What are EQ Filters
Every Equalizer uses filters in order to cut, boost, and shape the audio signals passing through it. There are different types of filters – the most common are Low Pass Filters (LPF), High Pass Filters (HPF), Notch Filters, Bell Filters, Bandpass, and Shelf Filters, to name a few. We will only cover a few of these filter types in this booklet – High and Low pass filters, Bandpass, and Notch Filters (the ones you need to be familiar with as a beginner audio technician). We will also explain some EQ basics, such as Q, Slope, and how Equalizer adjustments can affect the “Gain” of an audio signal. Filters can sound confusing and intimidating at first, so we will keep it as “simplified” as possible while still trying to ensure that the reader understands the concepts.
High Pass Filters (or Low-Cut Filter)
A High Pass Filter passes High Frequencies and “attenuates” the Low Frequencies below the “Cut-Off” Frequency point. There is some lingering confusion here because when you need to “Cut” some low frequencies, you would use a High-Pass Filter (which is also technically called and referred to as a “Low-Cut” filter). Both terms are used to get the same result. Make sure you understand the references noted above.
Low Pass Filters (or High-Cut Filter)
The opposite can be said regarding a Low-Pass Filter which will pass Low Frequencies and “attenuate” the High Frequencies below the “Cut-Off” point. Again, if you need to “Cut” some high frequencies, you would use a Low-Pass Filter (which is also technically called and referred to as a “High-Cut” filter). Similarly, these terms are both used to get the same result. Make sure you understand the references noted above.
Band Pass Filter
This type of filter removes (or attenuates) a specific range (band) of frequencies below or above a center frequency. A Bandpass filter can actually be thought of as a combination of High-Pass and Low-Pass filters. This type of filter is “controlled” by a High and a Low setting (Cut-Off).
Example of a typical 100Hz High-Pass Filter on a mixing board (shown below). When you press this switch, it activates a “steep” 18dB per Octave filter that attenuates the level of the Low-end frequencies only (Bass). This filter is mainly used in Live Sound situations to help “clean up” a mix – such as reducing stage rumble or even popping from microphones.
Filter Notes
What is Slope? Slope is commonly used interchangeably with “Roll Off.” Slope references how fast “attenuation” occurs by a filter once the frequencies pass the selected “Cut-Off” point. Slope is measured in dB / Octave (decibels per octave). Typical filter slopes are 6dB, 12dB, and 24dB.
Notch Filters – A type of filter that removes a single frequency or a very narrow band of frequencies, and has a very high “Q” value (narrow bandwidth). You will find Notch Filters on most larger Mixing Board input channels.
Equalizer Gain – This is the amount of “Cut” or “Boost” you use when setting up your EQ. The main point that you need to remember regarding EQ Gain is that it will affect the “overall” levels (gain) of the sound system.
What does “Q” Mean?
“Q” is short for “Quality Factor” as it relates to Equalization. It is the “width” of an EQ band that you adjust – the ratio of center frequency to bandwidth. When adjusting a “wider” area band of frequencies, it means a lower Q Value. These Lower Q Values allow a boost or a cut in a wider range of frequencies. When adjusting a “narrower” area band of frequencies, it means a higher Q Value (narrower bandwidths). Narrow bandwidth is ideal for removing (notching out) unwanted tones and for boosting pleasing / desired tones. While the subject of “Q” can continue, our goal is to ensure that you understand the concept. As a technician, you will most likely not be dealing with “Q” a whole lot. On the other hand, a Sound Board Operator must understand “Q” and how it affects their “mix.” And that usually takes years and years of hands-on training.
