Equalization

ToDo Needs review by Martyn

Equalization is a way of manipulating sounds by Frequency. It allows you to boost some frequencies and reduce other frequencies. 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.

The Graph

• Vertical Axis This shows the amount of amplification (above zero) or attenuation (below zero) that will be applied to the audio. The scales on the upper and lower parts of the graph can be adjusted independantly with the two sliders on the left hand side. A smaller range will give you more detailed adjustment, but only allow you to make more subtle changes to the sound.
• Horizontal Axis This shows the range of frequencies that will be affected by the equalisation. 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.
• Blue Line: This curve is constructed from the controls, either the control points on the Draw Curves interface or the sliders in the Graphic EQ. It represents the idea effect that audacity will try to get as close as possible to. Parts of the curve above the thin horizontal line mean boost those frequencies, parts below mean reduce those frequencies.
• Green Line: This is the curve Audacity will actually use, taking into account the limitations of the algorithm. It's usually pretty close, but the effect cannot deal with very sudden changes in amplitude over a small frequency range. If the curves are very different, check the setting for length of filter - a short filter cannot produce a sharply changing curve, although it will be quicker and may sound better.
• Control points: In the Draw Curves view you can click on the blue line to add a control point and drag it around the graph to change the shape of the curve to suit your needs. To get rid of a control point drag it off the edge of the graph, and watch the curve snap back into shape.

Parameters

• 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: 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. Best left at the default setting. Large values will tend to "smear out" ????? the sound. Small values will deal less well??? with 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.