Diffraction is the change in direction of waves as they encounter obstacles or apertures. Diffraction is most noticable if the wavelength is comparable to or greater than the size of the obstacle or aperture. If the wave length is much smaller than the obstacle or aperture, diffraction is difficult to detect. Because of diffraction of sound waves you can easily hear around corners and walls, and through open windows and doors, even though you are not in line with the sound source.

Thomas Young's famous “double slit” experiment... diffraction plus superposition produces a unique and almost universal pattern of two-source interference!

Bat echo location and ultrasound imaging are both examples of the use of very short wavelength, high-frequency sound waves to locate or visualize relatively  small objects. The wavelength of the sound wave needs to be smaller than the smallest features that are to be imaged.

Chladni Plates and Standing Waves in "Things"!


Standing Waves in Pipes!

Doppler Shift!