Amplitude in synth and sampler items
I'm preparing an experiment which includes a hearing screening in which I want to play pure tones of various frequencies at 20 dB amplitude. So I wonder how to set the amplitude at 20 dB in a synth item. I cannot find any information on what the relative value (e.g. "1.00 x maximum") means. What is "maximum"? And is there any way to define the amplitude in absolute numbers (i.e. decibels)?
Relatedly, in the main part of my experiment, I present speech stimuli in noise. I present the stimuli and noise simultaneously as separate sampler items. But I have to make sure that the SNR (signal to noise ratio) is correct and constant across stimuli. Can I be certain that if my file for sampler1 is at, say, 60 dB and the file for sampler2 is at 65 dB and I choose "1.00 x maximum" in the sampler settings, then both items are played back at exactly that amplitude?
Would be really grateful if somebody had some information on that!