When a wave encounters a change in the material in which it propagates, wave reflection
will occur at that boundary.
The way in which waves reflect at a boundary is determined by boundary conditions, such as the size of the boundary in comparison to the incident wavelength, the relative smoothness of the surface, and/or the relationship between media properties on both sides of the boundary.
If a reflecting surface is smooth over several wavelengths in all directions, the angle an incident wavefront makes with the surface is equal to the angle of reflection (referred to as specular reflection).
On the other hand, if a reflecting surface is quite irregularly shaped, wavefronts will scatter in many different directions (sometimes referred to as diffuse scattering).
Specular vs. diffuse scattering behaviours will depend on incident wavelengths. Low-frequency (large wavelength) sound wave components will reflect specularly from a choppy sea surface but high-frequency (short wavelength) components will scatter diffusely.
Components of a wavefront with wavelengths much larger than the size of an obstacle will diffract
or bend around the object almost as if it isn't there. For wave components with wavelengths shorter than the size of an obstacle, a “shadow” region will result behind the object.
Diffraction is caused by the “removal” of some portion of the incident wavefront and can be qualitatively understood in terms of Huygens principle (Fahy, 2001).
A sudden or progressive change in wave speed will produce a change in propagation direction or a “bending” of the waves. This is known as refraction.
The Doppler effect
occurs when a sound source and/or listener are moving relative to one another. When the source and listener move toward each other, the emitted sound is perceived by the listener to increase in frequency. When the source and listener move away from each other, the sound is heard to decrease in frequency.
The perceived Doppler frequency shift results from an effective change in wave propagation speed due to the moving source and/or listener.
