How many Hz is an Octave?
Racer
Joined: Oct 2002
Posts: 452
Likes: 158
From: Newark, DE
Re: How many Hz is an Octave? (korvetkeith)
The human ear interprets sound on a logarithmic scale. An octave is a doubling of frequency. For instance, 60 Hz is a b flat. The next b flat up is 120 Hz, then 240 Hz, then 480 Hz, and so on. To find the difference between certain notes, use:
F = F0*2^(diff/12)
That's kind of cryptic, so I'll explain it. F0 is the original frequency. diff is the number of half-steps (on the western 12-note chromatic scale) that you want to move. So, if you want to know what the C below A440 is, you get:
F = 440*2^(-9/12)
F = 261.63 Hz
You can replace (diff/12) with the number of octaves you want to move. If you want to know what a half-octave below 35 Hz is, just use -1/2 there.
You can figure out how many octaves are between two different frequencies by using:
Octaves = log2(F/F0)
log2 means log base 2. I don't think I can do subscripts here. So if you have a low-pass first order filter that cuts off at 80 Hz and you want to find how much it attenuates 100 Hz signals, just do:
Octaves = log2(100/80)
then
dB = Octaves * -6dB/octave
I can't do that math on the crappy calculator program that windows has, but you get my point.
And yes, ported enclosures roll off at 12dB per octave.
[Modified by GTLocke13, 11:14 PM 12/17/2002]
F = F0*2^(diff/12)
That's kind of cryptic, so I'll explain it. F0 is the original frequency. diff is the number of half-steps (on the western 12-note chromatic scale) that you want to move. So, if you want to know what the C below A440 is, you get:
F = 440*2^(-9/12)
F = 261.63 Hz
You can replace (diff/12) with the number of octaves you want to move. If you want to know what a half-octave below 35 Hz is, just use -1/2 there.
You can figure out how many octaves are between two different frequencies by using:
Octaves = log2(F/F0)
log2 means log base 2. I don't think I can do subscripts here. So if you have a low-pass first order filter that cuts off at 80 Hz and you want to find how much it attenuates 100 Hz signals, just do:
Octaves = log2(100/80)
then
dB = Octaves * -6dB/octave
I can't do that math on the crappy calculator program that windows has, but you get my point.
And yes, ported enclosures roll off at 12dB per octave.
[Modified by GTLocke13, 11:14 PM 12/17/2002]
Thread Starter
Melting Slicks
Joined: Jul 2000
Posts: 2,112
Likes: 2
From: Peoria IL
Re: How many Hz is an Octave? (GTLocke13)
Intermediate
Joined: Oct 2002
Posts: 37
Likes: 0
From: Medford OR
Re: How many Hz is an Octave? (korvetkeith)
There is no specific number of Hz in an octave. And octave represents a doubling of frequency. Hence 20 Hz in an octave above 10 Hz which is a 10 Hz change. But, 2000 Hz is an octave above 1000 Hz which is a 1000 Hz change.
