I'm making an experiment where I play pure tones of three different frequencies (220 Hz, 440 Hz, 880 Hz) at 75 dB SPL. I've noticed that they don't sound equally loud, and I would like them to. I found a number of webpages on equal loudness contours, but from the graphs it is still unclear to me by how much I should change the gain of the tones in order to make them appear subjectively equal (to the average listener).
I'm using a program called Presentation, which allows me to set an attenuation parameter to the tones. The attenuation parameter has the following description:
You can control the volume of a sound by setting the sound stimulus attenuation parameter. This parameter can have a value greater than or equal to zero with the following meaning.
Applied Attenuation = attenuation * 100 Db (Decibels)
If you do not define this parameter, the value of the default_attenuation header parameter will be used. If attenuation = 0, there is no attenuation and the sound will be played as it is given in the wave file. If attenuation = 1, the sound will be attenuated by 100 Db which effectively results in silence.
From this description I would not expect a linear relationship between the size of the attenuation parameter and perceived sound intensity changes... But I know very little about digital signal processing, so I'm not sure.
Could anybody point me to some references where I could find a solution to my problem? Or better yet, solve my problem :)