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.
Refraction: A change in direction of a wave due to a change in its speed. When an ultrasonic wave passes through an interface between two materials at an oblique angle, and the materials have different indices of refraction, both reflected and refracted waves are produced.