Diffraction:

 An obstacle is no match for a sound wave; the wave simply bends around it. For example, if a stereo is playing in a room with the door open, the sound produced by the stereo will bend around the walls surrounding the opening. This bending of a wave is called diffraction. All waves exhibit diffraction, not just sound waves. Without diffraction, the sound from the stereo could only be heard directly in front of the door. Instead, the air in the doorway is set into longitudinal vibration by the sound waves from the stereo. This means that each air molecule is a source of a sound wave itself. This results in each molecule producing a sound wave and emitting it outward in a spherical fashion. The final result is the diffraction of the sound wave around the doorway. The sound outside of the room has varying intensity depending on where you stand. Directly in front of the center of the doorway the intensity is a maximum. As you move further away from the center, the intensity decreases until it is at zero, then increases to a maximum, falls to zero, rises to a maximum...and so on. Each maxima gets progressively softer further away from the center. Waves diffract differently depending on the object they are bending around. If we let angle x be the location of the first minimum intensity point on either side of the center, W be the wavelength, and D be the width of the doorway, the equation gives x in terms of the wavelength and the width of the doorway. For a circular opening, the equation is slightly different. Angle x, W for wavelength, and D for width are all still the same. The equation looks like this: So, looking at these two equations you can tell that the extent of the diffraction depends on the ratio of the wavelength to the size and shape of the opening. If the ratio of W/D is large, then x is large. In this case, the waves are said to have a wide dispersion and the sound waves are spread out wider through the opening. Conversely, if the ratio of W/D is small, then x is small and the waves are said to have a narrow dispersion and the sound waves go through the opening without spreading out very much. So, it makes sense that lower-frequency sounds typically have a wide dispersion and sounds with small wavelenths have a narrow dispersion.