(57) An automatic gain active noise reduction arrangement for reducing the level of acoustic
noise within the internal cavity or enclosure of an ear-defender or earphone structure
(2), the arrangement comprises a noise pick-up microphone (4) and a noise-cancelling
transducer (9) mounted within said cavity or enclosure, the noise pick-up microphone
(4) being adapted to produce an electrical signal output in response to an acoustic
noise field within said cavity or enclosure, and phase inverter means (5), filtering
means (6) and amplifying (8) means connected in a feedback control path extending
between the noise pick-up microphone (4) and the noise-cancelling transducer (9) and
effective in response to the generation of an electrical signal output by the noise
pick-up microphone (4) to produce a noise-cancelling signal output which is fed to
the noise-cancelling transducer (9), in which the output from the noise pick-up microphone
(4) is also applied to signal processing (10,11) and control means (7) for producing
an electrical output which is dependent upon the microphone output and a predetermined
non-linear control algorithm and which is applied to variable loop gain control means
coupled with the feedback control path for controlling the loop gain in accordance
with a preselected parameter of the microphone output.
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