Should the slope of your filter be 6, 12, 18 or 24 dB per octave?

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of

Tuesday August 13, 2019

Low-pass and high-pass filters often have a 'slope' control. What is the slope of a filter? How do you select the right slope?

Pass band and stop band

A low-pass filter, for example, is a circuit (or digital equivalent) that allows low frequencies to pass and cuts high frequencies. What's interesting is what happens in the transition between pass and cut.

Most filters do not simply pass certain frequencies, then suddenly at a certain point cut the rest of the frequency band completely. A so-called 'brick wall' filter is neither useful nor sounds good in most circumstances.

Instead there is a 'pass band' in which signals get through unaltered, then a gentle transition to the 'cutoff frequency' where the level is 3 dB lower than in the pass band. Then the response falls off further and further down into the 'stop band'.

6, 12, 18, 24 dB/octave slopes

If the simplest possible filter is constructed from electronic components - one resistor and one capacitor - then the rate at which the level drops in the stop band will be 6 decibels per octave.

This means that for each successive doubling of frequencies above the cutoff frequency, the response falls 6 dB. (This is the same as saying 20 dB per decade - a drop of 20 decibels for every tenfold increase in frequency).

A slightly more complex filter design can achieve 12 dB/octave; further increases in complexity can yield 18 dB/octave and 24 dB/octave. Oddly enough, in-between values are more complex to achieve, and therefore pointless to attempt.

What use are these different filter 'slopes'?

A 6dB per octave slope is useful for gentle shaping - a little less bright, a little less heavy, depending on whether the filter is high-pass or low-pass. 12 dB/octave is more useful in a creative musical context. 18 dB/octave even more so as you can cut out great swathes of frequencies and hear hardly anything that you don't want left behind.

24 dB/octave is a little harsh for most purposes. It's like a cliff edge rather than a steep hill, and you can hear the 'edge' where the response suddenly starts to fall.

24 dB/octave in synthesizers

However in synthesis - subtractive ('analog') synthesis, then 24 dB/octave is perfect as it can remove high frequencies while still leaving 'bite' in the sound. Some synthesizers have filters with even steeper slopes.

Filters in loudspeakers

Loudspeaker crossovers also have filters. 12 dB/octave and 18 dB/octave slopes are commonly used to separate bands of frequencies between woofer, mid-range and tweeter. The steeper 24 dB/octave slope is usually only found in PA systems where an active crossover (for which a 24 dB/octave filter is easier to implement) divides the frequency bands before the signals pass through to the power amplifiers.

Like, follow, and comment on this article at Facebook, Twitter, Reddit, Instagram or the social network of your choice.

Come on the Audio Masterclass Pro Home Studio MiniCourse - 60 great hints and tips to get your home recording studio MOVING

It's FREE!

Get It Now >>

An interesting microphone setup for violinist Nigel Kennedy

Are you compressing too much? Here's how to tell...

If setting the gain correctly is so important, why don't mic preamplifiers have meters?

The Internet goes analogue!

How to choose an audio interface

Audio left-right test. Does it matter?

Electric guitar - compress before the amp, or after?

What is comb filtering? What does it sound like?

NEW: Audio crossfades come to Final Cut Pro X 10.4.9!

What is the difference between EQ and filters? *With Audio*

What difference will a preamp make to your recording?

Watch our video on linear phase filters and frequency response with the FabFilter Pro Q 2

Read our post on linear phase filters and frequency response with the Fabfilter Pro Q 2

Harmonic distortion with the Soundtoys Decapitator

What's the best height for studio monitors? Answer - Not too low!

What is the Red Book standard? Do I need to use it? Why?

Will floating point change the way we record?

Mixing: What is the 'Pedalboard Exception'?

The difference between mic level and line level

The problem with parallel compression that you didn't know you had. What it sounds like and how to fix it.

Compressing a snare drum to even out the level

What does parallel compression on vocals sound like?

How to automate tracks that have parallel compression

Why mono is better than stereo for recording vocals and dialogue