Near Field

The near field extends from the source surface to a distance roughly equal to the largest dimension of the source divided by pi. For a loudspeaker with a 380 mm (15-inch) woofer, the near field extends approximately 120 mm (5 inches) from the cone. Within this region, different parts of the cone radiate at different phases, and the resulting interference creates an irregular pressure pattern. Near-field measurement is a practical technique that exploits the physics of close-proximity capture. When a microphone is placed very close to a driver (within a few millimeters for small drivers, a few centimeters for large woofers), the direct sound from that driver overwhelmingly dominates. Room reflections are negligible because the direct-to-reverberant ratio is extremely high. This allows accurate low-frequency measurement in any room. The technique was described by Don Keele in 1974 and is widely used in loudspeaker design and testing. By measuring each driver in the near field and combining the results with far-field measurements (which are accurate at higher frequencies), a complete frequency response can be assembled without an anechoic chamber. For subwoofers and bass drivers, near-field measurement is particularly valuable because low-frequency room modes severely contaminate far-field measurements below 200-300 Hz. The near-field technique sidesteps this completely. SonaVyx's SPL and transfer function measurements can be performed at any distance. Understanding near-field behavior helps interpret measurements taken close to sources and explains why readings at different distances may not follow the expected 6 dB per doubling.