theoretical question... room without a ceiling

[eagle]

Just wondering... let's say you had a room located on the top of a mountain... no other buildings or trees around it. The top of the walls of this room are the highest things around. The walls are of absolute amazing sonic performance in terms of soundproofing... essentially no sound is getting through the walls... (6 foot thick concrete or whatever). Now, let's say that this room has NO ceiling or roof of any kind. The top of the room is wide open to the sky. A band is playing inside the room creating a very high sound level. If a person was standing outside this room on the ground, would they hear any sound? My thought is that the sound would just shoot straight up into the sky, nothing around to reflect it back downward. Thus the person on the ground would not hear anything at all, even though the band is blasting away with no top on the room. Would this be correct? (I of course am also assuming that the walls and/or ground is not vibrating at all.) Anyway, if it is true that you would not hear sound eminating from within the room in this particular case, then would it also be true that in certain situations, for rooms that are up high with nothing around them, that the ceiling/roof part of the room would not need to be as heavy-duty as the side walls and floor/ground in order to achieve maximum soundproofing?

[zemlin]

For the first part of your question - I think diffusion (or is it dispersion?) - at least at lower frequencies - would allow some of the sound to propogate to someone outside the room.

Sound Waves are pressure waves - and when this pressure is no longer contained by the 4 walls it will push outside the bounds of the walls - when the pressure front moves outside the walls, the surrounding air will react in all directions - including down.

... as heavy-duty as the side walls and floor/ground in order to achieve maximum soundproofing?

I suppose you mean ADEQUATE rather than Maximum - you can't have Maximum if you are less than all you can have. Then the question is how do you define adequate?

[David French]

Yes, only the lower frequencies would diffract around the edge of the walls and come down to the ground.