The time delay value only partially simulates the transfer function between error microphone-headphone transducer, but is adequate to compensate for the phase shifts between from the transfer functions of the error mike and the speaker that cause instability in conventional adaptive filters. The adaptive filter must be stable in order to converge to its steady state solution. A single delay has a straight-line phase response which may be unable to stabilize the composite phase of a system. Passive attenuation in an earcup is a function of the front and rear cavity volumes and the driver compliance below the free air resonance. If the front cavity is kept small to maximize efficiency, then increasing the driver compliance can compensate for the reduction of passive attenuation.
Efficiency is a major issue with portable NCHs which are battery-powered.
High compliance drivers are sensitive to overpressure, which can occur when the user moves or takes off the headphones - the pressure waves due to the movement can pull the voice coil outside the gap or distort the diaphragm. Forming grooves or indentations in the diaphragm can strengthen it and allow it to quickly recover when distorted. Another option is to stiffen the diaphragm by constructing it from different material laminates for greater strength and to prevent natural resonances. This method has the advantage of extending the operating range of the ANR without phase shifts because the system resonance frequency is raised when the diaphragm compliance is less than the compliance of the rear cavity. The fundamental resonance of the diaphragm can be dampened with a dampening disk positioned below the diaphragm.