MEMS Microphone Back Volume Thermal Noise Reduction
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Solution Overview
Problem
Microphone assemblies with MEMS transducers face limitations in achieving high acoustic signal-to-noise ratio (SNR) due to thermal-acoustic noise, particularly at small sizes and low frequencies, where the thermal boundary layer thickness becomes significant, affecting the back volume and noise contribution.
Innovation Solution
Designing MEMS acoustic transducers with a gap height between the counter electrode and diaphragm that is less than two times the thermal boundary layer thickness across the audio frequency band, ensuring that any point within the back volume is within a single thermal boundary layer thickness, thereby reducing thermal-acoustic noise and enhancing SNR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the microphone assembly is miniaturized, then the package size is reduced, but the acoustic signal-to-noise ratio deteriorates due to increased thermal-acoustic noise
Solution Approach 1:
The patent applies parameter changes by precisely controlling the gap height between the diaphragm and counter electrode to be less than two times the thermal boundary layer thickness. This specific dimensional parameter optimization reduces thermal-acoustic noise generation while maintaining the miniaturized package size, thereby improving SNR without sacrificing compactness.
2Object-generated harmful factors
If the gap height between counter electrode and diaphragm is reduced, then thermal-acoustic noise is minimized, but the back volume is constrained
Solution Approach 1:
The patent resolves this contradiction by optimizing the gap height parameter to be less than two times the thermal boundary layer thickness. This precise parameter control minimizes thermal-acoustic noise while the overall back volume is maintained through optimized transducer geometry, achieving both noise reduction and adequate volume for low-frequency response.
3Volume of stationary object
If the gap height is increased, then the back volume is enlarged for better low-frequency response, but thermal-acoustic noise increases
Solution Approach 1:
The patent applies parameter changes by setting the gap height to be less than two times the thermal boundary layer thickness, which optimizes the balance between back volume and thermal-acoustic noise. This parameter optimization ensures adequate back volume for low-frequency response while minimizing thermal noise generation through the constrained gap dimension.
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 configuration results in improved acoustic SNR performance by minimizing thermal-acoustic noise, even at very small sizes, and maintains sensitivity across the audio frequency range, contrary to conventional wisdom, by optimizing the back volume and gap dimensions.
Implementation Method 1
microphone assemblies that include microelectromechanical systems (MEMS) acoustic transducers convert acoustic energy into an electrical signal
Implementation Method 2
thermal-acoustic noise, particularly at small sizes and low frequencies, where the thermal boundary layer thickness becomes significant
Data Source
AI summary
A microphone assembly can include a microelectromechanical systems (MEMS) transducer comprising a transducer substrate, a diaphragm oriented substantially parallel to the transducer substrate and spaced apart from the transducer substrate to form a gap, and a counter electrode coupled to the transducer substrate, the counter electrode positioned between the diaphragm and the transducer substrate. The MEMS transducer can generate a signal representative of a change in capacitance between the counter electrode and the diaphragm. A back volume of the MEMS transducer can be an enclosed volume positioned between the transducer substrate and the diaphragm. The microphone assembly can include an integrated circuit that receives the signal, wherein every point within the back volume is less than a thermal boundary layer thickness from a nearest solid surface at an upper limit of an audio frequency band that the integrated circuit is monitoring.