Notice For Designing The Microphone of Speaker Phone

Due to the global pandemic in 2020, many people have had to adapt to an entirely new work style: Working from home. WFM has increased the demand for Speakerphone and video conferencing products that require superior sound quality even in a noisy environment.

 

It’s important to accelerate the development time, but the engineer needs to spend a lot of time on the mechanical design and sound adjustment of the microphone and speaker; it also requires frequent going to the acoustic lab for fine-tuning. You will miss out on the time to the market if you are not familiar with the design criteria of acoustic components,

 

The microphone, a highly sensitive acoustic component, not only receives tiny sound signals in the air but also conductive resonance and harmonic distortion from the speaker vibration and mechanism of the product. These non-linear signals will make the DSP can’t effectively process the AEC signal, and you will hear the echo sound easily. It will decline the quality of the call or the voice you’d like to hear.

 

Therefore, here are some tips for designing the microphone's duplex hands-free calling products.

 

1. Mic Array:  Mic Array is formed by 2 to 8 pcs microphones and combined with DSP & Beamforming algorithm; it could achieve Voice Far Pickup & Noise reduction to improve the quality of voice calls. figure 1

Figure 1. Human ears can distinguish the direction of the sound; we could use two microphones to achieve directional sound receive.

 

 

2. Mic Sensitivity & Fr: The sensitivity of each microphone of the Mic Array should be close to the Fr. Ideally ±1dB, which can reduce the deviation of DSP during calculation, and the total length of the sound tunnel structure should not exceed 5mm and avoid the resonance frequency of the microphone channel <12KHz. Figure 2 & 3

Figure 2. Microphone frequency response matching.

Figure 3. Microphone rubber holder anbezelel

 

3. Mic Phase: Every Mic phase should be closer to each other and will be better, ideally is ±5∘, it could reduce the time of deviation when DSP is working on calculation and ensure the accuracy of Beam & Directivity. Figure 4

Figure 4. Microphone phase matching test.

4. Mic Rubber Holder Sealing:  The microphone rubber holder anbezelel must be truly sealed, and the sound insulation must be at least 20dBSPL to prevent the sound of the internal speaker of the product from entering the microphone's sound tunnel through the rubber gap. Figure 5a & 5b

Figure 5a. Microphone rubber holder and bezel sealing.

Figure 5b. The microphone records the sound of the speaker (microphone airtight frequency response test)

5. Mic vs Speaker location and shock-proof:  Considering the speaker amplification and microphone's AEC signal processing, usually we will put the speaker placed in the middle, and the microphone should be placed far away from the speaker, the microphone also needs to use the rubber to shock-proof and airtight. The box speaker and the shell were in contact with the surface, the screw's hole needs to use a sponge or rubber holder to avoid the vibration, or the microphone will receive the THD signal from the speaker, which will affect the quality of the AEC. Figure 6a & 6b.

Figure 6a. The position and the distance of the microphone and the speaker.

Figure 6b. The microphone records the sound of the speaker (microphone THD test)

 

If customers followed the notice above and the Audio Codec Design Guide, the debugging time could be greatly shortened, enabling the product to be launched on time.

 

Refer to relevant acoustic components

Please click https://www.kingstate.com.tw/application/33