Acoustic gasket and sealing guidelines

AN4427

12/19

DocID025705 Rev 1

These simulated geometries are helpful when the gasket designer must introduce different

volumes. This constraint can occur when the whole application forces the placement of the

microphone far from the chassis or perpendicular to the chassis. The following table

summarizes the resonances frequency position according to the simulated geometries.

The simulations performed so far have been focused on the contribution of the geometry on

the microphone frequency response. There is another important parameter to be introduced

to provide a proper analysis of the gaskets: the acoustic impedance of the involved material.

Basically, while the position of resonance frequency depends on the geometry of the whole

structure, the magnitude of the resonance peak depends on the acoustic impedance of the

properties of each surface involved in the simulation. The goal of the following simulations is

to determine the relationship between the resonance peak magnitudes with respect to the

acoustic properties of the material. This target will be achieved by setting impedance

proper acoustic impedance of the material will be set. The simulations will be performed,

firstly setting the pressure at the structure sound inlet opening (1Pa), setting the acoustic

impedance of the MEMS membrane (120*150) and setting the silicon cavity impedance

(2000*1500). The above parameters are common to the entire real situation while the

gasket surfaces acoustic impedance can vary according to the designer’s preference. As a

matter of fact, the gasket can consist of plastic, rubber or other materials. Additionally the

cover can be made of aluminum or iron or plastic as well.

Referring to the following figure, the yellow, pink and green surfaces will be set and kept

fixed according to their real acoustic impedance while the blue surface will be modified

according to the properties of the different materials.

Table 3. Resonance peak vs. geometry

Geometry

Resonance frequency (Hz)

(a): L = 4 mm; R = 300 µ

16600

(b): L = 4 mm; R1 = 300 µ; R2 = 600 µ

11600

(c): L = 4 mm; R1 = 300 µ; R2 = 1 mm

7500

(d): L = 4 mm; R1 = 300 µ; R2= 600 µ; R3 = 300 µ

14600

(e): L = 4 mm; R1 = 600 µ; R2 = 300 µ

22700

(f): Tank L = 4 mm; R = 300 µ

15600

(g): Tank L = 4 mm; R = 300 µ; W = 600 µ

12200

a.

The acoustic impedance is defined as the product of the density of the material and the speed of sound in that