[linux-audio-dev] scaling jackinput to dbSPL

Dan Mills dmills_00 at yahoo.co.uk
Tue Aug 15 14:25:06 UTC 2006

--- conrad berhörster <conrad.berhoerster at gmx.de>

> Hello all, 
> Does anybody know, how i can scale the incoming jack
> signals to dbSPL, 
> which is in the range of 0 to 120. An is it possible
> to calculate from dbFS 
> (which is used in normal soundapp in range -inf to
> 12db)  into dbSPL. 
> thanks c~

Look up what voltage corresponds to 0dbFS, then given
a known sensitivity figure for the microphone and a
known gain for the preamp do the math.

For example if 0dbFS is specified as corresponding to
+22dbu (actually a voltage reference BTW), and you
establish that 100dbSPL gives an output of say -50dbu
from the mic, then if your preamp has a gain of say
50db, the math looks like this:

100dbSPL = -50dbu at mic, so 0dbSPL = -150dbu at mic
(burried in the nose floor in all probability), the
preamp has 50db gain so the output will be -100dbu at

At 120dbSPL (assuming a well behaved mic and preamp),
you will have +20dbu, now the Full sacle value is
specified as +22dbu, so a 20dbu input will read -2dbFS
, therefore in this example SPL = 122 + DBFS (always

This calibration obviously needs to be made for each
microphone/preamp/card combination if you want decent

In practice, I would try to set up for the top of my
measurement range to be about 10db or so below 0dbFS
as you typically want RMS measurements and 0dbFS is a
peak value and a very unforgiving place.

Some of the better card manufacturers specify things
like 0dbu output for -20dbFS which is obviously saying
that 0DBFS = +20dbu

Regards, Dan. 

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