Equalization

ToDo Needs review by Martyn. What do you call the equivalent of a "dropdown" box on Linux? I presume they just scroll? - Gale

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.

Graph

• 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.
• 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 (see below), 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. When in "Graphic EQ" mode, operating the EQ sliders creates the blue curve for you.
  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.

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:

1. Click at the point that is opposite both -10 dB on the vertical scale and 100 Hz on the horizontal scale.
2. Click at the point that is opposite both +10 dB on the vertical scale and 5000 Hz on the horizontal scale.
3. 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.

Controls

• 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: Selecting this radio button gives you the "Draw Curves" view shown in the image above, in which the equalization curve is drawn by manipulating control points.
• Graphic EQ: This button switches to the simple "Graphic EQ" view shown in the image below.
  
  
  In this view, 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 (or centered on) the frequency stated on the slider. Drag the slider up to increase volume by up to 20 dB, or move it down to reduce it by up to 20 dB. You can also increment a slider in 1 dB steps with the arrow keys on your keyboard, or navigate from one slider to another by pressing TAB.
  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.
• 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. The "Graphic EQ" view only displays a logarithmic scale.
• 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 arrow to select from a list of pre-set equalizations. These are mostly playback equalizations for gramophone records, with which you could equalize a LP or 78 rpm disk which you had recorded into Audacity without equalization. Because of differences in calculation between the "Draw Curves" and "Graphic EQ" modes, the box displays "custom" at all times, except when you are displaying an unmodified pre-set curve in "Draw Curves" mode.
• Save as: Allows you to save any curve on the graph as a pre-set equalization with a custom name of your choice.
• Delete: Permanently removes a pre-set equalization from the drop-down list. Only curves saved with a custom name can be deleted when in "Graphic EQ" mode.

The last curve visible on the graph is always remembered, even in a new Audacity session, along with any custom presets. If you accidentally overwrite or delete any of the named presets Audacity shipped with and want to restore them, delete your stored "EQCurves.xml" file. This is in the same "Audacity" folder as your Preferences configuration file. Note however that deleting the xml file will also delete any custom curves you had saved.

• Flat: 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.