Sennheiser NoiseGard HMDC 322 Instructions For Use Manual page 65

In response to these problems, Sennheiser has developed an active noise compensation
system – NoiseGard™ – which, combined with a high-quality passive hearing protector,
provides consistent noise attenuation over the entire audio range. The overall noise
level is reduced so that the radio volume can be turned down but enough noise still
remains for the pilot to safely monitor the aircraft engines.
NoiseGard™ active noise compensation is achieved by generating a signal identical in
sound pressure level but exactly reversed in phase to the noise signal, the effect being
that the out-of-phase signal cancels most of the noise signal.
Active noise compensation is accomplished in the following manner: Each earcup
includes a microphone, a feedback control circuit, and a transducer to reproduce both
the communication and the noise cancelling signal. The feedback control microphones
sense the total sound pressure within each earcup resulting from both the desired radio
signal from the receiver and the undesired noise that has come through the earcup. The
microphone signal is amplified and the radio signal is subtracted from it. The remaining
signal (noise) is then filtered and inverted and the radio signal is added back in.
Eventually, the entire signal is amplified and fed back to the transducer in each earcup.
Since the noise component of the signal is inverted, it cancels the noise signal coming
through the earcup. The radio signal remains unaffected, as it was not processed
through the cancellation circuits.
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