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.
Reflection of sound. The return of sound waves from surfaces on which they are incident. The geometrical laws for reflection of sound waves are the same as those for light waves. The apparent differences involve only questions of scale, because the average wavelength of sound is about 100,000 times that of light.