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Understanding acoustic design for MEMS microphones

17 Apr 2014  | Alessandro Morcelli, John Widder

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Changing the acoustic impedance of the gasket did not affect the resonant frequency, which is determined by the geometry of the acoustic path, but it did affect the Q of the resonance. Although the acoustic path continues to resonate, softer gasket materials damp the resonance and reduce its impact near the resonant frequency. Even using iron surface material for the sound inlet reduced the peak magnitude of the response significantly from the results obtained with sound-hard boundary conditions, showing that using sound-hard boundary conditions gives unrealistically severe results.


Case study: Analysis of the full acoustic path of a tablet with a bottom-port microphone
Figure 11 shows an example of an acoustic path for a bottom-port microphone in a tablet. In this example the bottom port microphone is mounted on a PCB and a soft rubber gasket is used to provide the seal between the PCB and the product housing.

Figure 11: Sound path design for a tablet microphone and 3D model of the acoustic cavity.


The correct acoustic properties were used for all of the components in the acoustic path in this simulation. Figure 11(b) shows the 3D model of the acoustic path of the structure shown in figure 11(a). The materials used for this simulation are those typically found in consumer electronics: the printed circuit board is FR4, the gasket is made with a soft rubber, and the case material is aluminium.


Figure 12: Tablet microphone acoustic path simulation results.


Figure 12(a) shows the frequency response for the microphone acoustic path with a resonant peak at about 21.6kHz, and figure 12(b) shows the distribution of pressure in inside the acoustic path at the resonant frequency. The maximum pressure at the resonant frequency is at the MEMS membrane.


Summary
The following guidelines will help optimise the frequency response of a microphone's acoustic path:

 • Keep the acoustic path as short and as wide as possible. Widening the sound path at the exterior end will help the frequency response but widening it next to the microphone will hurt the response.
 • Try to eliminate any cavities in the acoustic path. If a cavity is unavoidable then it should not be directly adjacent to the microphone sound inlet.
 • Bends in the acoustic path do not seem to have a major effect on the frequency response.
 • Softer gasket materials damp the resonance and may improve the frequency response.


About the authors
Alessandro Morcelli is microphone applications engineer at STMicroelectronics SRL.

John Widder is with audio & MEMS microphone marketing team at STMicroelectronics Inc.


To download the PDF version of this article, click here.


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