I think that is is also important to point out that to correlate 0dbfs in the computer to dbSPL outside of the computer requires many other deciding factors, including an amplifier setting, and a position in the listening space.  For instance, in a specific calibration, one could decide to use the following settings: 0dbfs in Csound equals 90dbSPL when the amplifier knob is at 80%, and the listener/measuring device is 1 meter from the sound source and set to the dB/A scale.   Creating a group of settings such as this gives the audio engineer a set reference point from which to measure other points or convert values.   If the engineer wants to test the range of the amplifier, he can now change that setting, and measure its effect on the dbSPL at the listening position. Or change the microphone's distance from the sound source and test the effect of that.  Each of these factors along with any other points of amplitude/gain modification need to be
 considered when trying to correlate 0dbfs and dbSPL, and it's not really possible to convert directly from 0dbfs to dbSPL without a set reference point for the entire system.

                             Mark Jamerson
   
   

----- Original Message ----
From: David Akbari 
To: csound@lists.bath.ac.uk
Sent: Tuesday, April 29, 2008 11:19:23 AM
Subject: [Csnd] Re: Re: Questions about dB opcodes

Thank you gentlemen for your clarifications regarding dBFS and how it
is implemented in Csound 5.

I'm still confused by the unit of measurement used in the ampdb()
opcode, however. One way I could ask this question more clearly is:

What is the reference equivalent value assigned to 0 dB in the ampdb()
opcode ... is it assumed to be 20 µPascals (ie. a measure of sound in
air)? If not, do the "non-fs" dB family of opcodes still reference the
value defined by the 0dbfs opcode? What formulae might someone use to
convert from specifically from decibels in Full Scale to Sound
Pressure Level?

You have all done a great job at establishing that the 0dbfs opcode
sets the reference value for the 'fs' family opcodes, but what about
ampdb()? Does this opcode also use the value defined by 0dbfs as its
chosen reference value for amplitude scaling?

I'm sorry for all of the questions, but I am very interested in these
values specifically as I plan to use Csound to implement Adaptive
Forced Choice procedures in experiments involving hearing research. I
need to know these values so I can understand how to better approach
the calibration of the testing equipment [software].


Thank you for your time and consideration,

David Akbari


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