Sound frequency and human are key to understanding equalisation. The frequency of an item can be defined as the number of times that an event occurs with in a given time period.
In the context of sound the given time period is one second, and the frequency, or event, is the number of times a wave form goes through one complete cycle. The time period is known as the periodic time.
By subtracting T1 from T2 (See Graph) we get the periodic time in seconds. The frequency can then be calculated by dividing the periodic time by one. It is also possible to calculate the time from the frequency by doing the reverse.
When talking about frequency it can be expressed as the number of complete cycles a wave form goes through in one second. The simplest wave form is a sine wave. A Sine wave can be seen in the period graph shown above.
Here is an example for a periodic time of 13ms:
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This player demonstrates a Sine wave sweep from 20hz to 20Khz
When talking about frequency it is also important to understand that the human hearing range is from 20 to 20,000 cycles per second. The higher a given Frequency, the shorter its wave length.
It is much harder to localise the location of low frequency sounds than higher frequency sounds.
You might also like to read this, Setting Mix levels in the Studio
Or possibly this Balanced Audio connections