Equalizers are in most cases described to the reference of what is called a “flat response curve”. This means that system or a piece of equipment responds equally to all frequencies within the specific range. This range is usually set to 20 Hz to 20kHz. In case that some frequencies are boosted or cut, it is useful to describe the frequency response graph with the help of a visual analogy.
Shelving Equalizer 101
In a shelving EQ, a band of frequencies is boosted or cut either in the high-frequency end of the spectrum 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 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.
The shelving equalizer offers the peak/deep response on the side of the selected centre frequency, and a flat cut or boost region. A broad equalization desire might be to add brightness to the sound in general. A high-frequency shelving equalizer that is increased for 6 dB at 8 kHz will raise the output from about 8 kHz and above. It is not limited to a centre frequency or and its associated bandwidth. The resulting alternation in frequency response is flat (like a shelf) beyond the selected frequency. The concept of shelving equalizer applies to low frequencies as well as high, and cuts as well as boosts. In all cases, there is a flat region beyond that selected centre frequency, which is boosted or attenuated.
The transition back to the unprocessed region is simply half of the bell curve of a peak/dip contour. Therefore, the idea of the 3 dB down point and the concept of the bandwidth and the Q apply to the shelving EQ as well. Boosting by 9 dB at 10 kHz and above is, in fact, a 3 dB down (+6dB) at 10 kHz, achieving the full boost (9 dB) a little bit above 10 kHz. Below 10 kHz, this shelving EQ is thought not to be processing the signal. This is true well below 10 kHz, but just below 10 kHz, the shelving EQ starts to lift the signal amplitude up.
A high-Q shelf abruptly transitions from the region of unaltered frequency response to the region of the 9 dB boost. A low Q spreads that transition out across a broader spectral region. Both approaches have production value, in right circumstances, so the engineer must pay a careful attention and listen intensely.
Easy to use
Given that the shelving equalizers have few controls and work on the two most easily identifiable components of the audio spectrum (bass and treble), they tend to be one of the easiest equalizers to use. However, what the shelving equalizer gains in ease-of-use it lacks in respect of flexibility. This is effectively limiting you to just tailoring the basic timbral qualities of a sound. Whether adding a little bit of a high-end sparkle or some extra bass-end weight.
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