Last week we looked briefly at filters. In the third part of our EQ mini series, we will take a quick look at three types of EQ response curves.
A low shelf response allows the signal below the corner frequency to be boosted or attenuated. The corner frequency can be very simply defined as the point at which the signal falls to 3dB below the boost or gain value. In Figure 1 this point would be around 160Hz. This -3dB reference is not strictly correct for all applications (there is no -3dB point for a boost of 1dB for example), however it does serve as a good guide and a true derivation is beyond the scope of this brief introductory series.
A high shelf is the opposite of a low shelf, boosting or attenuating all frequencies above the corner frequency. Some example plots are shown in Figure 2.
A bell curve boosts or attenuates its center frequency, and a range of frequencies around it. The range of surrounding frequencies it affects is determined by its Q value. A low Q spans a larger range of frequencies and is generally said to result in a more ‘musical’, or pleasant sound. A narrow Q affects fewer surrounding frequencies and can be used for more precise frequency shaping but can tend to sound less natural, especially at higher gain settings. Figure 3 illustrates the shape of a bell response.