What severe limitation did fixed analog ANC systems face, often termed 'hallowing'?

Answer

Acoustic feedback where the secondary source interfered with the reference microphone.

Fixed analog systems, prevalent before the digital revolution, suffered from a critical instability known as acoustic feedback, or 'hallowing.' This phenomenon occurs when the noise generated by the secondary noise source (the loudspeaker emitting the anti-noise) travels back acoustically and is picked up by the reference microphone intended only to sense the primary, unwanted noise. This creates a loop where the system becomes unstable, effectively reinforcing rather than canceling the sound, because the system is reacting to its own output rather than solely the original noise input. Later analog refinements focused on geometric arrangements to minimize these feedback paths.

What severe limitation did fixed analog ANC systems face, often termed 'hallowing'?

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