VoIP Acoustic Design - VLSI Solution

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VLSISolutionHHyVoIP Acoustic DesignVSMPG2. HOW TO COMBAT FEEDBACK2.3.8 Meas +21.8 dB: Mic Removed from Circuit Board0−10−20F=466.1865HzLevel=−1.9277dBS/(N+THD)=−17.8183dBS/N=−17.8252dBHD=16.1388dBAmplitude (dB)−30−40−50−60−700 500 1000 1500 2000 2500 3000 3500 4000Frequency (Hz)Figure 2.12: +21.8 dB: Free-Hanging Microphone, Closed ChassisTo further combat potential Equipment Vibration FES, the microphone was disconnectedfrom the microphone circuit board. Then it was connected back with thin electrical wiresas shown in Figure 2.13. The microphone was secured to the main chassis with adhesivematerial.As can be seen in Figure 2.12, preventing Equipment Vibration FES from the circuitboard helped: while it didn’t affect frequencies below 500 Hz, it greatly reduced frequenciesin the most problematic area at 800. . . 1300 Hz. Now signal level was +21.8 dB.Main chassisRubber protector/directorMicrophoneMic pins + solderCircuit boardThin electrical wiresFigure 2.13: Better Speaker Phone Microphone EncapsulationRev. 2.11 2008-09-18 Page 14(22)
VLSISolutionHHyVoIP Acoustic DesignVSMPG2. HOW TO COMBAT FEEDBACK2.3.9 Meas +16.2 dB: Mic Removed from Circ. Board + Chassis Hole0−10−20F=39.0625HzLevel=−9.6489dBS/(N+THD)=−19.8129dBS/N=−19.8133dBHD=7.4257dBAmplitude (dB)−30−40−50−60−700 500 1000 1500 2000 2500 3000 3500 4000Frequency (Hz)Figure 2.14: +16.2 dB: Free-Hanging Microphone, Open ChassisRemoving the microphone from the main circuit board helped a lot, but there was stillmuch room from improvement.An extra hole, roughly the size of the SD port hole, was added to the main chassis. Ascan be seen in Figure 2.14, this moved the high peak frequency range from 300. . . 600 Hzto 700. . . 1000 Hz, but above all it lowered signal level. Now the total signal level was+16.2 dB above the reference level. At frequencies 2800. . . 3400 Hz it is in fact better thanthe reference unit in Chapter 2.3.3, although at lower frequencies it still is significantlyworse. The signal level was now as low as it was in Chapter 2.3.5 where no chassis wasused. This was low enough for the VoIP speaker phone to work, although even lowerlevels would be preferable.Rev. 2.11 2008-09-18 Page 15(22)
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VoIP Acoustic Design - VLSI Solution

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