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.
Diffraction is the bending of waves when they hit a barrier. Sound is the result of vibrations in molecules causing the molecules to compress and retract, making sound a longitudinal wave. The vibrating source must push some material (medium, e.i. air, metal, water). for the sound wave to travel.