thank you again for the wonderful information and taking the time to inform, Dr. Sid!
just to be clear though (as i'm feeling still a little confused by this), in sh4's simplistic model, that '20%' is the final value of the probability (ie a reduction by 80%)... so you're suggesting then that attenuation can cause anywhere up to 100% signal loss under 'normal' conditions (as in the case you mention of shallow angles)? this certainly seems to be born out by even just that first image from your simulator you posted (barring that slight scatter immediately below the emitter)... but i want to make sure that i've got my head on straight with this.
also, i'd love to play around with your sound propagation simulator (the images you shared look terrific), so thanks as well for that! does it give numeric output as well as visual representation? what sort of initial-state parameters are configurable? 'looks great!
cheers
hc