The general term applied to the study of a variety of atmospheric sound propagation phenomena, including frequency-dependent molecular absorption, refraction by wind and temperature gradients, diffraction by topography, buildings, trees, etc. , and scattering by turbulence. Also of interest are sounds, some of atmospheric origin, ranging from infrasonic emissions by thunderstorms and large explosions to ultrasonic emissions by high-power turbines and transducers used for materials testing.