I believe the "acoustic" ones are basically what Ed Said... A small piece of material (probably metal) that is coild around, and as it is subjected to more humidity, the coil expands, and pushing the needle or whatever further into the "wet" zone on the hydrogometer... something like that. I spent an hour trying to figure it out a few days ago... Neat idea, them theres are.
Check out my friends band... The vendors! Alternative/pop-punk.