A partially open window presents an opening 20cm wide.What kind of sounds from outside will go across the room

Question

in a well defined beam? What kind will spread evenly throughout the room? Roughly what frequency marks the boundary between the two cases?

Answer 1

Ignoring room resonances, the higher frequencies tend to behave as beams while the lower frequencies tend to spread. This is called acoustic diffraction (ref.). I'd think the boundary would be roughly where the window size is a half wavelength, or F ~= 850 Hz.