It is recommended that the external sound holes on the whole case be as close to the MIC as possible, which can simplify the design of gaskets and related mechanical structures. At the same time, the sound hole should be kept as far away as possible from speakers and other noise sources to minimize the impact of these unnecessary signals on the MIC input.
If multiple MICs are used in the design, the selection of the MIC sound hole position is mainly limited by the product application mode and use algorithm. Selecting the position of the MIC and its sound hole early in the design process can avoid the damage caused by the later change of the casing. Cost of PCB circuit changes.
sound channel design
The frequency response curve of the MIC in the whole machine design depends on the frequency response curve of the MIC itself and the mechanical dimensions of each part of the sound inlet channel, including the size of the sound hole on the casing, the size of the gasket and the size of the PCB opening. In addition, there should be no leakage in the sound inlet channel. If there is leakage, it will easily cause echo and noise problems.
A short and wide input channel has little effect on the MIC frequency response curve, while a long and narrow input channel can generate resonance peaks in the audio frequency range, and a good input channel design can achieve a flat sound in the audio range. Therefore, it is recommended that the designer measure the frequency response curve of the MIC with the chassis and the sound inlet channel during design to judge whether the performance meets the design requirements.
For the design using the forward sound MEMS MIC, the diameter of the opening of the gasket should be at least 0.5mm larger than the diameter of the sound hole of the microphone to avoid the influence of the deviation of the opening of the gasket and the placement position in the x and y directions, and to ensure that the gasket acts as a seal. For the function of MIC, the inner diameter of the gasket should not be too large, any sound leakage may cause echo, noise and frequency response problems.
For the design using the rear sound (zero height) MEMS MIC, the sound inlet channel includes the welding ring between the MIC and the PCB of the whole machine and the through hole on the PCB of the whole machine. The sound hole on the PCB of the whole machine should be appropriately larger to ensure that it does not affect the Frequency response curve, but in order to ensure that the welding area of the ground ring on the PCB is not too large, it is recommended that the diameter of the PCB opening of the whole machine range from 0.4mm to 0.9mm. In order to prevent the solder paste from melting into the sound hole and blocking the sound hole during the reflow process, the sound hole on the PCB cannot be metallized.
Echo and Noise Control
Most of the echo problems are caused by the poor sealing of the gasket. The sound leakage at the gasket will allow the sound of the horn and other noises to enter the interior of the case and be picked up by the MIC. It will also cause the audio noise generated by other noise sources to be picked up by the MIC. Echo or noise problems.
For echo or noise problems, there are several ways to improve:
A. Reduce or limit the output signal amplitude of the speaker;
B. Increase the distance between the speaker and the MIC by changing the position of the speaker until the echo falls within the acceptable range;
C. Use special echo cancellation software to remove the speaker signal from the MIC end;
D. Reduce the internal MIC gain of the baseband chip or main chip through software settings
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Post time: Jul-07-2022
