EQ Applications and Understanding
Although most equalization is done by ear, it’s helpful to have an idea about which frequencies affect an instrument in order to achieve and particular effect (hence my tables above). Let’s start at the very beginning: what is frequency? Frequency is the wavelength of sound. That is to say, it is the rate at which a sound wave completes a cycle of positive and negative amplitude. The number of cycles that occurs in one full second is the frequency of a sound wave and that is measured in Hertz (Hz). You will also frequently see the term Kilohertz (kHz) used when talking about sound waves; 1 kHz is equal to 1000 Hz.
When sound waves overlap they combine into a new wave and the frequencies interfere, enhance, and cancel each other. This is where equalization (EQ) comes into play. Equalizers allow you to control the frequencies of sound and thus allow you to shape your music so it sounds better (or worse). It is not enough to just have a bunch of great samples and sounds and throw them together thinking it will sound good. Once sounds overlap they change each other and you must be able to control that change or you end up with a sonic mess. Equalization will allow you to cut and boost certain frequencies within each of your sounds/instruments so that they do not interfere with each other (at least not as much).
On the whole the audio spectrum can be divided into four frequency bands: LOW (20-200Hz), LOW-MIDDLE (200-1000Hz), HIGH-MIDDLE (1-5kHz), and HIGH (5-20kHz):
LOW: 20-200 Hz
This range is often known as the sub bass and is most commonly taken up by the lowest part of the kick drum and bass guitar; although at these frequencies it's almost impossible to determine any pitch. Anything below 40 Hz is not heard at all by human ears but can be felt, especially in the chest. Sub bass is one of the reasons why 12" vinyl became available: low frequencies require wider grooves than high frequencies - without rolling off everything below 50 Hz you couldn't fit a full track onto a 7" vinyl record. However I do NOT recommend applying any form of boost around this area without the use of very high quality studio monitors (not home monitors - there is a vast difference between home near-field and studio far-field monitors costing anywhere between $5,000 and $20,000). Boosting blindly in this area without a valid reference point can and will permanently damage most speakers, even PA systems. You have been warned!
LOW-MIDDLE: 200-1000 Hz
This is the range you're adjusting when applying bass boost in the upper ranges to add some presence or clarity to your low end instruments. This is also the main culprit area for muddy sounding mixes. Most frequencies around here can cause psycho-acoustic problems: if too many sounds in a mix are dominating this area, a track can quickly become annoying.
HIGH-MIDDLE: 1-5 kHz
Human hearing is extremely sensitive at these frequencies, and even a minute boost around here will result in a huge change in the sound (almost the same as if you boosted around 10 dB at any other range). This is because our voices are centered in this area, so it's the frequency range we hear more than any other. Most telephones work at 3 kHz, because at this frequency speech is most intelligible. This frequency also covers TV stations, radio, and electric power tools. If you have to apply any boosting in this area, be very cautious, especially on vocals. We're particularly sensitive to how the human voice sounds and its frequency coverage.
HIGH: 5-20 kHz
This is the range you adjust when applying the treble boost on your home stereo. This area is slightly boosted to make sounds artificially brighter when mastering a track before burning it to CD. The high end area also includes the higher frequencies of cymbals and hi-hats, but boosting around this range, particularly around 12 kHz can make a recording sound more high quality than it actually is, and it's a technique commonly used by the recording industry to fool people into thinking that certain CDs are more hi-fidelity than they'd otherwise sound. However, boosting in this area also requires a lot of care - it can easily pronounce any background hiss, and using too much will result in a mix becoming irritating. Anything over 20 kHz cannot be heard by human ears.
One way to zero in a particular frequency using an equalizer is to set the amount of boost for all frequencies to near maximum and drop out each affected range one at a time until the problem frequency is found. If you find yourself boosting one frequency and then subsequently boosting most of the other frequencies to bring your sound back to how you want it you should probably just increase the overall gain of that sound. If an overall gain increase doesn’t sound satisfactory, it may be that one range of frequencies is too dominant and requires attenuation.
Recording with EQ is a highly debated subject. Some people record with EQ in order to make up for bad microphone placement or room acoustics. If you are unsure of how you want your finished sound to be heard then you should probably record without EQ so that you can make dramatic changes to the sound sample later. It is easy to boost frequencies, but it is a thousand times more difficult to cut frequencies out, and you may find that you need your sound to take on totally different qualities once it is thrown into the big picture of your song. Recording with more than one mic is the perfect solution as you can EQ one and leave another flat. Later you can take what bits of EQ you want from each and combine them.
An equalizer is one of the most powerful tools of the musician and used properly it can greatly enhance or restore the sonic balance of your mix. Experimentation is the key so don’t think I gave you any real knowledge just now; this is just a guide and a suggestion. Keep in mind an equalizer won’t be able to make up for dodgy recording techniques. It is simply a tool for correcting the minor problems of acoustics, enhancing what you have, and for cleaning the extra junk out of your mix.
.