Måns,
Perhaps the following information will explain what I am seeing.
Files
Stereo white noise created by SoX; a 24bit 96kHz file at -12dBFS Peak and 10s duration, dithered with TPDF dither to 24 bits.
SoX invocation: "sox -V -b 24 -r 96000 -n sox,noise,white,stereo,24b,96000Hz,m12dBFSpeak,tpdf,10s.wav synth 10 whitenoise whitenoise gain -12 dither"
Stereo white noise created by iZotope RX6; a 24bit 96kHz file at -12dBFS RMS and 10s duration, dithered with TPDF dither to 24 bits.
Screenshots
Brief RX6 analysis of the SoX file; notice the peak level is -12dBFS while the RMS level is -13.76dBFS.
Brief RX6 analysis of the RX6 file; notice the peak level is -10.24dBFS while the RMS level is -12dBFS.
An FYI shot of the RX6 dialog box to point out that it only allows amplitude specifying as RMS; there is no way to enter a peak value.
An FYI shot to show that RX6 offers 4 flavors of white noise. I chose white uniform for this exercise because that appears to match what SoX produces. As a side question, it would be nice if someone here could confirm what flavor of white noise SoX produces.
To recap what I am trying to do, so that my original question is not lost: I want SoX, ideally natively via some appropriate command-line switch, to output a white noise file with a specific RMS amplitude.
Thanks.