Impmooc – Week 5 – Filters

Hi there! My name is Karyn Ellis. I am a professional songwriter / performer from Toronto, Canada.

This week I am going to discuss the three main types of EQ filters (there is a fourth called a graphic EQ, but I won’t delve into that category in this lesson as it is really just a variation of the Parametric EQ.)

This Music Production course has been a tremendous (and fun) learning curve for me, but as a complete newby to the world of DAWS I found it too challenging to set up a working demonstration this week. Instead I did a theoretical assignment. Feel free to dock me points under “presentation” if you feel like I should have done a DAW demonstration anyways (I tried! Really! Clicks and stutters; that’s all I could manage!), but hopefully this review of filters is equally helpful to you.

An EQ filter works by increasing or decreasing the amplitude of certain frequencies within the signal. This can be done to affect the timbre of the signal, to direct the listener’s attention to a particular instrument or vocal, to thin or thicken a sound so that it sits better within a mix or to get rid of unwanted frequencies such as low rumbles and noise.

There are three kinds of EQ filters that can be used to change the frequency characteristics of an audio signal: PASS FILTER, SHELF FILTER and PARAMETRIC EQ.

As the name of this category suggests, this filter allows frequencies to pass through unaffected when they are within a certain range that is set by a CUTOFF frequency. Amplitudes of frequencies outside of that range are attenuated according to the rate – or slope – described as # of dBs per octave. This means for every octave away from the cutoff frequency, the signal will be reduced by that given number of decibels. (Amplitudes can also be boosted, but this is highly uncommon with this type of filter.)

There are three kinds of Pass Filters:

HIGHPASS:  This allows frequencies above the cutoff frequency to pass through the filter. Everything below the cutoff frequency will be attenuated.


LOWPASS: This allows frequencies below the cutoff frequency to pass through the filter. Everything above the cutoff frequency will be attenuated.


BANDPASS: This filter has two cutoff frequencies: one at the low end, one at the high end. It allows the frequencies between these two cutoffs to pass through. Everything above or below the cutoff frequencies will be attenuated.


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A pass filter can be described as a REMOVAL FILTER. Because of their dramatic effect on the signal, the lowpass and the bandpass are rarely used in a mixing context. On the other hand, the highpass  filter is often used to reduce rumble and noise below the fundamental frequency of a particular instrument.

Note that the CUTOFF FREQUENCY is not the point where attenuation starts to occur, but where the filter is already reducing the amplitude by 3dB.

This filter can be described as a REDUCTION (or BOOST) FILTER. At the cutoff frequency, the level is reduced or boosted a set number of decibels, then stays at that level throughout the rest of the frequency spectrum. This results in a filter pattern that looks like a shelf.

There are two types of shelving filters.

HIGH SHELF: This filter reduces or boosts the amplitude above the cutoff frequency.


LOW SHELF: This filter reduces or boosts the amplitude below the cutoff frequency.


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The high shelf is helpful in the mixing context to direct attention. Our ear is naturally attracted to brighter sounds, so a slight boost of high end frequencies can direct the listener to particular instruments or the vocals.

The low shelf is helpful for boosting bass instruments and giving a mix a little more ooompf at the low end.

    What’s in a name? A highpass but a low shelf… ?

    Pass filters describe which frequencies pass through the filter unaffected — ie highpass filters let higher frequencies above the cutoff pass unchanged.

    Shelf filters, on the other hand, are named according to which frequencies are affected — ie high shelves boost/attenuate the amplitudes of frequencies above the cutoff.


This filter creates a bell-shaped notch in the frequency spectrum. Both the width and depth of the notch is adjustable. This filter is most often applied in the low-mids (between 100hz and 2000hz) or the high-mids range (between 400hz and 8000hz.)

Instead of a cutoff frequency, the parametric filter uses a center frequency for its calculations. There is also a range of frequencies to either side that is affected by the filter. This bandwidth – known as Q – can be broad (low Q) or narrow (high Q). In other words, Q is a measure of how selective the filter is: a higher Q factor will give a more narrow notch. The gain control on the filter determines how much amplitude will be boost or cut at and around that center frequency.


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Although a parametric EQ can be used boost particular frequencies or ranges of frequencies, it is often used as a NOTCH FILTER to remove unwanted resonances in a piece of audio. In this case the filter is set with a high Q factor (a very narrow range of frequencies.)


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Thank for taking the time to read and critique my lesson. Any feedback you have would be great.

I know that this topic wasn’t one that Louden suggested we do as our assignment, but I was struggling to get the basics happening with the DAW I’m using (I downloaded the trial version of Reaper and have been slowly working my way through how to set up tracks, to record sound and so forth.) I don’t think it would have been much use for either you or me to bash my head against any of the topics we were given this week. It’s enough for me to figure out how to arm a track, let alone learn the quirky features of the DAW-specific effects plug ins and then do a video about it!

So in order to get as much out of this six-week course as possible, I have chosen to focus on the theoretical side of things. When this course is over I will review the course material, and apply a more hands on approach to what we’ve covered.

About the topic I chose: filters are cool! I’ve sat in on mixing / mastering session before so I have an idea of how powerful EQ is. I can’t wait till I feel more comfortable moving around the mixer and can really check out the different filters myself. Very interesting to learn about their different uses.

Thanks again! Hope you enjoyed!
Best, Karyn


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