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Acoustic gasket and sealing guidelines

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Acoustic gasket and sealing guidelines

1.1

Best practices for sealing MEMS microphones in consumer

applications

MEMS microphones, owing to their form factor, are suitable components for many

consumer products such as laptops, smartphones, tablets and portable devices in general.

Consumer devices have progressively become very small and thin and the sound inlet of

the microphone is not placed in direct interface with the environment. As a matter of fact, the

devices hosting the microphone commonly have a plastic cover. This mechanical

construction requires interposing, between the cover of the device and the sound inlet, a

gasket which serves to guide the sound wave. Additionally, if the microphone is a bottom

port, the PCB width also increments the acoustic path between the sound source and the

microphone sound inlet. The picture below shows an example of a gasket used in a tablet.

Figure 1. Application example

Basically the acoustic cavity created by the components involved (cover, gasket and the

PCB, if bottom-port package) modifies the frequency response of the microphone. The

equations regulating the behavior of a stationary wave inside an acoustic cavity are complex

and depend on the geometry of the volumes involved such as the hole in the cover, the hole

produced in the gasket and, for bottom-port configurations only, the diameter of the PCB via.

Additionally, the behavior of the entire frequency response depends also on the material of

these components. For these reasons, ST provides a simulation of the frequency response

experiments using this tool to determine basic guidelines for designing a proper gasket.

1.2

Acoustic theory

In microphone applications, a gasket placed on top of the sound inlet works a resonator. An

acoustic resonator works in the following manner: when air is forced into a cavity, the

pressure inside increases; when the external force pushing the air into the cavity is