EQs are built around filter circuits which allow some frequencies to pass and reject others. You might well have noticed in the diagram that the response the other side of the centre or ‘cutoff’ frequency does not just drop to zero instantly, but gradually decreases. This is because it is not possible to build real-time filters that are infinitely steep, and the resulting shape is called the ‘slope’ of the filter, measured in dB per octave. So for example, a 6dB per octave filter’s slope will reduce or increase gain by 6dB for every doubling of frequency.
