You may perceive diffraction to have a dual nature, since the same phenomenon which causes waves to bend around obstacles causes them to spread out past small openings. There are other factors such as the higher air absorption of high frequencies involved, but diffraction plays a part in the experience. The thunder from a distant strike will be experienced as a low rumble since it is the long wavelengths which can bend around obstacles to get to you. The thunder from a close bolt of lightning will be experienced as a sharp crack, indicating the presence of a lot of high frequency sound. Another common example of diffraction is the contrast in sound from a close lightning strike and a distant one. The fact that diffraction is more pronounced with longer wavelengths implies that you can hear low frequencies around obstacles better than high frequencies, as illustrated by the example of a marching band on the street. Diffraction in such cases helps the sound to "bend around" the obstacles.
The fact that you can hear sounds around corners and around barriers involves both diffraction and reflection of sound. Important parts of our experience with sound involve diffraction. If D is greater than the wavelength there is at least one angle where destructive interference occurs the diffraction patterns in such cases are similar to the interference patterns produces by two sources close together.Diffraction of Sound Diffraction of Soundĭiffraction: the bending of waves around small* obstaclesand the spreading out of waves beyond small* openings.* small compared to the wavelength If D, the width of the opening, is less than the wavelength than there is no place where the interference is completely destructive. It turns out that the points where this destructive interference occurs are all along one line, at an angle (measured from a line perpendicular to the opening) given by: Similarly, the wave from the part of the opening next to the side will interfer destructively with the part of the opening next to the center, and so on - the waves from one half of the opening completely cancel the waves from the other half. This is the condition for destructive interference: the wave from the side of the opening will interfere destructively with the wave from the center of the opening. The waves from each piece of the opening are sent out in phase with each other at some places they will interfere constructively, and at others they will interfere destructively.Ĭonsider a point that is half a wavelength further from the center of the opening than from one side of the opening. If the opening is divided into many small pieces, each piece can be thought of as an emitter of the wave.
Interference, both constructive and destructive, is important to understanding why diffraction occurs. The most interesting cases (i.e., the ones with interesting patterns of maxima and minima) are those in which the size of the openings or obstacles is about the same as the wavelength of the wave. When the sources send out waves in phase, constructive interference will occur at a particular point if the path lengths from the two sources to that point differ by an integral number of wavelengths.ĭestructive interference occurs at a particular point if the path lengths from the two sources to that point differ by an integral number of wavelengths + 1/2 a wavelength.ĭiffraction is the bending of waves that takes place when the wave encounters openings or obstacles. For a particular separation and wavelength, the pattern is as shown in the diagram, with constructive interference taking place at certain angles and destructive interference taking place at other angles. The sources are in phase with each other.
Consider first the special case of two sources separated by a small distance d, sending out waves of the same frequency. When two (or more) waves of the same frequency interfere, a variety of different results can be obtained. The interference of two waves of the same frequency