XO Wave has two basic EQ effects: a simple Butterworth Filter, and a Multi-band Parametric EQ, which offers several pre-configured arrangements of multiple Butterworth filters. We'll start with the standalone Butterworth Filter, and then review the multi-band options. You may also want to read How to use EQ for some pointers on using EQ in your mix.

Butterworth Filter

Butterworth filters are essentially the same filters found on most analog and digital equalizers. The basic operation and sound of these filters should be familiar to anyone who has used an equalizer.

XO Wave's EQ comes in several flavors (subtypes): High Cut (also called a "low pass filter", and labelled H-Cut inside XO Wave), Low Cut (also called a "high pass filter", and labelled L-Cut inside XO Wave), Band Pass (B-Pass), Band Stop (B-Stop), Boost/Cut (B/C, the default Butterworth Filter), Low Shelf (L-Shelf), and High Shelf (H-Shelf). You will probably find yourself using different types of filter at different times, so we'll describe them all, along with possible uses for each. We'll save Band Pass and Band Stop for the end because they tend to be used least often in audio production.

The Butterworth Filter allows you to change the filter subtype with a pop-up menu at any time.

High and Low Cut filters

As their name implies, the Low Cut and High Cut filters eliminate frequencies at the extremes. The Low Cut filter, for example, eliminates low frequencies, while leaving high frequencies mostly unchanged. This is extremely useful for eliminating rumbling sounds from a track. It can also be used as a more noticeable effect, such as thinning out a lead instrument so it cuts through a mix better.

The High Cut filter, though it's typically less used than the Low Cut filter, can be used for reducing high frequency noise, such as is found on bass and kick drum tracks. It can also be used to soften a harsh-sounding track such as an overly distorted guitar track.

High Cut and Low Cut filters offer only one control: frequency. This is the "knee" point past which most sounds are eliminated. For example, a Low Cut filter with the frequency set to 200 Hz will eliminate most sounds below 200 Hz and leave sounds above 200 Hz more or less untouched. Similarly, a High Cut filter with the same frequency setting will reduce most sounds above 200 Hz and keep most sounds below 200 Hz.

High and Low Shelf filters

High and Low Shelf filters are useful for increasing or reducing a broad range of either high or low frequencies. Shelving filters are useful when the overall balance of high and low frequencies is off. For example, if the bass is a little weak, you could use a Low Shelf to bring it up a bit. Similarly, if a mix seems a little dark, you might be able to use a High Shelf to add a little high frequency. If the mix is, instead, a little harsh, you can use a High Shelf to reduce the high frequencies without completely eliminating some frequencies, as would happen with a High Cut.

High Shelf and Low Shelf filters have two controls: frequency and level. The frequency control, like the frequency control of the high and Low Cut filters, controls which frequencies will be altered by the filter. The level control adjusts how much of the selected frequencies will be added or taken away. For example, if you have a Low Shelf filter with the frequency set at 1,000 Hz (1 kHz), and the level set at -5 dB, all sounds below 1,000 Hz will be reduced by approximately 5 dB, and other frequencies will remain unaffected.

Boost/Cut filter

The Boost/Cut filter is probably the one you will use most often. It allows you to pick out a range, or band, of frequencies and increase or decrease the sounds inside that range. This is extremely useful for a wide range of applications. For example, you can add mid and high frequencies to a track to help it stand out in the mix better, or you can reduce a certain frequency that is conflicting with another instrument which is also strong in that range. It is also useful if, for example, a particular instrument has an unpleasant quality that falls in a particular frequency range. For example, if a particular vocal track sounds very nasal, you can use a Boost/Cut filter to reduce the offending frequencies (probably somewhere between 400 Hz and 2,000 Hz).

The Boost/Cut filter has three controls: frequency, Q (Sharpness), and level. The frequency control adjusts the center of the range of frequencies which will be boosted (increased) or cut (reduced). Q controls the range of frequencies that will be boosted or cut. The higher the Q, the narrower the range of effected frequencies. Level controls how much of the given range of frequencies should be added or removed.

For example, a frequency setting of 500 Hz, a Q of 1.3, and a level of 7dB, will increase a fairly broad range of frequencies around 500 Hz by 7 dB.

Band Pass and Band Stop

You probably won't find too much use for the Band Pass and Band Stop filters, but sometimes they can be very useful. A Band Stop filter is like an extreme version of a band cut filter -- it eliminates a range of frequencies almost entirely. A Band Pass filter is the opposite: it allows only a small range of frequencies to pass through.

Band Pass filters, though seldom used, can be useful for special effects or for training your ear as to what certain frequencies sound like. One special effect can be created by using a low Q and a frequency roughly in 1000 Hz range, which makes the track sound like it's coming over a telephone line. Using a higher Q allows you to isolate certain frequencies, which can produce a "ringing" effect.

A Band Stop filter can be used for eliminating specific frequencies. This might be used for eliminating narrow band noise, such as 60/50 Hz electrical noise or noise from machinery. When eliminating electrical noise, it's important to recognize that a significant amount of the noise may be at harmonics, which are whole-number multiples of the fundamental (or main) frequency. For example, the US uses 60 Hz power but it is not uncommon to have a substantial amount of noise at 120 Hz or even 180 Hz or 240 Hz.

Multi-band Parametric EQ

The Multi-band Parametric EQ is essentially nothing more than several Butterworth filters put together. When creating this effect, you will be asked for the EQ type, which lets you select how many "bands" you want, where one band is equivalent to one Butterworth Filter. In the case of some EQ types, you will have the option of having the lowest frequency filter be either a low cut filter or a Boost/Cut filter. While you can change the exact filter types and center frequencies later, it is a good idea to start with the filter as close to what you need as possible. Most engineers find that with a parametric EQ, they seldom need more than 5 bands, and 3 is often fine, so if you are unsure how many bands to use, start with between 3 and 5. Note that you cannot add or take away bands later, but just about everything else can be changed.

The main window of the parametric EQ shows you the level controls for each Boost/Cut filter and the frequency control for each High or Low Cut filter. For more fine-grained control of each filter, you can either use the tabs at the top to switch to the band you want. If you want to see all the controls at once, you can click the "All-In-One View" button.