Experimental characterization of the near field zones of silence in an active sound cancellation scheme

A filtered X-LMS active sound cancellation (ASC) scheme consisting of a primary source, a secondary source, and an error microphone was used to experimentally determine the near field destructive interference patterns. The patterns for single tones of 80 and 125 Hz were mapped, and the patterns for an additive combination of 80 Hz and 125 Hz were mapped. The interference patterns for a measurement plane, parallel to the support surface and centered on the error microphone, were determined. The plane was discretized radially and circumferentially, and the average sound pressure level (SPL) was determined at each point. Level curves were generated to provide a comparison between SPLs in the near field both with the primary source acting alone and with the destructive interference generated by ASC. The primary field was substantially reduced in the entire region beyond the error microphone and away from the primary source. Although the magnitude of reduction for each of the primary source waveforms was different, the general interference pattern that occurs for an ASC system was similar