Why is possible to hear in an open space someone's voice even if he's not facing me?
Is it because of refraction, diffraction or reflection of the sound wave?
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Generally that would be diffraction - the spreading out of a wave. Reflection is bouncing off objects - e.g. an echo of someone's voice. Refraction is about a wave changing direction e.g. water looks shallower than it is when viewed from above in air because the light from the bottom is refracted (bent) at the water/air surface.
In this case - diffraction - the key thing is the ratio of the width of somthing the wave passes through to the wavelength of the wave. - e.g. light passing through a window does not diffracted - visible light has a wavelengths of tens of millionths of centimetres - whereas most windows are tens of centimetres wide - this gives no diffraction that is easy to see. Audible sound waves have wavelgnths of centimeters and metres (roughly speaking) and the opening of the mouth is about the same size or smaller - which gives lots of diffraction.
If the space is truly "open", then reflection should not play (since there is nothing to reflect off), and refraction can only play if there is a change in refractive index (which "normal air" would not have).
That seems to leave two things: the fact that the entire body vibrates when you speak - and thus there is some fraction of the sound being directed towards you even if the speaker is facing away) and diffraction - the bending of sound around the head of the speaker.