Issues in calibration. Primary reference standards. A reference standard for a unit of measurement is an artifact that embodies the quantity of interest in a way that ties its value to the reference base. At the highest level, a primary reference standard is assigned a value by direct comparison with the reference base.
Refraction of Sound. Refraction is the bending of waves when they enter a medium where their speed is different. Refraction is not so important a phenomenon with sound as it is with light where it is responsible for image formation by lenses, the eye, cameras, etc. But bending of sound waves does occur and is an interesting phenomena in sound.
1. Reflection of sound waves. If a source of sound is directed at a vertical surface some distance away, an ECHO may be heard. Sound waves bounce off the vertical surface, and are REFLECTED back towards the source. Reflection of sound waves obey the laws of reflection. The human ear cannot distinguish between two sound bursts if they are less than about 0.1 s apart. For that reason, in order to hear an echo, the reflecting object must be more than 17 m away from the hearer (assuming sound travels at 340 mÂ·s-1).