Sound waves move out spherically from a point source of sound, and as they do so they become less intense.
Sound pressure is inversely proportional to distance from the source as long as the sound does not encounter obstacles, like the head and external ears for example.
Obstacles, which create a change in the medium, impede or resist the propagation of sound.
When a sound waved encounters an obstacle or change in medium, a portion of the sound wave is reflected from the surface.
That portion of a sound wave not reflected from an obstacle is absorbed and continues to be propagated through the new medium.
Reflectance of a sound is at the heart of our understanding of the action of the middle ear, whose purpose is to overcome the impedance mismatch at the interface of air and fluid of the inner ear.
Reflected sound may encounter the original sound wave and, depending on the relative timing of the two, they may either reinforce or cancel one another.
Sound waves may also be diffracted, which means that, depending on the frequency of the sound, they are able to wrap around small or medium-size objects.
Reflectance and diffraction are two principle ways that sound waves are altered by the head.
