ECM and MEMS Microphone -The Operating Principles and Design of Acoustic Ports

This guidance concerns the size of an opening (a port) that transmits sound pressure changes to the diaphragm of an electret condenser microphone (ECM) and Micro-ElectroMechanical Systems (MEMS). microphone: how does that port’s diameter impact the sensitivity and frequency response, and how the port’s volume (a function of its depth and geometry) affects the sensitivity and frequency response.

Port geometry exerts a significant influence on the acoustic performance of both ECM and MEMS microphones. The following discussion presents the physical intuition, the key formulas used to estimate effects, some worked examples, and practical design rules that apply to ECM and MEMS microphones.

Summary:

o    Very small diameters produce:

• Much higher viscous losses and higher
• Lower low-frequency sensitivity, increased low-frequency roll-off, and higher damping.
• The Helmholtz resonance frequency shifts lower, but the resonance becomes heavily damped and may be difficult to observe in the response.

o    Large diameters produce:

• Lower acoustic resistance and lower
• Improved low-frequency sensitivity and less
• May increase the microphone’s sensitivity to wind noise local pressure gradients, and acoustic leakage.

• Port depth (neck length) and the cavity volume behind the port together form a Helmholtz resonator. That resonance frequency and the low-frequency shape depend on the neck area, neck effective length, and cavity volume.

o    Longer necks or smaller areas

• These increase the acoustic
• They lower the Helmholtz resonance frequency (or make low-frequency roll-off worse).
• They often degrade usable low-frequency response, a result of increased viscous damping.

o    Larger cavity volume

• This Increases
• It lowers the Helmholtz resonance
• It tends to extend low-frequency response when losses are not
• Multiple small ports in parallel
  • They behave as one single port with larger equivalent area (areas add) but keep the same combined length effects per port.
  • Their acoustic mass decreases as area
  • Their viscous losses likely remain high if individual ports have very small area.