Contents
AN4427
Contents
1
Acoustic gasket and sealing guidelines . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1
1.2
1.3
Best practices for sealing MEMS microphones in consumer applications . 5
Acoustic theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Gasket design recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix A Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
MP34DT01 X-ray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
MEMS microphone cavity to be simulated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
MP34DT01 frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
MP34DT01 frequency response vs. tube length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
MP34DT01 frequency response vs. tube radius. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Simulated complex geometries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
MP34DT01 frequency response vs. complex geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Subdomain materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Resonance peak magnitude vs. materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Example of tablet - design and acoustic cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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List of tables
AN4427
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Resonance peak vs. tube length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Resonance peak vs. tube radius . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Resonance peak vs. geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Acoustic properties of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Resonance peak magnitude vs. materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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Acoustic gasket and sealing guidelines
1
1.1
Acoustic gasket and sealing guidelines
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
behavior using the professional tool COMSOL
®
. The following sections describe
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
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