[sdiy] db to semitones, octaves.

[karl dalen]

Some people say for instance:
The fist bandpass filter are spaced 1 octave
above the 2 bandpass filter, both wich have
about ca 20db slopes.(they asume Cf to Cf i suppose).

So if we have a BP filter wich we dont know exatly
the slope db but want to figure out how many
semitones fitts in that range the BP covers
by the means of centerfrequency (easy to see on
a scope.)how do we do that?

Ohh, did i answer my own question perhaps?!


Reg
KD

 --- Magnus Danielson <cfmd at bredband.net> skrev: >
From: Roman <modular at go2.pl>
> Subject: Re: [sdiy] db to semitones, octaves.
> Date: Wed, 26 May 2004 10:03:38 +0200
> Message-ID:
> <20040526080338.AB09143E5B at rekin8.go2.pl>
> 
> > db or dB like decibels?
> > 
> > 1 octave means times-two, what in dB equals
> 6.0206dB, 1 semitone is about
> > 0.5dB
> > 
> > 1 dB = 1.993 semitone
> 
> Actually, it may not be that simple. Bell is a
> power-ratio scale, such that
>            P
> L = log  (----)
>        10 Pref
> 
> However, often is the deci-Bell scale used, so then
> it is
> 
>                 P
> L = 10 * log  (----)
>             10 Pref
> 
> For voltage and current related stuff, you need to
> convert it into power first,
> which implies squaring. The Square can then be
> isolated out from the log
> expression and becomes a multiplication by two, so
> this leads to
> 
>                 V
> L = 20 * log  (----)
>             10 Vref
> 
> The question now comes how you relate frequency to
> power. There are many
> different power-laws which changes with frequency by
> different exponents.
> 
> So, is the doubling in frequency twice as powerfull
> or twice as high amplitude?
> That gives different answers to the question. I
> would actually lean towards the
> answer that there is no answer, you never do this
> for musical situations. dB
> for frequency is meaningless. Octaves, semitones and
> cents fills this purpose
> already and another logarithmic scale is as such not
> very meaningfull.
> 
> Cheers,
> Magnus - adjusting the centibells and milibells
accordingly 

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