Hello Takashi,
what is acceptable noise level for mic boost? My laptop has three levels of mic boost (+36dB). Level 3 is useless since noise level will be about 0db. With level 1 i get -12dB noise, so there is almost 12dB room for data... but it is not enough for normal speech, which is about 20dB.
Are there any ALSA guide for HW designers, which will say some thing like acceptable levels are 90-20dB? There are some Skype and Microsoft prescriptions for certification. For example M$ prescribe 18dB or more for speech to noise ration for build in mics. It sound like good number to start.
In my case, i need to remove mic boost completely to guarantee minimum 18dB SpNR. Suddenly i didn't found correct way to do it. I will be thankful for some tip.