This page shows a crude simulation of sound travelling from a point source in a room (6x4m). As the sound travels away from the source the energy is spread out according to the inverse square law.
The default settings are for a tone-frequency of 1000Hz and perfect absorption (value of 1.0, no reflection) at the walls. For these values the sound wave spreads out equally in all directions. However, if the wall absorption is set to zero the sound reflected off the walls interferes with the direct sound to produce peaks and dips (nodes and anti-nodes) in the sound level in the room. You can see this interaction as 'fuzzy' lines in the pattern away from the sound source. Notice also how the wavelength of the sound changes as the frequency changes.
[Note, occasionally the animation may appear to go in reverse with the waves travelling towards the source - this is a bug and seems to depend on when in the cycle the parameters are changed. If this happens, refresh the page to reset things. Note also, this is a very crude simulation and does not include all reflections from each wall and does not include multiple reflections]