ADI’s bottom sound hole MEMS microphone can be soldered directly to the PCB by reflow soldering. A hole needs to be made in the PCB to pass sound into the microphone package. In addition, the housing that houses the PCB and microphone has an opening to allow the microphone to communicate with the external environment
In a common embodiment, the microphone is exposed to the external environment. In harsh external environments, water or other liquids may enter the microphone cavity and affect microphone performance and sound quality. Liquid infiltration can also permanently damage the microphone. This application note describes how to prevent the microphone from being damaged by this, making it suitable for use in wet and dusty environments, including full immersion.
design description
Providing protection is easy, just put a piece of soft rubber or something like a seal in front of the microphone. Compared to the acoustic impedance of the microphone port, this seal in the design substantially reduces its acoustic impedance. When designed properly, the seal doesn’t affect the microphone sensitivity, only slightly affects the frequency response, limited to the treble range. The bottom port microphone is always mounted on the PCB. In this design, the outer side of the PCB is covered with a layer of flexible waterproof material such as silicone rubber. This layer of flexible material can be used as part of a keyboard or numeric keypad, or can be integrated into industrial designs. As shown in Figure 1, this layer of material should create a cavity in front of the sound hole in the PCB, improving the mechanically consistent compliance of the film. The flexible film acts to protect the microphone and should be as thin as possible.
The toughness of the film increases with the thickness of the cube, so choosing the thinnest possible material for the application minimizes the effect of the frequency response. A large (relative to the microphone port and the hole in the PCB) diameter cavity and the thin flexible film together form a relatively low impedance acoustic loop. This low impedance (relative to the microphone input impedance) reduces signal loss. The cavity diameter should be approximately 2× to 4× that of the sound port, and the cavity height should be between 0.5 mm and 1.0 mm.
Post time: Sep-07-2022
