EQ is always described with reference to what we call a 'flat frequency response'. This means that a system or piece of equipment responds equally to all frequencies within a specified range, normally 20 Hz to 20 kHz.
When certain bands of frequencies are boosted or cut, it is useful to describe the frequency response graph by means of a visual analogy.
Hence the term 'shelf' or 'shelving EQ' for the frequency response graph shown.
In a shelving EQ, a band of frequencies is boosted or cut either in the high frequency end of the spectrum as shown, or in the low frequency end. 'Shelving' is not a term that is ever applied to a mid-range boost or cut.
So in a shelving EQ, all frequencies above or below a certain frequency are boosted or cut by the same amount. The same change in level is applied all the way to the limit of the frequency spectrum.
Other forms of EQ include the 'bell' where a certain band is boosted, but higher and lower than that the response returns to normal. Another type steadily increases the response towards the extreme end of the spectrum, however this is not common in sound engineering practice.
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