The purpose of a filter is to let certain frequencies pass through and to block others. There are three types of EQ filters, Low Pass, High Pass, and Band Pass. Each of these have a different sound and function, but they all have the same general controls.
There are also a couple of special filters that can be applied in EDM production as well, namely Comb filters and Formant filters.
EQ Filter TypesEdit
High Pass Filter:Edit
A high pass (or Low-Cut as some manufacturers call it) filter does exactly what the name implies. It allows the highs of the track to pass through, and attenuates the lows by a designated range. There are typically 2 controls for a high-pass filter. Cutoff Frequency and Q. The cutoff frequency is NOT the frequency where the filter starts rolling off the lows, but rather the frequency where the lows are 3dB lower than the original signal. So a HPF with the cutoff set at 100Hz means that 100Hz is 3dB down from the original, source track.
The Q on a HPF is typically set in dB/octave. So a filter that has a 6dB/Octave slope means that the filter rolls off 6dB for every halving of frequency. In this example, If the cutoff is set to 100Hz, then you can expect the frequency of 50Hz (One octave, or halving below the cutoff) to be at -9dB (due to the cutoff being 3dB below the original)
A Few Examples of a HPF:Edit
This is an example of the stock Pro Tools EQ set to -6dB/Octave with a cutoff frequency of 100Hz. Note the relatively gentle roll-off as the frequency gets lower and lower
Here is an example of the EQIII set to a slope of -18dB/Octave. Note how the roll-off is much sharper, and in effect you lose the lows much quicker than the previous example.
The Low Pass filter (Or hi-cut) is a staple of EDM. All that it does is filters out all of the high frequencies of your track so only the lows pass through. Also, on a side note, EDM production is unique in that it uses the LPF much more frequently than pop/rock mixes. Due to most of the aural excitment in a track living in the high frequencies, cutting them out on a recorded instrument is essentially mix suicide. However, in EDM when you cut out all the highs, it adds a wonderful tension to a track that can either be relieved by gradually building them back in, or bringing them all back in at once with a drop.
The LPF has the same controls as a HPF, but in the opposite direction. The cutoff frequency is still 3dB down, and the slope is still determined in dB/Octave.
Some Examples of a LPF:Edit
Just as with the HPF, this is the same plugin, with the same -6dB/Octave slope setting, and the cutoff frequency set to 2kHz. However, note how it is now filtering out the high end of the frequency spectrum, and leaving the low end alone.
Again, same EQ as the HPF example, but this time set to -18dB/Octave.
Band Pass Filter:EditThe band pass filter is simply a hi-pass and a low-pass filter acting at the same time. This filters out the low end of the spectrum and the high end of the spectrum, giving the track a very low-fi or old timey sound.
Here is an example of what a band-pass filter might look like. However, the only way to really dial in the sound you're going for is by tweaking the Q and cutoff frequencies of both until you find the right amount of passing happeneing.
Shelving filters are usefully as a sort of "broad paintbrush" approach to changing frequency content. Interestingly enough, car stereos frequently use shelving filters for the "Treble" and "Bass" settings if they don't allow individual EQ'ing. All that a shelving filter does is boost or cut a set of frequencies at a certain point. They typically have 2-3 options, boost/cut, frequency, and sometimes slope. Slope is just another word for how steep the filter is in relation to the center frequency.
An extreme example of a Low-Shelf filter
Some EQ Filters (such as the one in Ableton) have a resonance frequency option. The idea behind this is that the engineer on the song wants to cut out all of the frequencies below or above their cutoff point, but there is some reason why they chose to put their cutoff point where they did. In other words, the cutoff point is just below (or above) some very important frequency to the mix, which is why the engineer chose not to cut it out.
In light of this, some manufactuers offer the option of boosting the frequency right before the cutoff point, as a potential tool for whoever is mixing the track looking for that specific sound.
This is Ableton's EQ set as a resonant HPF
There are a couple of filters that don't find as much common usage in EDM but are still worth talking about. The first, a comb filter, is a filter as the result of delays applied to a track. The second, a formant filter, has to do with the vowel sounds that we associate with EQ and human speech.
A comb filter is one that results from very short delays. What a comb filter is, in essence, is an exact copy of a source track, play at the same volume, but delayed by a small amount. This delay puts the source signal 90 degress out of phase with the original, and thus some frequencies are doubled, while others are attenuated to the point of notching them out. It gets its name from the distinct shape that the resulting EQ response gets, looking like a comb
This is the graph of a 1ms comb filtered signal. Note the extreme notches at certain frequencies, and also note the 6dB boosts at others, notable 1kHz, 2kHz, 3kHz, and so on. This sort of comb filter can result in a very exciting sound, since aurally those frequencies are the ones that we love to hear.
A formant filter is one that boosts the frequencies typically associated with human speech. "Talking Bass" synth lines use formant filters to create sounds that are almost words, but not quite. For a more in depth look at formant filters, refer to the Wiki article on it