Digital Active Noise Reduction Ear Plugs

Abstract

Abstract : A digitally-controlled Active Noise Reduction (ANR) ear plug prototype system was developed. In contrast to available ANR headsets that implement fixed analog filters, the prototype defines a digital filter that is optimally defined for the user's current acoustical environment. An above-the-ear (ATE) and an in-the-ear (ITE) prototype ear plug system were developed and demonstrated. A passive headset is worn over the ANR ear plug to increase passive attenuation. A reverberant room was built and a KEMAR mannequin was used to test the ANR ear plugs in band-limited random noise with a spectrum similar to jet noise. Under these conditions, the ATE and ITE respectively gave 30 dB and 40 dB of overall passive attenuation and both gave an additional 10 dB of overall active attenuation. The ATE cancelled noise from 20 to 400 Hz and the ITE cancelled noise from 20 to 3 kHz. Currently only the ATE prototype is fully human compatible and capable of performing in 140 dB external sound levels. Previously-available ANR headsets fail when external sound pressure levels exceed 130 dB. Successful ANR is limited to low sound levels because the ANR system's controlled secondary sound source is normally required to operate in a linear regime while matching the amplitude of the primary noise.