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Equalizing for dummies!
Equalizing for dummies! By Christian Zechner
OK, so you have worked for hours, days, weeks on this GREAT track of yours but no matter what you do, it just sounds flat or «muddy», you can't get the kick-drums to stand out, the vocals are almost impossible to interpret and it doesn't sound as clean, transparent and punchy as you hoped it would. Here I've tried to provide an interesting and useful read about the sound frequency spectrum in general, equalizers, filters and how to utilize these in your mix.
Contents:
1. Introduction – the sound frequency spectrum
2. Music and frequency ranges – Equalizing
3. What is an equalizer?
4. Filter types
5. General frequency ranges
6. Helpful tips
1. Introduction – the sound frequency spectrum
What is frequency? If a digital audio signal has a frequency of 50 Hertz (50Hz) this tells us that the signal in question is cycling from its starting point (0) to positive amplitude to negative amplitude and back to the starting point 50 times per second. The lower the frequency is, the slower the signal will oscillate. (In synthesizers, oscillators with very low frequency settings are used as control parameters to control other aspects of a sound. These oscillators are called LOW FREQUENCY OSCILLATORS or LFOs.)
The OPTIMAL human ear picks up sound in the frequency range of 16Hz to 24 000Hz (24 kHz), but the average infant/adolescent person can hear sounds between 20Hz to 20kHz and the range for adults are generally 50Hz to 16kHz. This will gradually get narrower as you grow older. Some of you have probably heard of the popular ring-tone "mosquito" which exploits this fact. It's a very high-frequent sound and cannot be heard by most adults, so it is a very popular ring-tone used in classrooms, f.ex.. The upper limit of human hearing is caused by the middle ear acting as a LOW-PASS FILTER. If ultrasound is fed directly to the skull bone, much higher frequencies can be heard (<200kHz).
The INFRASONIC spectrum consists of sounds that have a frequency too low to be picked up by the human ear. This is everything between 0.001Hz up to approximately 20Hz. These sounds can often be felt physically, but not heard. These are very energy-loaded sounds, and they can travel great distances (after the great Krakatoa eruption in the 19th century, infrasound travelled seven times around the Earth) and through/around objects with ease. Earthquake- and tornado-warning systems and nuclear bomb monitoring systems utilize this.
Examples of this in nature are: sounds emitted during and prior to a volcanic eruption or earthquake, avalanches, ocean waves, tornadoes and other winds. It is also the preferred communication method by elephants, giraffes, alligators, rhinos and whales. Animals are thought to pick up infrasound prior to and during natural disasters, this is thought to be the case in 2004 when the great tsunami hit the shores of countries around the Indian Sea. The animals fled from the shores long before the tsunami hit.
Infrasound is also sometimes utilized in soundtracks and music, as it can produce feelings of fear, sorrow and anxiety when (unconsciously) detected by humans.
The ULTRASONIC spectrum, however, consists of the sounds with a frequency too high to be heard by the human ear (i.e. >20KHz). We've all seen how ultrasound is utilized in medical situations when examining internal organs is part of determining a diagnose, or to see the fetus inside a pregnant womb. Ultrasound is also used in dog whistles (16-22kHz) as dogs have a higher upper limit of hearing, and in sonar/echolocators on boats. Natural occurrences of ultrasound can be found f.ex. in the natural sonar used by whales and dolphins and the echolocation method used by bats.
2. Music and frequency ranges
In music, we will most of the time utilize the sonic range of 20Hz to 20 000Hz (most often written as 20kHz, or sometimes only 20k). We want to filter out any frequencies we can't hear to make room for and exploit those we CAN hear.
3. WHAT IS AN EQUALIZER?
An equalizer can be explained as a volume control. But unlike a normal volume control (like the knob on your stereo system), an equalizer can be assigned to raise or lower the volume on a specific frequency instead of changing the overall volume. There are two types of equalizers, GRAPHICAL and PARAMETRIC. Parametric equalizers have a graphic preview of the EQ slope which most of the time can be directly tweaked by clicking on it, and you can assign any frequency and any bandwidth (also known as the «Q factor») to each band. Graphic equalizers on the other hand, have already assigned frequencies and bandwidths on each band and are therefore generally less controllable.
4. FILTER TYPES
A filter is, as the name suggests, a method used when filtering out various frequencies. They are not as advanced and versatile as an equalizer, but the principles are the same. When you are working with an equalizer you can sometimes set various filter settings on different bands, most common on the lowest (left-most) and highest (right-most) frequency bands. This depends on what equalizer you are using. We have seven different types of filters, all of which can be used when doing only slight equalizing or as an effect. Different filter plugins will sound different, so experiment and find one that suits your taste. The filters are the following:
LOW-PASS (LP, also known as HIGH-CUT)
A low-pass filter will let through the frequencies BELOW the set cutoff frequency. This is the most commonly used filter type in electronic music, mainly as an effect. Electronic low-pass filters are used in subwoofers and other speakers, to filter out the higher frequencies that can't be translated well by the respective speaker. Useful as a simple EQ on bass sounds or for filtering out noise from bad recordings.
HIGH-PASS (HP, also known as LOW-CUT)
This is the opposite of a low-pass filter. A high-pass filter will let through all the frequencies ABOVE the chosen cutoff frequency. This is useful as a simple EQ on hi-hats/cymbals and other percussive drum sounds as well as for reducing/removing the lower frequencies in other non-bass instruments.
BAND-PASS (BP)
A band-pass filter will let through the frequencies on and around the selected cutoff frequency. Nice for percussive sounds and supporting basslines.
BAND-REJECT (also referred to as a BAND-STOP or NOTCH filter. A notch filter has a generally high Q factor)
Being the opposite of a band-pass filter, this will filter out the frequencies on and around the set cutoff frequency.
PEAK
A peak filter is very similar to a band-reject filter, but in opposition to a band-reject filter a peak filter allows you to either boost OR cut on and around the specified frequency, rather than just cut.
LOW- and HIGH SHELF
Shelving filters are useful when you want to boost/cut every frequency below (low shelf) or above (high shelf) a certain frequency.
We also have the COMB filter (which are essentially several peak filters) and the FORMANT filter (>2 bandpass/peak filters on already assigned frequencies. This is more used as an effect rather than EQ so I will not cover it here).
5. GENERAL FREQUENCY RANGES
Play with your favourite equalizer plugin and try out these various EQ settings. The following frequency range descriptions are meant to be used only as guidelines and are not to be followed literally.
16Hz – 60Hz = SUB BASS
This is the super low-end that can be felt physically by your body on a good subwoofer/sub-bass system. Sounds with these frequencies are the most powerful ones, and they will take up a lot of room in the mix. Use this range to fatten up your kick drums or sub-bass patches. Too much volume in this range makes your mix sound «muddy.»
60Hz – 250Hz = BASS
This is where basslines and kick drums have their most important sounds. A common problem is that the bassline and kick cancel each other out due to PHASE problems (easily demonstrated when DJ-ing, if you play two tracks and have them beatmatched, it's important to cut one of the tracks' bass level or else the kick drums will cancel each other out and the overall bass level is lowered). A useful trick then is to try PHASE INVERSION on either the bassline or the kick drum, compressing the kick and bass together and/or avoiding to place a bass note on top of a kick drum. This range should also be lowered in most other sounds like guitars, synth lines and vocals so they don't interfere with the kick and bassline. Too much volume here makes the mix sound «boomy.»
200Hz - 400Hz
Too much volume here will cause vocals to sound muddy and unclear. Cut this to thin out drum parts like snares, hi-hats, percussions and cymbals, boost to make them sound warmer or more «woody.»
250Hz – 2kHz = LOW MID or MID-LO
Most instruments have their «darkest» parts here; guitars, piano, synthlines. Boosting around 500Hz – 1kHz can sound «horn-like» while boosting 1kHz – 2kHz can sound metallic.
400Hz - 800Hz
You can reduce some of these frequencies on the master mix to make your overall bass level sound tighter. Boost or cut here to fatten up or thin out the low end of guitars, synthlines and vocals.
800Hz – 1kHz
Here you can also fatten up vocals and make them sound warmer, in a different way than the previously mentioned method. Boosting around 1kHz helps add to the «knocking» sound of a kick drum.
1kHz – 3kHz
This is the edgy part of a sound, boost (gently!) here to define guitars, pianos, vocals and add clarity to basslines. Cut here to remove painful mid-frequencies in vocals. This frequency range is very hard on the ears, so be careful not adding too much volume here!
2kHz – 4kHz = HIGH MID or MID-HI
Vocals have a lot of sound in this area, the sounds «B», «M» and «V» lie here.
3kHz – 6kHz = PRESENCE
Plucky, fingered guitars and basslines can be more defined by boosting in this range. Cut in the lower part to remove the hard sound of vocals. Cut in the upper part to soften/round off sounds, and boost to add more clarity or presence to a sound. Boosting here helps defining most instruments and vocals.
6kHz – 10kHz = HIGH
Boost this area to add more air and transparency to a sound. Crispness and and sparkle can be added by boosting this range on guitars, strings and synth sounds. Snares and bassdrums also benefits from boosting this area. In vocals, cut some of these frequencies (a de-esser plugin does this easily) to remove the hissing sounds. The sounds «S» and «T» lies between 6kHz and 8kHz and too much volume there will make the vocals stressful on your ears.
10kHz – 16kHz = HIGH
This frequency range is where the crispness and brightness of sounds lie, and hi-hats and cymbals are the dominant drum parts. You can boost here to add even more air and transparency to sounds, and cut here to remove noise and hissing sounds which is unwanted in a bassline, for example. Pads and atmospheric sounds benefits from a boost in this range to make them sound brighter. Be careful not to boost too heavily, or else the mix will sound noisy.
6. HELPFUL TIPS
A rule of thumb is to remove unwanted frequencies before raising the levels of those you want, but remember: the more frequencies you raise in a sound, the harder it is to place in the mix.
90% of the time it's better to cut rather than boost frequencies in a sound.
When listening for a bad, sharp frequency (maybe your newly recorded fat guitar riff, or the beautiful vocal hook is really hard on the ears when listening on loud volume) a good tip to find that horrible frequency is to put on a peak filter with a very narrow bandwidth, high gain settings, and then sweep the peak filter across the frequency spectrum until you find the right spot where it sounds like knives are being stabbed in your ears (be careful with the speaker volume when doing this!). Now invert your gain settings to a minimum and cut this frequency. You can also increase the bandwidth a bit if it still sounds sharp. Another way is to apply a narrow peak filter, and start with the gain all the way DOWN (instead of raising it) and sweep across the spectrum until your instrument sits nicely in the mix.
It is really important to listen to the channel you are working on in both solo mode and together with the rest of the track. If it sounds weird in solo mode, it doesn't necessarily sound weird when played together with all the other tracks, so use your ears!
Assigning each instrument to its own frequency range will help making the mix sound clearer overall (e.g. Basslines to the low end, guitars to mid-lo and/or mid-hi, vocals to mid-hi/high frequencies).
You cannot modify a frequency in recorded audio if the frequency isn't present in the sound itself, so if you f.ex. have a «muddy» recording to start off with, removing frequencies can help making it sound clearer.
Removal of subsonic rumble from your mix is essential for getting a nice, transparent sound image (especially when you are mastering for a vinyl record. When writing audio data to a vinyl record, too much information applied will write a wide ridge in the record and the needle will get unstable and skip.) The application of a highpass filter will fix this. Set it to cut everything below 20-50Hz.
In general, the EQ slope on a master mix should look much like a «smile». Raise the bass and treble levels and/or cut the mid range. This will ensure a good low end while maintaining a good, clear treble, and the mix won't be too hard on your ears.
And lastly: if it's not broken, don't fix it!
I hope this was a useful lesson, please don't hesitate to ask if you want more tutorials/infos on music production! I'll try to do some more stuff in the future if you've found this helpful 
(DISCLAIMER: This text is based on my own way and my own methods of working with equalizers, and it is not necessarily the RIGHT way to do it. Factual errors could occur in this text. Trial and error is, by no doubt, the best way of learning. This text is subject to change without notice.)
REFERENCES:
WikiPedia
About.com
Computer Music
Digital Recording Techniques
... and my own brain 
___________________
www.cz-audio.com
www.myspace.com/christianzechner
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Last edited by nephilim on Nov-21-2006 at 20:16
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