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Understanding acoustic design for MEMS microphones

17 Apr 2014  | Alessandro Morcelli, John Widder

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It's clear from these simulations that adding a gasket hurts the high frequency response. The addition of a gasket (and a PCB in the case of a bottom-port mic) increases the effective length of the sound inlet, which lowers the resonant frequency and causes the sensitivity to rise at higher frequencies. Thicker gaskets increase the length of the resonator neck, resulting in a lower resonant frequency and poorer high frequency response.

The effect of the gasket hole diameter on the frequency response
The next set of simulations show the effect of a gasket with a fixed thickness (2mm) and varying hole diameters on the frequency response. Figure 6 shows the simulation results for different gasket hole diameters.

Figure 6: MP34DT01 frequency response vs. gasket hole radius.

These simulations show that increasing diameter of the hole in the gasket increases the resonant frequency, improving the overall frequency response.

The effect of different geometries on the frequency response
The results up to this point have been in line with what could be predicted by examining the equation for the Helmholtz resonant frequency. The next set of simulations look at the effect of changing the shape of the microphone's acoustic path, which is not so easy to predict. The structure shown in figure 7(a) is a simple acoustic path with a constant 4mm length and 600µm diameter that serves as a point of comparison for the other simulations. The other acoustic paths studied increase the complexity by adding cavities with different radiuses, lengths, and shapes that simulate variations in the widths and shapes of the holes in the product housing, the gasket, and the PCB.

Figure 7: Acoustic path shape variations.

Figure 8: MP34DB01 frequency response with different acoustic port shapes.

The effect of different materials on the frequency response
The simulations performed up to this point have focused on the effect of the sound path geometry on frequency response and have used sound-hard boundary conditions for all of the surfaces. The next set of simulations examines how the acoustic impedance of the gasket affects the frequency response. The correct acoustic impedances were used for the materials used for the inlet (yellow), sensor body (pink), and sensor membrane (green) surfaces in figure 9 while the acoustic impedance of the blue surface was varied. The acoustic impedance of a material is defined as the product of its density and the speed of sound in that material (Z = ρ • c). Gaskets are usually made from rubber or other elastomeric materials, while typical housing materials include plastic, aluminium, and steel.

Figure 9: Acoustic path surfaces.

Figure 10: The effect of gasket material on the magnitude of the resonant peak.

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