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.
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.