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Amplitude in synth and sampler items

Hi everyone,
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!



  • edited August 2017

    Hi Sophia,

    I have never done any auditory experiments (and therefore won't be able to help you with your SNR), but I do have a bit of a background in music.

    Abstract: that's not how decibels "work".

    Volume controls work in the opposite way to what we intuitively think. We use them to 'pump up the volume', but in fact they are attenuators. So a volume knob turned all the way to the right lets all of the sound through (i.e. "1.00 x maximum'), and you trim/attenuate this by dealing it to the left or decreasing the amplitude value in your synth item. Your output can never be more than 100%. (Gain is something different, as that controls an amplification circuit. The reason why some music/tv ads sounds louder than other is that low amplitudes are pushed all the way to the maximum with compression and gain. Decibel meters will give the same reading but it sounds louder to our ears. I digress).

    The decibel is an acoustic measurement, depending on volume settings, distance, room acoustics, angle, etc etc. By definition, you can not set this output volume. The only way to find out how much dB your sounds are is by measuring it. Also, keep in mind that the dB scale is not exactly intuitive (at least not to me), as it is a base-10 logarithmic scale (I seem to vaguely remember that it's 10*log10(P) hence DECI-bel, but don't quote me on that). In practice, this means that 60dB is not twice as loud as 30dB and that adding a 65 dB source to a 60 dB source will result in a total of about 66 dB.

    I hope this helps with the first part of the question.

  • Hi cesco,

    Thank you very much for your explanations! That's a very clear intro to the subject which is quite new to me (as you might have noticed ;) ).

    I used an audio programme to equalize the amplitudes of all my stimulus files and had to choose which amplitude I wanted to set them to (in dB). I think that's what first got me on the wrong track. And on the other hand, this programme can determine the (average) amplitude of any given sound file and gives the output in dB. So if I understand you correctly, that output value would then be the maximum amplitude that is reached when both the computer (i.e. operating system) volume and the synth item amplitude are at maximum. (?)

    Oh, and thanks for pointing to the log scale! That's an easy one to forget.

    Anyone else who knows if two simultaneously run sampler items are reliably played back at the correct ratio?

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