Freefield

Free-field is a term used in acoustics to describe a sound field in which there are no reflections from boundaries that would alter the direct sound from a source. In a free-field environment, the sound pressure at distance r from a source decreases approximately with 1/r, and the direct sound dominates over any reflected components. Real free-field conditions are idealized; in practice they are approximated in anechoic chambers or in outdoor locations with minimal reflective surfaces and controlled ground effects. Measurements conducted in free-field conditions are used to characterize loudspeakers and microphones, assess directivity, and calibrate sound levels because the direct sound path is unaltered by reflections.

The concept is distinguished from diffuse-field or reverberant-field conditions, where multiple reflections create a more uniform distribution of sound energy. Free-field measurements rely on environmental control and standardized procedures to suppress reflections, such as in true anechoic chambers or outdoor line-of-sight setups at sufficient distance from boundaries. The term may appear in standards and calibration guides for acoustical equipment, though practical implementations are approximations of the ideal free field. The idea also informs models of outdoor sound propagation and has analogues in other disciplines that consider unobstructed propagation of waves.