Free Field

The free-field concept is the baseline reference for acoustic measurements and calculations. In a free field, sound behaves in the simplest possible way: it radiates outward from the source, intensity decreases with the square of distance, and there are no reflections to create interference patterns. Real-world free-field conditions are approximated outdoors, elevated well above the ground and far from buildings. Even outdoors, the ground provides one reflective surface, creating a half-space (semi-free-field) condition. True free-field measurements require an anechoic chamber. The free-field assumption underlies many standard acoustic calculations. SPL prediction models start with free-field propagation and then add corrections for boundary reflections, absorption, and barriers. Speaker specifications measured in free-field conditions provide the baseline data for these predictions. Free-field microphones are calibrated to have flat response when the sound wave passes by without obstruction. Pressure-field microphones are calibrated for flat response at a reflective boundary. Using the wrong microphone type introduces measurement error at high frequencies where the microphone body causes diffraction. For SonaVyx measurements, understanding the difference between free-field and reverberant conditions helps interpret results. An SPL measurement in a free field gives the true source level. The same measurement in a room includes reverberant contribution that elevates the reading. SonaVyx's SPL meter displays the combined level; knowledge of the room's acoustics helps separate the direct and reverberant components.