Describe diffraction in sound, and how it works.

Diffraction is the bending of waves around small obstacles and apertures and the spreading out of waves beyond small openings. the extent of spreading out depends on how the width of the aperture compare to that of the wavelength of the wave. as sound waves have a larger wavelength than light waves, diffraction in sound waves is much more pronounced compared to diffraction in light waves because at large scale, e.g. the opening of a door the wavelength of sound waves is comparable to the gap or the obstacle and hence sound waves diffract. this allows sound waves to bend around obstacles, allowing us to hear sounds around corners and barriers. 

The answer is frictional force