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.
When a wave moves from one medium into another, its direction of propagation changes; this is termed refraction. In addition to the transmitted wave continuing from one medium into another, there is also a reflected wave back into the original medium. For example, when your radio is playing music in your room, the sound waves pass through the walls as well as reflect from them. Similarly, some of the light from your lamp passes through the glass of the window in your room, and some also is reflected from the glass.