MEMS microphones featuring high performance and small size are especially suitable for consumer electronics such as tablets, notebooks, and smartphones. However, the microphone sound holes of these products are usually hidden inside the product, so the device manufacturer must design a sound path between the outside and the microphone to transmit the sound signal to the MEMS microphone diaphragm. The design of this sound path has a large impact on the overall performance of the system.
The picture below shows the microphone sound path of a typical tablet:
Figure 1 – Typical Application Example
The sound path between the outside and the microphone diaphragm consists of a product housing, an acoustic seal, a printed circuit board, and a microphone. This sound path acts as a waveguide to build the overall frequency response of the system. In addition, the acoustic impedance of the sound path material also affects the frequency response. If you want to accurately predict the performance of acoustic design, you need to establish a sound path model, using COMSOL and other professional-level simulation tools to simulate the frequency response characteristics of the sound path. However, this article provides the reader with some basic principles for optimizing the microphone sound path.
The structure formed by the narrow sound hole connected to the hollow chamber generates acoustic resonance when excited by sound waves. This resonance occurs when we blow air over the mouth of the empty bottle. This structure is called the Helmholtz resonator and is named after the inventor of the phenomenon, Hermann von Helmholtz. Helmholtz uses resonators with different resonant frequencies to identify frequency components in complex sounds such as music.
The center frequency of the Helmholtz resonance is determined by the following program: 
Where c is the air velocity; AH is the cross-sectional area of ​​the sound hole; LH is the length of the sound hole; VC is the volume of the cavity. The equation assumes that the resonator is a simple structure consisting of a cavity connected to a pipe of equal cross-section. If the cross-sectional area and material of the sound path of the microphone are different, the equation describing the acoustic characteristics of the sound path is much more complicated. Therefore, acoustic simulation experiments must be performed on the entire sound path to accurately predict the overall performance of the acoustic design.
In this paper, we have performed frequency response simulation experiments on different sound paths by changing the thickness and inner diameter of the microphone seal, the sound hole diameter of the product casing, the sound hole diameter of the printed circuit board, the sound path bend and the acoustic impedance of the path material. . The experimental results allow the designer to grasp in advance the extent to which these parameter changes affect the overall performance of the sound path.
The low frequency response of the MEMS microphone is determined by the following main parameters: the size of the venting hole between the front and rear sides of the sensor diaphragm; the volume of the rear chamber. The high frequency response of the MEMS microphone is determined by the Helmholtz resonance generated by the microphone front chamber and the sound hole.
For most MEMS microphones, when the sensitivity of the microphone drops to a low frequency and then rises to a high frequency, the frequency response curve is roughly the same due to Helmholtz resonance. However, different MEMS microphones vary greatly in sensor design, package size, and construction, so the overall frequency response, especially the high frequency response, varies widely. Most of ST's microphones place sensors directly on the sound hole to minimize front chamber volume and ensure excellent high frequency response.
Figure 2 – X-ray image of the sound hole microphone and its sound chamber on the STMicroelectronics MP34DT01
The following simulation results describe the frequency response of the STMicroelectronics MP34DB01 MEMS microphone itself. The simulation tool solves the equation at each discrete point of the sound path model. After the simulation is finished, the data collected at all useful points is drawn. Graphics.
Figure 3 – Sound Chambers for MP34DB01 and MP34DT01 MEMS Microphones
The MP34DB01 microphone simulation results show that the frequency response curve is very flat in the high frequency part, and the typical sensitivity rise is about +3 dB at 20 kHz because the center frequency of the Helmholtz resonance is very high. The simulation results are very close to the actual measured frequency response of the MP34DB01.
Figure 4 – MP34DB01 MEMS microphone frequency response simulation results and actual measurement results
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