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.
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.he refraction of sound in the atmosphere is due to spatial variations in the air temperature, wind speed, and wind direction.