Equalization

From Audacity Manual

Equalization is a way of manipulating sounds by Frequency. It allows you to increase the volume of some frequencies and reduce others. This is a more advanced form of the EQ and Tone controls on many audio systems. As an example of equalization, the curve shown below changes the balance of high and low frequencies in the audio to make it sound like an AM radio broadcast. High frequencies (above 6000 Hz) and low frequencies (below 100 Hz) are reduced in volume by 20 dB.

Accessed by:

The Equalization effect has two modes: "Draw curves" and "Graphic EQ". These modes are selected by the radio buttons below the graph. The graph scale with sliders and most Equalization settings are common to both modes.

Graph Scale and Sliders

Vertical Scale: This scale is in dB and shows the amount of gain (amplification above 0 dB or attenuation below 0 dB) that will be applied to the audio at any given frequency.
Horizontal Scale: This shows the frequencies in Hz to which volume adjustments will be applied. Dragging the Equalization window wider displays some additional points on the scale and makes it easier to plot the graph accurately.
Vertical scale sliders: By default the vertical scale reads from + 30 dB to - 30 dB, but these two sliders to left of the scale let you adjust the upper and lower dB values so as to change the visible range on the graph. Note that moving either slider changes the horizontal position of the 0 dB line. Reducing the visible range lets you make a finer adjustment to how loud the frequencies sound, but the changes will be more subtle because the volume adjustment will be less.

Draw Curves

Equalization Curves and Control Points: If you look closely at the curve in the image above, you'll see it's composed of a blue curve joining together a number of white circles, and a green curve which follows the general shape of the blue curve. The white circles are called "control points". When in "Draw Curves" mode, the blue curve is drawn by either clicking in the graph at any position, or clicking on the blue curve and dragging it to a position. Doing either creates a control point at that position, then creating further control points draws the curve. To remove a control point, drag it outside the graph.
The green curve is the one that Audacity actually uses to perform the effect, taking into account the limitations of the equalization algorithm. The green curve usually follows the blue curve closely, but will be forced to a smoother path if there are sudden changes in amplitude over a small frequency range.
Linear Frequency Scale: When this box is unchecked, the horizontal frequency scale is logarithmic, giving more detail at the lower frequencies. This corresponds roughly to our greater sensitivity to lower frequencies. When unchecked, the frequency scale is linear, displaying equal frequency ranges for each unit of the scale. This can be useful for precision adjustments at high frequencies. This control is available only in "Draw Curves" mode.

Example: You want to make an audio selection sound "brighter" by reducing the frequencies below 100 Hz by 10 dB, and increasing those over 5000 Hz by 10 dB:

If the line in the graph is not already horizontal at the 0 dB position, click "Flat" (see below).
Click at the point that is opposite both -10 dB on the vertical scale and 100 Hz on the horizontal scale.
Click at the point that is opposite both +10 dB on the vertical scale and 5000 Hz on the horizontal scale.
Create extra control points if desired between 100 Hz and 5000 Hz to modify whether particular frequencies between those two levels should be reduced or increased in volume.

Note: No account is taken of whether your equalization curve will result in the selection as a whole being amplified beyond the clipping (distortion) level. If your curve involves amplifying any frequencies, especially lower frequencies which are normally the loudest ones, normalize the track(s) first to -3 dB or lower.

Graphic EQ

Frequency Sliders: In "Graphic EQ" mode, the equalization curve is drawn by manipulating a set of sliders. Each slider adjusts the gain of a specific range of frequencies, the gain being maximized at (centered on) the frequency stated on the slider. Click and drag the slider up or down to increase or decrease the volume by a maximum of 20 dB. You can also TAB between each slider. There are several tricks to get to an exact slider value. Clicking above or below the slider makes it jump by 8 dB then 4 dB increments. Using the arrow keys on the keyboard increments by 1 dB. Holding down SHIFT then either clicking or using the arrow keys increments by 0.1 dB. The current value of the slider can be seen by hovering over it with the mouse.

In "Graphic EQ" mode only, a dropdown box lets you choose between three different interpolation methods. B-spline tends to reduce somewhat the amount of gain set on the sliders, whilst spreading it to more of the surrounding frequencies. Cubic affects the surrounding frequencies the most, introducing a small opposite gain (for example an attenuation if you specified an amplification) at frequencies furthest from the frequency stated on the slider.

Equalization settings and presets

Draw Curves: Selecting this radio button gives you the "Draw Curves" mode, in which the equalization curve is drawn by manipulating control points.
Graphic EQ: This button switches to the simpler "Graphic EQ" mode, in which the equalization curve is drawn by adjusting the frequency sliders.
Length of filter: Sets how much audio Audacity processes with each step. Generally, it's best left at the default value of 4001. However if the green curve Audacity uses to perform the effect is very different from the blue curve you created, try increasing the length of the filter. Note that the effect will take longer to process with a longer filter. Also, using a shorter filter to produce a smoother green curve may actually sound better, unless you are modifying very low frequencies.
Select curve: Click the drop-down triangle to select from a list of preset equalizations. These are either your own saved presets (see "Save/Manage Curves" below) or built-in presets, mostly playback equalizations for gramophone records. These could be used to equalize an LP or 78 rpm disk recorded into Audacity without equalization. A curve will display as a "custom" curve if it is not yet saved as a preset, or if it is a modified preset. There will also be the following instances where curves can only be matched approximately, so will display as "custom":
- In "Graphic EQ" mode, built-in presets will display as "custom" because they were built using the differently calculated "Draw curves" mode.
- Your own saved presets will always display with their saved name if loaded in the same mode you saved them in. In "Graphic EQ" mode, only presets saved in that mode will display with their saved name. In "Draw Curves" mode, a curve created in "Graphic EQ" mode can be displayed as its saved name if loaded from within "Draw Curves". If that curve is loaded in "Graphic EQ" mode and then switched to "Draw Curves", it will display as "custom".
- Any curve in either mode will display as "custom" if there are points in the curve that are outside the range of the Horizontal Scale. For example, if you have a point at 10 Hz in a curve saved in linear view (where the scale starts at 0 Hz) then switch to logarithmic view (where the scale starts at 20 Hz), the curve will switch to "custom". In that case Audacity will put a point in at 20 Hz at the dB level you wanted at 10 Hz.
Save/Manage Curves: This brings up the "Manage Curves" dialog. See below for details.
Flatten: A quick way to set a "level response curve". This means the curve on the graph is drawn from left to right at 0 dB on the vertical scale, so that no frequencies will have their volume level modified.
Invert: Turns the current curve in the window upside down, changing positive gains at a particular frequency into negative, and vice-versa. To negate incorrect RIAA equalization of a 78 rpm record that was recorded with a modern turntable, you can apply the already supplied "Inverse RIAA" curve in the "Select Curve" drop-down, without using the "Invert" button.

Manage Curves

Move Up: Move the selected preset up in the list.
Move Down: Move the selected preset down in the list.
Rename: Give the preset a new name.
Delete: Remove the preset from the list.
Import: Import new presets from an XML file, adding them to the current list.
Export: Export the current preset list to a new XML file. You could use this to export your custom EQ curves to share them on the EQ Curves Download page on the Audacity wiki.
Get More: Clicking this button takes you to the EQ Curves Download page on the Audacity wiki, where you can download the latest default curve set, or other curve sets created by users for specific purposes such as applying the proper equalization to 78 rpm records.
Defaults: Restore the list to the default setting. Warning: you will lose any custom presets you have created.
Cancel: Close the dialog without making any changes.
OK: Close the dialog and accept the changes.

To save a new preset:

Select "unnamed" in the "Curves" list
Click the "Rename" button
Give the new preset a unique name
Click the "OK" button

Note that you cannot modify an existing preset curve and then save it under the same name. For example, if you select the "amradio" curve in the "Select Curve" dropdown menu, modify it, then click the "Save/Manage Curves" button, then click "amradio" in the Curves list of the "Manage Curves" dialog, then click "OK", all that will happen is that the original "amradio" curve will be loaded into the Equalization effect.

EQCurves.xml file: This is a text file which permanently stores the last curve visible on the graph, plus all the named presets. It's located in the same "Audacity" folder as your Preferences configuration file.