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 Waves. The speed of a wave depends on the elastic and inertia properties of the medium through which it travels. When a wave encounters different medium where the wave speed is different, the wave will change directions.
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).