Re: [boost] Decibels and SI Units (was Units (and operators.hpp, too))

From: Toon Knapen (toon_at_[hidden])
Date: 2001-07-01 11:50:18

Bill Seymour wrote:
> 
> Deane wrote:
> >
> > There are logarithmic units such as decibels and pH.
> > I'm not familiar with how these are used in formulas.
> > I think these are sufficiently strange so that they
> > should be treated specially.
> >
> 
> I'm a wires-and-pliers guy by training and early vocation,
> so I can tell you about decibels in electronics (I'm ignorant
> of acoustics).
> 
> A Bell (after Alexander Graham Bell) is the common log
> of a power ratio.  A more useful unit, it turns out, is
> a tenth of a Bell, or a decibel (with just one 'l' for
> historical reasons):
> 
>     1 dB = 10 log(P1/P0)
> 
> Audio engineers like to measure power in decibels over
> one milliwatt:
> 
>     1 dBm = 10 log(P/1mW)
> 
> and RF engineers like decibels over one kilowatt:
> 
>     1 dBk = 10 log(P/1kW)
> 

In acoustics the multiplier (in your case 10) for the log
function is '20' for some specific quantities.
Also in acoustics one has dBA, dbB and dBC which are Db's
that are filtered. The idea behind this is that one type
of dB-scale is more sensitive in the ranges heard by the human
ear, whereas the other .... Taking these into account
is even much more difficult as the filter needs to be known
to the mechanism.

toon