Nice little practical test! Well done. It sounds like you proved that they do exactly what they are supposed to do... which is good cause I have a set!
I have read an article on the web somewhere once about using 50mm rubber bath plugs for the same purpose... It would be nice to see exactly the same test done with these to compare results... not that I would particularly like to know that I could have invested less, but it might just either veify or debunk a myth!
(Just love debunking myths!)
Al.