Equalization is the process of adjusting audio by increasing or decreasing specific frequencies. We can pinpoint extremely small frequency bandwidths for adjustment or very wide regions and everything in between.
We can raise certain frequencies while decreasing others simultaneously. There are many reasons for doing this. With music, equalization can make a song sound better and create a more pleasant listening experience. With recorded voices, equalization can make the voice more intelligible.
For example, if the 2500 hz range is too low, consonants like the “k” sound may not be intelligible. If there isn’t enough 4000 hz, the articulation of “t” may be inaudible.
Here is an image of a spoken word file being equalized.