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Some History
Dating as far back as the 1930's, the equalizer is the oldest and probably the most extensively used signal processing device available to the recording or sound reinforcement engineer. Today there are many types of equalizers available, and these vary greatly in sophistication, from the simple bass and treble tone control of the fifties to advanced equipment like the modern multi-band graphic equalizer and the more complex parametric types. Basically, an equalizer consists of a number of electronic filters which allow frequency response of a sound system or signal chain to be altered. Over the past half century, equalizers design has grown increasingly sophisticated. Designs began with the basic 'shelving filter', but have since evolved to meet the requirements of today's audio industry.

Understanding EQ and its Effects on Signals
There are two areas of equalization that I want to cover. Those two areas are vocals and music. I'd like to discuss the different effects of frequencies within audio signals. What do certain frequencies do for sound and how we understand those sounds. Why are some sound harsh? Why do things sound muddy? Why can't I understand the vocals? I'll try and answer all of these question and hopefully bring some light to the voodoo world of EQ.

Roughly speaking, the speech spectrum may be divided into three main frequency bands corresponding to the speech components known as fundamentals, vowels, and consonants.
Speech fundamentals occur over a fairly limited range between about 125Hz and 250Hz. The fundamental region is important in that it allows us to tell who is speaking, and its clear transmission is therefore essential as far as voice quality is concerned.
Vowels essentially contain the maximum energy and power of the voice, occurring over the range of 350Hz to 2000Hz. Consonants occurring over the range of 1500Hz to 4000Hz contain little energy but are essential to intelligibility.
For example, the frequency range from 63 to 500Hz carries 60% of the power of the voice and yet contributes only 5% to the intelligibility. The 500Hz to 1KHz region produces 35% of the intelligibility, while the range from 1 to 8KHz produces just 5% of the power but 60% of the intelligibility.
By rolling off the low frequencies and accentuating the range from 1 to 5KHz, the intelligibility and clarity can be improved.
Here are some of the effect EQ can have in regards to intelligibility. Boosting the low frequencies from 100 to 250Hz makes a vocal boomy or chesty. A cut in the 150 to 500Hz area will make it boxy, hollow, or tube like. Dips around 500 to 1Khz produce hardness, while peaks about 1 and 3Khz produce a hard metallic nasal quality. Dips around 2 to 5KHz reduce intelligibility and make vocals woolly and lifeless. Peaks in the 4 to 10KHz produce sibilance and a gritty quality.

Effects of Equalization on Vocals
For the best control over any audio signal, fully parametric EQ's are the best way to go.

80 to 125

160 to 250

315 to 500

Sense of power in some outstanding bass singers.

Voice fundamentals

Important to voice quality

630 to 1K

Important for a natural sound. Too much boost in the

315 to 1K range produces a honky, telephone-like quality.

1.25 to 4K

5 to 8K

Accentuation of vocals

Important to vocal intelligibility - Too much boost between 2 and 4KHz can mask certain vocal sounds such as 'm', 'b', 'v'.
Too much boost between 1 and 4KHz can produce 'listening fatigue'.
Vocals can be highlighted at the 3KHz area; at the same time, dip the instruments at the same frequency.
Accentuation of vocals:
The range from 1.25 to 8K governs the clarity of vocals. Too much in the area of 5 to 16K can cause sibilance.

Mic'ing instruments is an art ... and equalizers can often times be used to help an engineer get the sound he is looking for. Many instruments have complex sounds with radiating patterns that make it almost impossible to capture when close mic'ing. An equalizer can compensate for these imbalances by accenting some frequencies and rolling off others. The goal is to capture the sounds as natural as possible and use equalizers to straighten out any non-linear qualities to the tones.
Clarity of many instruments can be improved by boosting their harmonics. In fact, the ear in many cases actually fills in hard-to-hear fundamental notes of sounds, provided the harmonics are clear. Drums are one instrument that can be effectively lifted and cleaned up simply by rolling off the bass giving way to more harmonic tones.
Here are a few ideas on what different frequencies do to sounds and their effects on our ears.

40Hz to 70Hz

These frequencies give music a sense of power. If over emphasized they can make things muddy and dull. Will also cloudy up some harmonic content.

80Hz to 125Hz

Too much in this area produces excessive 'boom'.

160Hz to 250Hz

This is the problem area of a lot of mixes. To much of this area can take away from the power of a mix but is still needed for warmth. 160Hz is a pet-peeve frequency of mine. Also, the fundamental of bass guitar and other bass instruments sit here.

300Hz to 500Hz

Fundamentals of string and percussion instruments.

400Hz to 1K

Fundamentals and harmonics of strings, keyboards and percussion. This is probably the most important area when trying to control or shape to a natural sound. The 'voice' of an instrument is in the mids.

To much in this area can make instruments sound horn-like.

800Hz to 4K

This is a good range to accentuate instruments or warm them up. Too much in this area can produce 'listening fatigue'. Boosts in the 1K to 2K range can make instruments sound tinny.

4K to 10K

Accentuation of percussion, cymbals, and snare drum.

Playing with 5K makes the overall sound more distant or transparent.

8K to 20K

This area is often what defines the quality of a recording or mix. This area can also help define depth and 'air' to mix. Too much can take away from the natural sense of a mix by becoming shrill and brittle.

Here are a few other pin point frequencies to start with for different instruments. In a live sound situation, I might preset the console's eq to these frequencies to help save time once the sound check is under way. These aren't the answers to everything... just a place to start at.
Kick Drum:
Besides the usual cuts in the 200Hz to 400 area, some tighter bandwidths at 380Hz, 5kHz and shelf at 10k may help. The point of these frequencies makes for space for the fundamental tones of a bass guitar or stand up. I have also found a high pass filter at 40Hz to 50Hz will help tighten up the kick along with giving your compressor a signal it can deal with musically.
Snare Drum:
The snare drum is an instrument that can really be clouded by having too much low end. Frequencies under about 150Hz are really un-usable for modern mixing styles. I would suggest a high pass filter in this case. Most snares are out front enough so a few cuts might be all that is needed. I like to start with 260Hz, boost at 5kHz, and some 12K. This are just frequencies to play with. Doesn't mean you will use all. If the snare is too transparent in the mix but I like the level it is at, a cut at 5K can give it a little more distance and that might mean a little boost at 10K to brighten it up.
High Hats:
High hats have very little low end information. I high pass at 200Hz can clean up a lot of un-usable mud in regards to mic bleed. The mid tones are the most important to a high hat. This will mean the 400Hz to 1K area but I've found the 600Hz to 800Hz area to be the most effective. To brighten up high hats, a shelving filter at 12.5K does nicely.
Toms and Floor Toms:
Again, the focus here is control. I like to high pass at 100Hz which give more focus about tone rather than size. Most toms could use a cut in the 300Hz to 400Hz area. And there is nothing real usable under 100Hz for a tom... unless you are going for a special effect. Too much low end cloud up harmonics and the natural tones of the instrument. Think color not big low end. Boost at 5kHz and shelf at 10kHz to make things more aggressive. .
Over Heads:
In my opinion, drum over heads are the most important mics on a drum kit. They are the ones that really define the sound of the drums. That also give the kit some ambience and space. These mics usually need a cut in the 400Hz area and can use a good rolling off at about 150Hz. Again, they are not used for power.... these mics 'are' the color of your drum sound. Roll off anything that will mask harmonic content or make your drums sound dull. Cuts at 800Hz can bring more focus to these mics and a little boost of a shelving filter at 12.5K can bring some air to the tones as well.
Bass Guitar:
Bass guitar puts out all the frequencies that you really don't want on every other instrument. The clarity of bass is defined a lot at 800Hz. Too much low end can mask the clarity of a bass line. I've heard other say that the best way to shape the bass tone is to roll off everything below 150Hz, mold the mids into the tone you are looking for, then slowly roll the low end back in until the power and body is there you are looking for. If the bass isn't defined enough, there is probably too much low end and not enough mid range clarity. Think of sounds in a linear fashion, like on a graph. If there is too much bass and no clarity, you would see a bump in the low end masking the top end. The use of EQ can fix those abnormalities. High pass at 40Hz to 50Hz to tighten things up. .
Guitar/piano/ etc.:
These instruments all have fundamentals in the mid range. Rolling off low end that is not needed or usable is a good idea. Even if you feel you can't really hear the low end, it still is doing something to the mix. Low end on these instruments give what I call support. The tone is in the mids. 400Hz and 800Hz are usually a point of interest as are the upper mids or 1K to 5K. Anything above that just adds brightness. Remember to look at perspective though. Is a kick brighter than a vocal? Is a piano bright than a vocal? Is a cymbal brighter than a vocal?
In Closing
Equalizers are one of the most over looked and mis-used pieces of gear in the audio industry. By understanding equalizers better, an engineer can control and get the results he or she is looking for. The key to EQ'ing is knowing how to get the results you are looking for. Also, knowing if its a mic character or mic placement problem. EQ can't fix everything. It can only change what signal its working with. Equalizers are also a lot more effective taking away things in the signal than replacing what was never there.
Written by Devin DeVore 
Additional research provided by Klark - Technik