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Understanding acoustic design for MEMS microphones

17 Apr 2014  | Alessandro Morcelli, John Widder

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The overall shape of the frequency response is generally the same for most MEMS microphones, with their sensitivity falling off at low frequencies and rising at higher frequencies due to the Helmholtz resonance. However, there are considerable differences in response between different MEMS microphones due to differences in sensor design, package size, and construction, especially at high frequencies. Most of ST's MEMS microphones have the sensor placed directly over the sound inlet in order to minimise the size of the front chamber and ensure excellent high frequency response.

Figure 2: An x-ray photograph of a ST MP34DT01 top-port microphone and its acoustic cavity.

The simulation below shows the frequency response of ST's MP34DB01 MEMS microphone by itself. The simulator solves the acoustic equations at every discrete point of the model. At the end of the simulation, it is possible to plot the collected data at any desired point.

Figure 3: The MEMS microphone cavity of the MP34DB01 and MP34DT01 MEMS microphones.

A simulation of the MP34DB01 microphone shows that its response is very flat at higher frequencies, with a typical rise in sensitivity of about +3dB at 20kHz. This is due to the very high centre frequency of its Helmholtz resonance. The simulation results are in excellent agreement with the measured frequency response of the MP34DB01.

Figure 4: Simulated and measured frequency response of the MP34DB01 MEMS microphone.

The effect of the gasket thickness on the frequency response
A gasket is needed to provide an airtight seal between the microphone sound inlet and the acoustic port in the product housing. When a gasket is placed over a microphone, it changes the resonant frequency by increasing the effective length of the sound inlet leading to the front chamber of the microphone. The following simulations show how gasket thickness affects frequency response by placing a cylindrical tube with a fixed diameter (400µm) and varying lengths over a microphone sound inlet.

Figure 5: MP34DT01 frequency response vs. gasket thickness.

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