Sound Attenuation LPF Slope

Hey guys,

I am currently producing a demo to complement a undergraduate research project and was intending on using the Low Pass Filter in the sound attenuation setting to simulate real sound decay. However I am trying to make it as close as possible to real world attenuation figures and one question that I could not find anywhere was what is the actual slope of the filter? is it 6db/octave, 12db/octave or 18db/octave? That would matter as the programming of sounds for realism would depend on setting the correct LPFFrequency at Min