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-x/dB Per Octave on a Filter?
Old 2nd December 2014
  #1
Gear Head
 
🎧 5 years
-x/dB Per Octave on a Filter?

Hi,

What does it mean exactly? Does it have relation in-terms of the harmonics of the sound e.g. if the fundamental is 440Hz and there is a harmonic that goes up to 880Hz does that mean that the harmonic is attenuated by -x/dB or what?

Thanks.
Old 2nd December 2014
  #2
Lives for gear
 
🎧 10 years
I'm not an expert, but I believe it is a description of the pole angle of the filter. If this is correct then a steeper filters would have higher values for x (f.e. 24db has a steeper drop to infinity than a 6db filter.

Again, I'm no expert, but hey I've got an opinion and this is the internet.
Old 2nd December 2014
  #3
Lives for gear
 
LNerell's Avatar
 
🎧 10 years
Quote:
Originally Posted by D.E.L.B. ➑️
Hi,

What does it mean exactly? Does it have relation in-terms of the harmonics of the sound e.g. if the fundamental is 440Hz and there is a harmonic that goes up to 880Hz does that mean that the harmonic is attenuated by -x/dB or what?
Most of the time you use filters to subtract from the sound. So, at 440hz, if using a 12db filter 880hz with be 12db quieter than 440hz. A 24db filter 880hz will be 24db quieter. This is the slope of the filter.
Old 2nd December 2014
  #4
Gear Head
 
🎧 5 years
So it is to do with the harmonics then? It lessens the volume of each harmonic that is an octave?
Old 2nd December 2014
  #5
Old 3rd December 2014 | Show parent
  #6
Lives for gear
 
abruzzi's Avatar
 
🎧 5 years
Quote:
Originally Posted by D.E.L.B. ➑️
So it is to do with the harmonics then? It lessens the volume of each harmonic that is an octave?
Not exactly. If you play a above middle c, the fundamental is 440hz, and the harmonics will sound at 880, 1320, 1760, etc. but there will also likely be some non-harmonic frequencies, noise, etc. If you set your filters cutoff to 440hz, then any sound at 880 will be attenuated by 24db, any sound at 1760 will be attenuated by 48db. But those points are just points on a downward slope. The shape of that slope may vary somewhat from filter to filter, but a frequency between 440 and 880 will have an attenuation somewhere between 0db and 24db.

The sonic effect is the more attenuation per octave, the sharper the slope, and the harder the filter. 12db per octave is more gradual and while it quiets higher harmonics, they are usually still audible, while 24db per octave is more of a brick wall, cutting out all everything above the cutoff pretty quickly.
Old 3rd December 2014 | Show parent
  #7
Gear Head
 
🎧 5 years
Quote:
Originally Posted by abruzzi ➑️
Not exactly. If you play a above middle c, the fundamental is 440hz, and the harmonics will sound at 880, 1320, 1760, etc. but there will also likely be some non-harmonic frequencies, noise, etc. If you set your filters cutoff to 440hz, then any sound at 880 will be attenuated by 24db, any sound at 1760 will be attenuated by 48db. But those points are just points on a downward slope. The shape of that slope may vary somewhat from filter to filter, but a frequency between 440 and 880 will have an attenuation somewhere between 0db and 24db.

The sonic effect is the more attenuation per octave, the sharper the slope, and the harder the filter. 12db per octave is more gradual and while it quiets higher harmonics, they are usually still audible, while 24db per octave is more of a brick wall, cutting out all everything above the cutoff pretty quickly.
Great post, cheers mate.
Old 3rd December 2014 | Show parent
  #8
Gear Head
 
🎧 5 years
Quote:
Originally Posted by abruzzi ➑️
Not exactly. If you play a above middle c, the fundamental is 440hz, and the harmonics will sound at 880, 1320, 1760, etc. but there will also likely be some non-harmonic frequencies, noise, etc. If you set your filters cutoff to 440hz, then any sound at 880 will be attenuated by 24db, any sound at 1760 will be attenuated by 48db. But those points are just points on a downward slope. The shape of that slope may vary somewhat from filter to filter, but a frequency between 440 and 880 will have an attenuation somewhere between 0db and 24db.

The sonic effect is the more attenuation per octave, the sharper the slope, and the harder the filter. 12db per octave is more gradual and while it quiets higher harmonics, they are usually still audible, while 24db per octave is more of a brick wall, cutting out all everything above the cutoff pretty quickly.
Great post, cheers mate.
Old 3rd December 2014
  #9
Lives for gear
 
ImNotDedyet's Avatar
 
🎧 5 years
Synth Secrets, Part 5: Further With Filters goes into a bit more detail on how filters work and on abruzzi's post.
Old 3rd December 2014 | Show parent
  #10
Gear Head
 
🎧 5 years
Quote:
Originally Posted by ImNotDedyet ➑️
Synth Secrets, Part 5: Further With Filters goes into a bit more detail on how filters work and on abruzzi's post.
Cheers, bookmarked will read at a later time.
Old 3rd December 2014
  #11
Deleted User
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On synths with two filters, you can put two 24db/octaves in series for a whopping 48db/octave!
Old 3rd December 2014
  #12
Lives for gear
 
tribeofenki's Avatar
 
4 Reviews written
🎧 10 years
With modular software, as Reaktor, you can create an eq with 108 fixed bands assigned to all semitone values and roll-off a single note if necessary.

It will need a good CPU.
Old 3rd December 2014
  #13
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redloheb's Avatar
 
🎧 5 years
Quote:
Originally Posted by D.E.L.B. ➑️
Hi,

What does it mean exactly? Does it have relation in-terms of the harmonics of the sound e.g. if the fundamental is 440Hz and there is a harmonic that goes up to 880Hz does that mean that the harmonic is attenuated by -x/dB or what?

Thanks.
Bel is log(P1/P2). Decibel is 10log(P1/P2).
db/oct is reduction of power for frequencies w and 2w.

Example: if filter 6db oct has actual reduction of power is 6/10 = log(P1/P2) i.e. ~3.98, amplitude is half of the power i.e reduction of amplitude is ~1.99
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