Hi. I'm currently doing a project with the Arduino Uno and Arduino GSM Shield.
A microphone will pick up a noise from a Burglar Alarm of 85 dB then it will go through an amplifier then passing through a rectifier diode and then a low pass filter. After that, signal is fed to the Arduino UNo and it will trigger the sms to be sent by the GSM Shield.
I built a pre-amp (small signal amplifier or bjt amplifier common emitter). A electret mic is connected to the input of the pre-amp. And the output of the pre amp is connected to a diode for rectification and then it goes through a low pass RC filter.
I tested the mic on the oscilloscope using a 10k ohm resistor and 150uF capacitor. The maximum output is about 4.2V from the mic which is quite high.
Testing such big signal on multisim, I have to change the coupling capacitor at the input of the pre amp to 1nF. Therefore, basically I build an attenuator not an amplifier anymore.
As the microphone will pick up a noise of 85dB so it should be a maximum voltage so I use 4V and 1kHz when testing with multisim.
The output of the pre amp after feeding 4V input on multisim is 1.7V. It's all good at this point. Then it comes to the rectification and filtering parts, the voltage just disappear. It drops down to 200mV so I dont know how can it drive the Arduino (as I know the signal must be 1V to 5V in amplitude).
I wonder what is the problem there and what are the other methods that I cab have to get a good constant output to feed into the Arduino Uno.
I use 9V supply as the electret mic needs 9V. So the Arduino and the pre amp are also used 9V to operate.
I read other similar topic that said I need an amplifier after the mic because the mic output is so small. But in my case, the tested mic gave me 4.2V.
I thought the project is all sorted but many problems come up 
Please help,
Kind regards,
Eric P