Also, the reverse wave-form will have areas of constructive and destructive interference that are difficult to predict and control in an area with lots of reflections, so the speaker will still be heard, and in many cases actually be louder because you added energy to the signal. The size of these areas of interference is related to the wavelength of the sound and it is possible if one ear is in an area of destructive interference, the other will be in an area of constructive interference.
Actually, Hydrogen only has 8% more lifting force than Helium: http://en.wikipedia.org/wiki/Lifting_gas#Hydrogen_and_helium
I ALMOST bought one. I tried to get four of them shipped to me, but after I entered all my information and submitted the form, it showed me someone else's shipping information. Makes me wonder if the site is designed to handle more than one order at a time. It is a pretty neat looking poster nonetheless.