Receiving path Fr simulation
KINGSTATE has COMSOL Multiphysics Software & High-Speed workstations. In the early stage of product development, the 3D/Rubber holder/Microphone parameters of the mechanism of the product can be input the COMSOL system to simulate the frequency response of the microphone's tunnel path. In the COMSOL system, you need to set the Microphone/Rubber holder and related parameters. After the setting finish, the COMSOL will start to calculate the result. (Fig.1.1~1.4). If the simulation curve does not reach the expected value, you can go back to the 3D model to correct the size and simulate the frequency response again. And then, you will have several different frequency response curves that can analyze on the computer and optimized frequency response curves that can be found. After that, make the mockup of the samples according to the best model. And assembly the model to measurement to verify if it complies with the COMSOL simulation curve. If not, adjust the 3D or model to be consistent with the simulation. In the end, it could be delivered to the customer for verification until they make a mold. Use COMSOL to save the time of the experiment. Also, submit the simulation results to customers for reference in advance.
Rubber holder design
The microphone is a sensitive component, it usually is placed between the PCB and the panel. It needs to be closely matched with the panel and airtight buffer material to ensure that the sound will only enter the microphone diaphragm from the microphone sound hole, and not enter the microphone from other paths. At the same time, it is necessary to ensure the diameter and length of the receiving tunnel to avoid resonance frequency falling within the working frequency range. Therefore, besides the basic characteristics of the microphone. To achieve the best microphone record characteristics of the product, the rubber holder also will be one of the important parts. The material of the rubber holder will also affect the sound insulation characteristics. Generally, we will choose Shore 50A for the silicon rubber holder. In addition to the better airtightness, it can also absorb the vibration and noise conducted by the housing or PCB. While given appropriate interference force to the rubber ring. It could meet the airtightness requirement at >15dBSPL.
Mic airtight structure design
The key to tightly matching the microphone rubber holder and the mechanism is a rubber ring. It can avoid receiving the pressure wave signal from the internal speaker of the product, which causes the DSP to process AEC poorly and makes the product have too many sounds during the phone calls. The echo or distortion phenomenon will decrease the communication quality. Therefore, to achieve airtightness of the rubber ring, the product structure must also give a certain pressure, so the rubber ring can fully interfere with the panel to achieve broadband airtight isolation performance. The AEC test items could pass easier when doing voice quality testing.
Hands free car mic module design
To avoid the danger of drivers are have to hold the phone during driving, a hands-free communication system for cars has been developed. Drivers can speak to the microphone module directly. It can be divided two major categories: car front-mounted and car after-mounted markets. The front-mounted microphone module is mostly installed in the roof map lights, the speaker is the existing speaker system in the car; the after-mounted type is clipped to the sun visor or fixed behind the steering wheel commonly. Most of the products are equipped with their speakers, both types are AEC function is required to avoid excessive echo during a call in the car, which will affect the quality of the call. The microphone module for the car is shown in Fig.4.1 The microphone is usually composed of ECM or MEMS microphone to the front-end receiving function. There are two types of microphone output interfaces, digital and analog. (Fig.4.2), and directivity can be divided omnidirectional and unidirectional (Fig.4.3). The advantage of the digital type is it has better anti-noise, but the distance to the Audio Codec can't be too far. However, as long as the analog type is used with noise isolated wires or twisted wires, the transmission distance can usually be more than 2M. Regarding directivity, a single-pointing microphone can independently attenuate noise in a 180-degree direction, such as the engine noise, while 2 pieces array microphone can further only receive the voice in the driving area to make the sound quality better, but it has to be with beamforming calculations Law & DSP. (E.g. Fig.4.4)
Uni-directional mic structure design
ECM has three types of directivities. According to its polar pattern, it can be divided Omni-Directional/Uni-Directional/Noise Cancelling. And the Omni-Directional microphone is the one that is used in the world commonly, the usage rate of noise-canceling type is low. When it is necessary to eliminate the noise in the rear direction, the Uni-Directional Mic is usually selected. Its receiving directivity waveform is similar to a heart shape. Uni-microphone usually used in car hands-free calls/ headset Boom microphone/ telephone headset Boom microphone/ conference call system, to reduce the noise of microphone with an angle in 180 degrees, the attenuation of the directivity at 1KHZ with an angle 180 degrees is generally up to 15dB, which can meet general consumer noise reduction purposes (Fig.5.1). The reason why the uni-directional microphone can be directivity, the most important parts are the hole behind the microphone and the internal hole's material of the damper, these two structures will have an acoustic resistance effect, so the sound from the angle 180 degrees will pass through the acoustic resistance material. To reduce its moving speed and reach the diaphragm simultaneously as the sound that bypasses the 0-degree sound hole on the front. The diaphragm has the same pressure wave and the diaphragm will stop vibrating, so there is no output signal, the sound will attenuation in the angle 180-degree that direction has achieved. And the formation of directivity characteristics. (Fig.5.2).
Wind noise resistant microphone design
When the microphone is used outdoors, it is often affected by the wind to make the microphone diaphragm produce noise. Because wind-shear noise is a low-frequency pressure wave, it will cause the microphone to the diaphragm to move to the limit greatly, and it is easy for the microphone to enter the non-linear working area. The microphone signal output produces a clipping or distorted signal, which degrades the quality of the call. To reduce the occurrence of this phenomenon, the microphones of consumer products are usually not like professional equipment. They use bulky windshield equipment to filter out wind-shear noise. While, we usually put a sponge as noise-canceling material that is placed in front of the microphone's sound hole, so it can be filtered and also attenuation the wind-shear simultaneously (Fig.6.1), or using the microporous structure company with the sponge as the noise-canceling method (Fig.6.2), it can also achieve a good anti-wind noise effect (Fif.6.3).
AEC optimized design
Communication products are usually equipped with microphones and speakers. When making a call, the microphone will receive the sound signal by the speaker and enter the system through the microphone again, it will cause the system to generate echoes that interfere with the call. To eliminate the system whistling sound, it is generally necessary to cooperate with the DSP to effectively eliminate it. The microphone can adopt a uni-microphone to reduce the signal from the rear speaker. The microphone must be as far away from the speaker as possible, and the two should not be installed on the same plane. However, the combination of the speaker and the mechanism should be designed to prevent the vibration of the speaker transmitted to the microphone, which will make the echo and distortion more serious. Finally, the DSP and AEC algorithm are used to calculate to eliminate the echo, so the echo can be attenuated to the minimum and improve the call quality (Fig.7.2).
IPX7 waterproof structure mic design
Many outdoor products need the waterproof function, especially the parts of sound components, such as microphones and speakers, need waterproof structure design to ensure that the products will not malfunction after water enters. The general waterproof grade can be divided 9 levels, the more common waterproof levels are IPX5, IPX6, IPX7, of which IPX7 must be able to withstand 30 minutes at 1m underwater of work without being damaged. In addition to the waterproof product itself, the housing needs to be waterproof, and the microphone and the mechanism must meet the waterproof specifications. The waterproof microphone usually uses waterproof mesh and adhesive, the mechanism also needs to combine with the microphone appropriately to prevent water enters. The waterproof mesh is designed in the middle layer of the mechanism (Fig.8.1). After the waterproof mesh is added, the high-frequency response will be attenuated because of the acoustic resistance (Fig.8.2).