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.
A Physic's definition of refraction involves a change in the direction of waves as they pass from one medium to another. This is due to energy conservation when a wave changes its wavelength, and thus wave speed, as it enters a new medium (like in light or sound waves traveling from air to water). Now, in terms of ocean waves, refraction also involves a change of direction when a wave experiences a shift in wavelength and wave speed.