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 you are standing against the wind, no matter how loud you shout, your distant friend cannot hear you at all, but you can hear him very well. Does wind scatter sound wave? Surprisingly, this phenomenon is related to refraction! Sound wave travels faster in the air when it is with the wind. Generally speaking, wind speed is lower near the ground because of the presence of blockages, and so it increases with height (Fig. 1). Therefore, when traveling with the wind, sound wave farther from the ground travels faster.