In alot of turist attracions I saw these flat cardboard bird whistle like these:http://www.youtube.com/watch?v=PHejLm7Lf_w They are made of a cardboard washer folded in half and clamped with a metal washer. In the middle of the cardboard there's a semi translucent film, that actually makes the whistling sound. What is it made of? Is there an ible on how to make these whistles? THANKS!