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.
When sound propagates with the wind, the rays are bent toward the earth, with the result that the audibility of the sound is substantially improved (Figure 2). The refraction of sound in the upper layers of the atmosphere can lead to the formation of zones of silence and zones of abnormal audibility.