The spreading of waves
when they pass through an opening, or around an obstacle into regions where
we would not expect them, is called diffraction. Diffraction
occurs if a wave encounters an object and if the wavelength is of the same
size (or greater than) the object size. An example of diffraction phenomena is
given by the spreading of waves around an obstacle. The animation below
illustrates how a travelling wave emitted from the upper left corner by an
aeroplane is diffracted by a sound barrier erected to shield
homes from the traffic noise. Interference patterns due to the
superposition of the incident wave and the diffracted wave are clearly
seen just before the barrier (bottom left of the animation). A shadow
region is observed just behind the barrier (bottom right of the
animation). It is characterised by low noise levels due only to the
acoustic diffracted wave. Thus, this solution for noise reduction is efficient only if the
houses are located within the shadow region of the sound barrier.
The animation below shows another example
of diffraction. The white region is a cross-section of the front
part of an aircraft engine, the sound wave is produced by the turbofan.
Diffraction determines the direction in which most sound will be
radiated, an important factor for the acoustical engineers who work to
make them as quiet as possible.