[LAU] Calculating absolute played(!) volume

[Fons Adriaensen]

On Thu, Oct 15, 2020 at 08:54:57PM +0200, Jeanette C. wrote:

> for a project I am wondering, if the absolute output volume of a pair of
> headphones, directly connected to a soundcard, can be gained from the
> settings?
> 
> I have a Delta 1010. Inside alsamixer the volume for the DACs is set in dB.
> Is there a straight relationship between these settings and a connected
> output device? My pair of headphones, IIRC, have a resistence of 60 Ohm.
> 
> Any help is appreciated. A good pointer will do. If there is a method
> without special measuring equipment, I'd be happy. If not... So be it.

This isn't simple :-(

Assuming that by 'absolute output volume' you mean the sound pressure
level (SPL), you need to know four things:

1. The sensitivity of your headphone, that is the SPL for some 
given voltage. This may be in the specs of your headphone. If not,
it can be measured, and that requires some rather specialised
equipment (dummy head, calibrated mic, calibrated level meter, 
calibrated signal source).

2. The 'gain' of your sound card at a particular DAC gain setting.
This is the relation between the output voltage (in dBu or dBV) and
the RMS level of the digital signal sent to the sound card. This
could be measured (using an RMS voltmeter) with a know software
generated signal.

3. How (2) depends on the volume settings (if any) of your sound card.
If these are in dB and accurate (I wouldn't trust ALSA for this) that
is easy.

4. The RMS level (in dB FS) of the signal you are playing. Zita-mu1
provides this but is probably of little use to you. What do you normally
use for metering ?

Ciao,

-- 
FA