Lateral Mode Capacitive Microphone Reducing Squeeze Film Damping
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Solution Overview
Problem
Capacitive microphones suffer from significant mechanical noise due to squeeze film damping, which dominates noise sources and affects audio quality, especially when the movable membrane moves close to the fixed backplate, leading to sensitivity drops at low frequencies due to air leakage.
Innovation Solution
A lateral mode capacitive microphone design where the movable membrane does not move into the fixed backplate, instead gliding laterally, reducing squeeze film damping and preventing air leakage through a comb finger configuration and air flow restrictors, maintaining mutual capacitance variation primarily along a defined direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the movable membrane moves close to the fixed backplate to increase capacitance sensitivity, then the mutual capacitance variation increases, but squeeze film damping occurs causing mechanical noise and sensitivity drops at low frequencies
Solution Approach 1:
The patent transitions from a conventional parallel-plate configuration where the membrane moves perpendicular to the backplate (one-dimensional motion) to a lateral mode configuration where the membrane moves parallel to the backplate (changing the dimension of motion). This dimensional change eliminates squeeze film damping because the membrane no longer compresses the air film between itself and the backplate, thereby reducing mechanical noise while maintaining capacitance sensitivity through lateral displacement.
2Measurement precision
If the gap between the movable membrane and fixed backplate is reduced to enhance capacitance effect, then the mutual capacitance increases, but air leakage occurs causing sensitivity drops at low frequencies
Solution Approach 1:
By changing the motion dimension from perpendicular to parallel, the patent allows the membrane to achieve sufficient lateral displacement for capacitance modulation without requiring a reduced gap distance. This maintains an adequate air seal between the membrane and backplate, preventing air leakage and preserving low-frequency sensitivity while still achieving the necessary capacitance effect through lateral movement.
3Measurement precision
If the membrane surface area is increased to improve sound wave detection, then the capacitance variation increases, but squeeze film damping becomes more significant increasing mechanical noise
Solution Approach 1:
The lateral mode configuration allows the membrane to move parallel to the backplate, eliminating squeeze film damping effects. This enables the use of larger membrane surface areas to improve sound wave detection and capacitance variation without the penalty of increased mechanical noise from air film compression, as the lateral motion does not compress the air between the membrane and backplate.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design significantly reduces equivalent noise level by 4 dB and improves Signal-to-Noise Ratio (SNR) by 10 dB, while maintaining flat frequency response across the desired range, minimizing sensitivity drops at low frequencies.
Implementation Method 1
The two conductors are configured to have a relative spatial relationship therebetween so that a mutual capacitance can be generated between them
Implementation Method 2
The relative spatial relationship as well as the mutual capacitance can both be varied by an acoustic pressure impacting upon the first electrical conductor and/or the second electrical conductor
Data Source
AI summary
The present invention provides a capacitive microphone including a MEMS microphone. In the microphone, the movable or deflectable membrane/diaphragm moves in a lateral manner relative to the fixed backplate, instead of moving toward/from the fixed backplate. The squeeze film damping is substantially avoided, and the performances of the microphone is significantly improved.