing the signal 12-15 DB. 
In extreme cases use a LPF Filter.  Set the filter's cutoff frequency at or close to the extreme (highest or lowest) frequency of the instrument.  Example: To remove cymbals from a kick drum, try a cutoff frequency of 4 kHz 
Reducing Leakage that is in the frequency range of the instrument
Reduce key frequency of leaking instrument using a peak equalizer with a Q of 1.4 to 2,8 with the maximum dB reduction that sounds natural. 
Accenting Key Instrument Frequencies
The Q setting of 1.0 (1 1/3 octaves), I call the "Magic Q".  In general this is a bandwidth that matches how the different instruments cover the different frequency ranges of audio.  For more percussive instruments (such as drums), you can narrow the bandwidth with Q settings as high as about 2.8.  For more melodic instruments, such as voices and stringed instruments, you can broaden the bandwidth down with Q settings as low as 0.7.  The amount of boost should be in the range of 3 - 6 dB.
Equalize After Careful Level Adjustment
Equalization is all to often misused to compensate for poor level settings.  If you find you want your settings to be more than 6 dB boost, investigate and see if a readjustment of levels could help, reducing the amount of boost at the equalizer.
Using Shelf EQ
Shelf equalizers are good for general tone changes in the bottom or top three octaves of the audio frequencies.  Thus a low-frequency shelf control could be used effectively at 320 Hz or below and a high frequency shelf control at 2500 Hz or higher.
Shelf equalizers are best used for reducing rather than boosting energy.  The very top octave (10 kHz - 20 kHz) often has more noise component (hiss) than useful sound.  The same is true for the lowest octave (20 Hz to 40 Hz) except the noise is rumble rather than hiss.
Natural Vs Hyped
The farther "out" in the mix (the louder the instrument is mixed above other instruments), the more natural the instrument's sound should be.  When an instrument is mixed "in" the mix of other instruments, key frequency should be boosted with sharper bandwidth and larger dB amounts.  Example:  For a voice mixed well-above the music, a 3 dB boost at 5 kHz with a Q of 0.7 - 1.0 would add a lot of presence; if the voice was mixed "in" a 6 dB boost with a Q of 2.0 - 2.8 may be needed.
Splitting The Frequency Range To Reduce Masking
To obtain more clarity and distinction between two instruments in the same range (like two guitars or a guitar and keyboard), use a 1/2 octave bandwidth (Q=2.8) and boost about 3 dB with center frequencies a 1/2 octave apart.  Use the high center frequency on the instrument that has more upper harmonics (the one that is brighter).

Basic Equalization Approaches: 

Equalization Techniques

1

Boosting Harmonic Frequencies
 
Boosting harmonics is one of the first techniques an engineer learns to increase clarity and distinction on instruments. This is a very valid method of equalizing.  Some of the suggested equalizer settings from equalization frequency chart used these techniques:
Instrument Frequency Description
Bass 400 Hz "Increase to add clarity to bass lines..."
Bass 1500 Hz " Increase for ‘clarity’ & ‘pluck..."
Guitar 3 kHz "Increase to add attack..."
Guitar 5 kHz. "Increase ‘brightness..."
Vocal 5 kHz "Increase for vocal presence."
Vocal 10 kHz "Increase to brighten vocals."
Notice that there are at least two frequencies in the harmonic range of the above instruments that could be accented for "clarity" or "brightness"

2

Boosting Fundamental Frequencies
 
The boosting of fundamental frequencies is also one of the first things a new engineer tries, but boosting of fundamentals should be the last thing ever considered.
Accenting fundamental frequencies usually makes the instrument indistinct and muddy sounding. The fundamental frequencies of two instruments playing the same part are the same, therefore, accenting the fundamental of   instruments playing the same part makes both instruments closer to sounding the same (lack of distinction). When two instruments are playing similar parts in the same key they also loose distinction when the fundamental of either instrument is boosted.
If an instrument sounds "thin" or "small" one can carefully boost fundamental frequencies to correct this. The microphone could have been poorly placed and/or the harmonics over-boosted with EQ.   Another application for boosting fundamental frequencies would be to do so when an instrument was playing by itself (in solos etc.).

3

Reducing Fundamental Frequencies
 
Reducing fundamental frequencies in an instrument tends to accent all of the harmonics and is a good alternative to boosting harmonics. The method is most often used in rock recording but works well for all styles of music. This technique found its way to the suggested frequencies chart:
Instrument Frequency Description
Bass 40 Hz "Reduce to decrease "boom" and increase recognition."
Guitar 100 Hz "Reduce to decrease boom and increase clarity."
Vocal 200 Hz "Reduce to decrease muddiness of vocals."

4

Complimentary Equalization
 
One of the hardest things to overcome in mixing is the hearing limitation known as masking.  Masking is one sound covering up all or part of another sound because the frequencies of the two sounds are close.  The sound that is slightly louder sort of "wipes out" the other sound.
The way this works with music is that one instrument will make the other instrument sound dull and indistinct.  It is frustrating to both the novice and the experienced engineer that an instrument sounds so great by itself and so "lifeless" in the mix. 
An equalizer is a "level control" for certain rangers of frequencies.   When you boost a frequency with EQ, you are making the dialed up frequency louder than others (as well as frequencies that are close to the frequency set on the equalizer).   When you dip or cut with an equalizer you are reducing level of frequencies in that range.
When you have indistinct sound between two instruments, you can use a method called "complimentary equalization."  The idea is to boost a certain frequency on one instrument and dip that same frequency on another instrument.  This will get both instruments distinct, when properly done.
Some key conflicts that come up often in mixes, and that can be improved with complementary equalization techniques,  include:
Foot Drum Vs. Bass Dip between 350 Hz and 400Hz on the foot drum (to remove the "cardboard" sound) and increase the same frequency on the bass (to add bass presence).
Lead Vocals Vs. Background Vocals Dip between 3 kHz and 4 kHz on the background vocals to give them an "airy" sound and increase the same frequency on the lead vocal.
When using this method you will be surprised that you get a lot of change with only a little amount of equalization.  Use between 3 dB and 6 dB of boost and cut.

Recommended Frequencies (For EQ and Filtering):

Use the following Recommended Expanded Frequencies Chart as a starting point. 

 

FREQUENCY:

USES:

50Hz

1. Increase to add more fullness to lowest frequency instruments like foot,  toms, and the bass. Peak equalization with a 1.4 Q.
2. Reduce to decrease the "boom" of the bass and will increase overtones and the recognition of bass line in the mix. This is most often used on loud bass lines like rock.  Shelf equalization.
3.  Filter with this as a HPF cutoff frequency to get rumble or bass-instrument leakage out of the guitar or other rhythm instruments as well as male vocals. 

100Hz

1. Increase to add a harder bass sound to lowest frequency instruments. Peak Equalization with a Q of 1.0 to 1.4..
2. Increase to add fullness to guitars, snare. Peak Equalization with a Q of 1.0..
3. Increase to add warmth to piano and horns. Peak Equalization.   For piano use a Q of 1.0.  With horn use a Q of 1.4..
4. Reduce to remove boom on guitars &  increase clarity. Peak Equalization with a Q of 1.0 to 1.4..
5. Filter with this as a HPF cutoff frequency to reduce leakage into lead instruments such as lead guitars and for female vocals.

200Hz

1. Increase to add fullness to vocals. Peak Equalization with a Q of 0.7 to 1.0..
2. Increase to add fullness to snare and guitar ( harder sound ). Peak Equalization with a Q of 1.4.
3. Reduce to decrease muddiness of vocals or mid-range instruments.  Peak Equalization with a Q of 1.0.
4. Reduce to decrease gong sound of cymbals.  Peak Equalization with a Q of 1.0.
5. Filter with this as a HPF cutoff frequency to reduce bass line leakage into higher-ranged instruments ior to to simulate telephone audio. 

400Hz

1. Increase to add clarity to bass lines especially when speakers are at low volume.  Peak Equalization with a Q of 1.0.
2. Reduce to decrease "cardboard" sound of lower drums (foot and toms).  Peak Equalization with a Q of 1.4.
3. Reduce to decrease ambiance on cymbals. Peak Equalization with a Q of 0.7 to 1.0.  Alternately try a shelf EQ with a 320 Hz frequency setting.
4. Filter with this as a HPF cutoff frequency to reduce foot and tom leakage into the cymbal mics.

800Hz

1. Increase for clarity and "punch" of bass. Peak Equalization with a Q of 1.4.
2. Reduce to remove "cheap" sound of guitars.  Peak Equalization with a Q of 1.0.

1.5KHz

1. Increase for "clarity" and "pluck" of bass.   Peak Equalization with a Q of 1.4..
2. Reduce to remove dullness of guitars. Peak Equalization with a Q of 1.0.

3KHz

1. Increase for more "pluck" of bass. Peak Equalization with a Q of 1.4.
2. Increase for more attack of electric / acoustic guitar.  Peak Equalization with a Q of 1.4.
3. Increase for more attack on low piano parts.  Peak Equalization with a Q of 1.0.
4. Increase for more clarity / hardness on voice.  Peak Equalization with a Q of 1.0.
5. Reduce to increase breathy, soft sound on background vocals.  Peak Equalization with a Q of 1.0.
6. Reduce to disguise out-of-tune vocals / guitars.  Peak Equalization with a Q of 1.0.
7. Increase for more attack on the snare or other drums.  Peak Equalization with a Q of 1.4 to 2.8.
8. Filter with this as a LPF cutoff frequency to simulate telephone audio. 

5KHz

1. Increase for vocal presence. Peak Equalization with a Q of 1.0.
2. Increase low frequency drum attack ( foot / toms). Peak Equalization with a Q of 1.4 to 2.8.
3. Increase for more "finger sound" on bass. Peak Equalization with a Q of 1.4.
4. Increase attack of piano, acoustic guitar and brightness on guitars (especially rock guitars).  Peak Equalization with a Q of 1.4.
5. Reduce to make background parts more distant.  Peak Equalization with a Q of 1.0.
6. Reduce to soften "thin" guitar.  Peak Equalization with a Q of 1.0.
7. Filter with this as a LPF cutoff frequency to reduce noise or leakage into the foot drum microphone. 

7KHz

1. Increase to add attack on low frequency drums ( more metallic sound ). Peak Equalization with a Q of 1.4 to 2.8.
2. Increase to add attack to percussion instruments.  Peak Equalization with a Q of 1.4 to 2.8.
3. Increase on dull singer.  Peak Equalization with a Q of 1.0.
4. Increase for more "finger sound" on acoustic bass. Peak Equalization with a Q of 1.4.
5. Reduce to decrease "s" sound on singers.  Peak Equalization with a Q of 2.8.   Sweep frequency slightly (between 7 kHz and 8 kHz)  to find the "exact" frequency of the S
6. Increase to add sharpness to synthesizers, rock guitars, acoustic guitar and piano. Peak Equalization with a Q of 1.0 to 1.4.
7. Filter with this as a LPF cutoff frequency to simulate AM radio (orb other low-qualkity) audio. 

  10KHz

1. Increase to brighten vocals. Peak Equalization with a Q of 1.0.
2. Increase for "light brightness" in acoustic guitar and piano. Peak Equalization with a Q of 1.0.
3. Increase for hardness on cymbals. Peak Equalization with a Q of 1.4.
4. Reduce to decrease "s" sound on singers. Peak Equalization with a Q of 1.4.
5. Filter with this as a LPF cutoff frequency to simulate reduce noise/leakage into guitar (or other midrange instrument) microphones. 

15KHz

1. Increase to brighten vocals (breath sound). Peak Equalization with a Q of 1.0.
2. Increase to brighten cymbals, string instruments and flutes. Peak Equalization with a Q of 1.0.
3. Increase to make sampled synthesizer sound more real. Peak Equalization with a Q of 1.4 to 2.8.
4. Filter with this as a LPF cutoff frequency to reduce analog tape hiss.

Copyright 2001, Robert Dennis, ALL RIGHTS RESERVED