NARROW-BAND DIGITAL ACTIVE NOISE-REDUCTION IN A SIREN-CANCELING HEADSET - REAL-EAR AND ACOUSTICAL MANNEQUIN INSERTION LOSS

Active noise cancellation (ANC) technology has become available for he aring protectors and communications headsets in efforts to augment the ir low-frequency passive attenuation performance. Several analog-elect ronics ANC earmuffs offer active attenuation of up to about 20 dB at f requencies less than 1000 Hz and rely on the muff's passive attenuatio n at higher frequencies. Such performance lends itself primarily to ap plication in wideband, low-frequency biased noises. However, operators in more narrow-band noise threats may also benefit from a new approac h to ANC-based hearing protection devices, i.e., that of the digital, supra-aural headphone. The design, testing, and application of digital ANC-based protectors (as contrasted with analog) are reviewed, and re sults of an attenuation experiment on a siren-cancelling headphone are presented. The insertion loss measurement scenario utilized one-third -octave, 30-s-average sound pressure levels measured by miniature micr ophones in real ears and on a KEMAR manikin. Temporally variant siren modes consisting of ''wail,'' ''yelp,'' and ''hi-lo,'' were reproduced at representative wideband 30-s-average sound pressure levels of 90, 95, and 100 dB. There was generally close agreement between the real-e ar and KEMAR insertion-loss data at midband frequencies from 500 to 63 00 Hz. At the peak siren frequency of 800 Hz, mean real-ear attenuatio n ranged from 8 to 22 dB. At 4000 Hz and some higher frequencies, atte nuations of 15 dB were found, but at the 100 dB levels for the Wail an d Yelp signals, significant reductions in attenuation (from those meas ured at the lower siren levels) occurred as a consequence of a pre-pro grammed signal-level-limiting feature invoked when the siren-cancellin g headset overloaded. The digital active-noise-control technology in t he siren-cancelling headset offers potential for providing selective-f requency hearing protection against certain tonal noise hazards, with concomitant benefits to communications and user comfort of a lightweig ht, supra-aural headphone.