Equalization

ToDo Needs review by Martyn

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 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 is composed of a blue curve which joins together a number of white circles called "Control Points", and a green curve which follows the general shape of the blue curve. 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 graph. 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.

Controls

• Vertical scale sliders: By default the vertical scale reads from + 30 dB to - 30 dB, but these two sliders to left of the scales 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/GraphicEQ: These two options select whether the equalization is defined by drawing a curve with control points or by using on-screen sliders as in a graphic equalizer. The image above shows the Draw Curves view where a graph of Frequency against Gain is drawn. The image below shows the Graphic EQ, where a set of sliders control the gain at pre-set frequencies. /// needs expanding as this will be most used..... explain graphic EQ is a log view somewhere.. what is the width of the bands..... what is the increment on the sliders...///
• Linear Frequency Scale: You can choose between a logarithmic display which roughly corresponds to the way we perceive pitch, and a linear display which shows equal frequency ranges on each unit of the scale. It's more usual to have a logarithmic frequency scale as shown above which gives more detail at the lower frequencies. A linear scale can be useful for precision adjustments at high frequencies.
• Length of filter: Sets how much audio Audacity processes with each step. Generally its best left at the default setting. However if you find the green curve (see above) that Audacity uses to perform the effect is very different from the blue curve you drew, try increasing the length of the filter so that the green curve runs closer to the blue. Note that the effect will take longer to process with a longer filter, and using a shorter filter to produce a smoother green curve may actually sound better, unless you are modifying very low frequencies.
• Select curve: Used to choose between pre-set curves that have already been defined.
• Save as: Used to remember a setting you have created as a pre-set curve
• Delete: Used to entirely remove a setting you don't want any more.
• Flat: 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.