Helmet to helmet in vacuum - can we talk?
Answers
Because sound requires a transmitting medium (I gather you know that from your question) we can eliminate any sound traveling (in absence of a radio installed the helmets - as radio waves can travel in the vacuum of space) between the helmets if they are not touching. However if the helmets touch there would be a transmitting medium, provided that there is air in the helmets, and they are not helmets for aliens who need a vacuum inside their helmets. As to if you would need to shout, this would probably depend on the type of transmitting medium is in the helmet and the helmet material (solid, liquid, gas or plasma). Different transmitting materials effect the speed of the sound wave, which will also effect the wavelength, which will effect what you hear. Now common sense will tell you that the Helmet wearers have a better chance of hearing each other if they don't whisper