Sennheiser PXC 350 Instruction Manual page 22

NoiseGard Advance
The NoiseGard Advance active noise compensation
system is based on the principle of cancelling out
sound using "anti-sound" (phase-inverted sound).
Tiny microphones pick up low-frequency ambient
noise (up to 1 kHz) near the ear. The NoiseGard
Advance electronics use this noise to generate a
sound wave which is the exact opposite (IE: at the
same level but polarity reversed – 180° out-of-
phase), the effect being that the polarity reversed
signal cancels most of the outside noise.
Additional features
Outstanding sound quality due to patented
adaptive baffle damping and Duofol diaphragms
Optimum protection against ambient noise due to
NoiseGard Advance active noise compensation
Great for travel: comfortable, rugged and foldable
headphone construction with single-sided cable
Designed for use with in-flight entertainment
systems and audio, video and game consoles
Without the detachable audio cable, the head-
phones can be used as passive hearing protectors
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