Lateral Mode Capacitive Microphone with Sandwich Backplate
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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 systemic noise issues.
Innovation Solution
A capacitive microphone design with a lateral mode configuration and a sandwich structure is employed, where the movable membrane does not move into the fixed backplate, and the conductors are arranged in a spatial relationship that maximizes mutual capacitance variation with acoustic pressure, reducing squeeze film damping and systemic noise through a comb finger configuration and air flow restrictors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the movable membrane is placed close to the fixed backplate to increase capacitance sensitivity, then the audio signal quality is improved, but squeeze film damping increases causing mechanical noise
Solution Approach 1:
The capacitor plate is segmented into multiple sections (first capacitor plate section and second capacitor plate section) that are laterally offset from each other. This segmentation allows the movable membrane to maintain close proximity to the fixed backplate for high capacitance sensitivity while the lateral offset prevents direct alignment that would cause severe squeeze film damping, thus reducing mechanical noise.
2Reliability
If the gap between the movable membrane and fixed backplate is reduced to enhance capacitance variation, then the frequency response is improved, but air leakage increases causing systemic noise
Solution Approach 1:
The invention transitions from a conventional vertical stacking arrangement to a lateral offset configuration where the first and second capacitor plate sections are positioned side-by-side rather than one above the other. This dimensional change allows the movable membrane to be close to the backplate for good frequency response while the lateral offset creates a longer air path, reducing air leakage and associated systemic noise.
3Ease of manufacture
If a parallel-plate capacitive design is used to simplify the structure, then the manufacturing is easier, but squeeze film damping becomes significant causing noise
Solution Approach 1:
The invention introduces asymmetry by laterally offsetting the first and second capacitor plate sections relative to each other, breaking the symmetric parallel-plate configuration. This asymmetric arrangement maintains manufacturing simplicity while effectively reducing squeeze film damping by preventing the movable membrane from moving directly into the fixed backplate, thus reducing mechanical noise.
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 effectively reduces mechanical noise and maintains high audio quality by minimizing squeeze film damping and preventing air leakage, resulting in improved frequency response and sensitivity across a broader range of frequencies.
Implementation Method 1
Two thin layers 101 and 102 are placed closely in almost parallel. One of them is fixed backplate 101, and the other one is movable/deflectable membrane/diaphragm 102, which can be moved or driven by sound pressure. Diaphragm 102 acts as one plate of a capacitor, the vibrations of which produce changes in the distance between two layers 101 and 102, and changes in the mutual capacitance therebetween.
Implementation Method 2
Squeeze film damping occurs when the moving component is moving perpendicular and in close proximity to the surface of the fixed component (e.g., between approximately 2 and 50 micrometers). The squeezed film effect results from compressing and expanding the fluid (e.g., a gas or liquid) trapped in the space between the moving plate and the solid surface. The fluid has a high resistance, and damps the motion of the moving component as the fluid flows through the space between the moving plate and the solid surface.
Implementation Method 3
air flow restrictors
Data Source
AI summary
The present invention provides a capacitive microphone including a MEMS microphone. In the microphone, a movable or deflectable membrane/diaphragm moves in a lateral manner relative to a fixed backplate, instead of moving toward/from the fixed backplate. The fixed backplate includes an electrical insulator sandwiched between two sub-conductors to cancel systematic/background noise. The squeeze film damping is substantially avoided, and the performance, such as signal to noise ratio, of the microphone is significantly improved.