Helmet to helmet in vacuum - can we talk? Usual SciFi scene. In a spacesuit, in vacuum, with the comms out for some reason. They want to talk, so the butt helmets together so sound can pass through. Questions are:
a) Would it really work in practice?
b) It it did, could they talk normally or have to shout?
c) Are all frequencies going to pass equally, or are there high and low frequency cutoffs, filters, resonances etc?
Assume helmets are spherical, which means the contact area is small
 A: I know this is a bit late to reply to this post, but still, better late than never I guess.
1) As most people answered here mention, it would definitely work. A very good example of such technology is the bone-conducting "headphones". One can find some examples in this, this and this links (the last has some more extra information). In addition, this technology has been in an experimental phase for the US Army. Although this is not in direct connection to the exact nature of the question, one could easily find more "common" every-day examples such as the transmission of noise through structural elements in buildings (it's quite common in bad constructions to be able to hear neighbours a couple of floors up- or downstairs). So yes, this should work.
2) This is way too complicated to be able to answer with one word (shout or normal voice). As we all know, sound transmission is a quite complex phenomenon to be able to describe it so easy (especially when psychoacoustics jump in). By utilising the knowledge we have from noise transmission we could (at least as a first approximation) consider the case you describe as another "transmission through a structure" case (it could serve as a qualitative way to approach the "problem). We know that when one constructs double-leaf partitions for sound insulation the studs are a major issue. A considerable (for sound insulation standards at least) amount of energy is transmitted through the studs to the other side of the partition to be re-radiated. In addition to that, considering that in our case both helmets will most probably be made from the same material (this means that we have impedance matching, which constitutes a very nice condition for transmission) internal reflections and/or attenuation are minimised.
On the other hand, the area that the helmets touch are most probably gonna be quite small. But, if we also think about flanking transmissions, we'll see that it is possible to have considerable transmission through this small area and the whole helmet act as a radiator (this is kind of speculative, as the whole structure is quite complex to be able to make conclusions). This means that most probably a reasonable amount of incident energy would be required to manage to get enough sound energy on the "other side", but I don't think that it would have to be out of the possibilities of a normal individual. Most probably above normal speaking voice (this is considered to be about $60 - 70 ~ dB_{SPL}$ in about $1 m$ distance) but not extremely loud.
3) Well, I am sure we all know that no medium has a "flat frequency response". As is quite common in pretty much the vast majority of media, solids too, exhibit increasingly higher attenuation as the frequency goes up. In the "sound insulation world", it is considered to be about $6 dB/oct$ for the region of the so-called mass-law and about $12 dB/oct$ above the coincidence frequency. In practice, the values seem to somehow approach a maximum value (or it is just because we can't measure the increasing attenuation due to our equipment and space noise limitations). So, that means that also, in this case, not all frequencies will be transmitted with equal magnitude/amplitude.
A: 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.
A: I don't think it would work.
Helmet is made of 5mm thick polycarbonate. According to mass law, such a material of 10mm thickness (2 helmets) would affect the frequency response by:




Frequency [Hz]
Transmission loss [dB]




31.25
4.18


62.5
10.2


125
16.22


250
22.24


500
28.26


1000
34.28


2000
40.3


4000
46.32


8000
52.35


16000
58.37




In human speech, most consonants, which are important for intelligibility, are in 2KHz - 4Khz. And you can't really yell out consonants so maybe these would be at the normal speech levels which is about 65dB at 0.5m.
Taking all these things in account:
Consonants 2Khz transmitted through helmet would be = 65 - 40 = 25dB (louder than leaves rustling, softer than whispering).
Space suits operate at 0.29 atmospheres. The Sound Pressure Level (SPL) at 1 atmosphere is 194dB while SPL at 0.29 atmospheres is 183dB. So there is already a -11db difference just from air pressure alone.
I don't know if you need to take this into account but if you do then it would be 25 - 11 = 14dB (louder than breathing, softer than leaves rustling).
The other astronaut would probably hear something, but it would be nigh impossible to understand what is being said.
A: Really Are you Trying to move Sound in a Medium less Region .
1. Sound Require Medium to Travel So No chance If they Are Hearing Each Other .
2. IF there is Small Contact Then There will be Small Passing Of vibration and result in A little whisper to Both of them if they Shout Or whatever they Do.
3.as i said Transfer Of vibrations will be Small and Hence frequency may can Vary ..
