[linux-audio-dev] scaling jackinput to dbSPL

[Dominique Michel]

Le Tue, 15 Aug 2006 18:03:17 +0100 (BST),
Dan Mills <dmills_00 at yahoo.co.uk> a écrit :

> 
> --- Dominique Michel <dominique.michel at citycable.ch>
> wrote:
> 
> > With jack, mixers as jackeq or jackmix are already
> > using db scales.
> 
> Yeeees, but they deal with gain which is unitless in
> the DB system.
> 
The db system always use a reference and a db of any kind have no unit.
dbFS use as reference the maximum possible digital signal and the other
term will be too a possible digital signal. dbSPL use as reference a
given sound pressure, but the other term will be too a sound pressure,
so the result in both cases is unitless. 

sound_pressure/sound_pressure give the rate between the 2
sound_pressure, and a rate have no unit, and that even after the
multiplication by 10 or 20 log base 10. Only the knowledge of the
reference give you the knowledge of the unit (as well as the math).

The db inside jack are dbFS with a maximum possible signal of 0 db.
Now, both jacqeq and jackmix give you a maximum conrol level at +6dB.
It mean at +6dB in those EQ is equal to 0dbFS in jack.

> What is being discussed is how to reference things to
> external signal levels (measured in volts, SPL or
> whatever), this requires a mapping from dbFS to the
> external signal level and that is heavily hardware
> dependent. 
> 
> Regards, Dan.

I know that. I said in a precedent message at he must measure the sound
level in a given point of the local to know the relation between the
dbSPL at that point and the dbFS in the box. 

Best,
Dominique