A parametric equalizer, as the name implies, has more parameters than the graphic equalizer, making it more flexible and useful for professional audio engineering. Figure 7.4 shows a parametric equalizer. The different icons on the filter column show the types of filters that can be applied. They are, from top to bottom, peak-notch (also called bell), low-pass, high-pass, low shelf, and high shelf filters. The available parameters vary according to the filter type. This particular filter is applying a low-pass filter on the fourth band and a high-pass filter on the fifth band.
[aside]The term “paragraphic EQ” is used for a combination of a graphic and parametric EQ, with sliders to change amplitudes and parameters that can be set for Q, cutoff frequency, etc.[/aside]
For the peak-notch filter, the frequency parameter corresponds to the center frequency of the band to which the filter is applied. For the low-pass, high-pass, low-shelf, and high-shelf filters, which don’t have an actual “center,” the frequency parameter represents the cut-off frequency. The numbered circles on the frequency response curve correspond to the filter bands. Figure 7.5 shows a low-pass filter in band 1 where the 6 dB down point – the point at which the frequencies are attenuated by 6 dB – is set to 500 Hz.
The gain parameter is the amount by which the corresponding frequency band is boosted or attenuated. The gain cannot be set for low or high-pass filters, as these types of filters are designed to eliminate all frequencies beyond or up to the cut-off frequency.
The Q parameter is a measure of the height vs. the width of the frequency response curve. A higher Q value creates a steeper peak in the frequency response curve compared to a lower one, as shown in Figure 7.6.
Some parametric equalizers use a bandwidth parameter instead of Q to control the range of frequencies for a filter. Bandwidth works inversely from Q in that a larger bandwidth represents a larger range of frequencies. The unit of measurement for bandwidth is typically an octave. A bandwidth value of 1 represents a full octave of frequencies between the 6 dB down points of the filter.
