This format of numbers and abbreviations (dB/oct = decibels per octave) is often used to refer to the frequency response behavior of a filter. A filter typically has a cutoff or corner frequency it is tuned to. It then reduces (filters) the frequency spectrum of a signal going through it so that its loudness is multiples of 18 decibels weaker for each octave further away you get from the cutoff frequency.
An 18dB/octave filter is often referred to as a “three pole” filter (as each pole of a filter’s design results in 6dB of attenuation). This filter response is most often associated with the vintage Roland TB-303 as well as EMS synthesizers (although in reality, their designs had 4 poles), and is often used a coded shorthand for when someone wants to refer to acid-type bass lines from a TB-303 without mentioning the instrument by name.
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