Figure 1-15. Sound traveling from air, to glass, to air
(1) As the sound passes from the solid back to the air, it changes its speed and direction
(2) In refraction, the angle of change is always perpendicular to its original path.
d. Diffraction. When sound encounters an object in its normal path of travel, it bends around
that object and continues back to its normal path, (fig 1-16). Example: You are placed in a room with an
open window and in the distance you can hear the sounds of a marching band. The sounds most readily
heard are those of the bass drum and other instruments which produce the lower frequencies of the audio
spectrum. The lower frequencies, due to their longer wavelengths, diffract more readily than the higher
frequencies. As sound from instruments that produce the lower frequencies enter through the window,
this opening or aperture causes these low frequencies to scatter in all directions.