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.
Reflection of Sound. The reflection of sound follows the law angle of incidence equals angle of reflection, sometimes called the law of reflection. The same behavior is observed with light and other waves, and by the bounce of a billiard ball off the bank of a table.