In audio filters, slope refers to how quickly frequencies are attenuated by the filter once the cutoff frequency is passed. Slope is given as a dB/octave figure. For example in a high pass filter with a cutoff frequency of 4000 Hz, and with a slope of 6 dB/octave, for each octave (doubling of frequency) above 4000 Hz, the level of frequencies will be diminished by an additional 6 dB. Slope is determined by the “order” of the filter, or the number of poles it contains. A first order, or single pole filter will have a slope of 6 dB/octave. A second order, or two pole filter will have a slope of 12 dB/octave, and so on (slope increases by 6 dB/octave per order or pole).
Creating the correct slope is very important in filter design. For example, it determines how accurately an EQ can cut or boost some frequencies without affecting others. Slope is also important in crossovers, where it is undesirable for frequencies beyond the cutoff frequency to be passed on to amplifiers and drivers (typical crossover filter slopes are in the 12-24 dB/octave range). Sometimes crossovers feature selectable filter slope so that response can be matched to particular speaker set ups.