Bats use high frequency (low wavelength) ultrasonic waves in order to enhance their ability to hunt.
As the wavelength of a wave becomes smaller than the obstacle that it encounters, the wave is no longer able to diffract around the obstacle, instead the wave reflects off the obstacle.
Sound has a much longer wavelength than light. Sound waves can even diffract around buildings or through doorways. This is why you can often hear people when you can't see them.
As with all wave diffraction the amount of diffraction will depend on the wavelength of the sound wave and the size of the gap the wave is travelling through.
Sound waves are diffracted as they leave their source, for instance a loud speaker. The size of the source will affect how much diffraction occurs.
Diffraction also happens at convexly curved terrain surfaces, e.g. at the edges of slopes and hills. Diffraction is further responsible for the incidence of sound energy into acoustical shadows that were formed by upward refraction by sound waves. Therefore it is not completely quiet in shadow zones.
In addition to diffraction, also scattering of sound waves leads to the incidence of sound energy into shadow zones.