Diffraction:

The waves can gang up, fight each other, and travel in illogical directions at unfair speeds. They can also turn their corners. Do you recall playing in the side yard whenever you were a child, hopeful that you would not hear your mother call you from the front door? Whenever the time came, the voice reached your ears anyway. How could sound locate its way about the house? Your mother was out of your view, and you were out of hers. Why must sound go places visible light could not? Doesn't sound pass through in straight lines, such as light? Was your mother's voice reflecting from other houses in the neighborhood? To find out, you conduct an experiment with a friend close to an old, discarded barn in the middle of nowhere, and her voice found its way about the structure even although there was nothing nearby from which the sound could reflect.

Sound turns from corners, particularly from sharp corners, since of waves' ability to diffract. Whenever a wave disturbance encounters a "sharp" obstacle, the obstacle behaves as a new source of energy at similar wavelength (as shown in figure below). The phenomenon can take place repeatedly. Even when you hide in the garage, you could yet hear that voice. In a side yard three houses downward the street, you could hear it. You observed that the sounds from musical instruments, car engines, lawn mowers, and all kinds of other noisemakers can diffract too. Since anyone who lives in a city knows that no one can hide from noise. Diffraction is one of the cause sound is so persistent.

Figure: Diffraction makes it possible for waves to "go around corners."

Long waves diffract more eagerly than the shorter waves. Since an edge or corner becomes sharper associative to the wavelength of sound, diffraction takes place more proficiently. As the frequency reduces, the wavelength raises therefore all edges and corners become, in result, sharper. This effect is not exclusive to sound waves. It occurs with water waves, as any surfer knows. It occurs with radio waves; this is why you can hear the broadcast stations on your car radio, particularly on the AM broadcast band, here the EM waves are hundreds of meters long, even whenever there are buildings or hills among you and the transmitter. It occurs with visible light waves also, though the effect is more delicate and can be noticed only under certain situations. All the waves diffract about sharp corners. One of the tests by which scientists as certain the wave nature of a disturbance is to view whether or not the result can be noticed from about a corner.

Whenever an obstruction is small in comparison with the wavelength of a disturbance, the waves diffract so well therefore they pass the object as if it is not there. The flagpole has no effect on low-frequency sound waves. The pilings of a pier similarly are ignored by ocean surf.