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.
Sound rays always turn toward the layer where the speed of sound is lower. The more rapid the change in the speed of sound, the more pronounced the refraction. Figure 1. (a) Paths of sound rays when temperature decreases with altitude, (b) paths of sound rays when temperature increases with altitude. The refraction of sound in the atmosphere is due to spatial variations in the air temperature, wind speed, and wind direction. The temperature usually decreases with altitude up to an altitude of 15â20 km.