Although it is not good as other microphone , it low cost for mass manufacturing had interest most of the people . In the first revision of the sound sensor module , this electret microphone is couple with “old school” circuit in order to filter and amplify the raw signal from electret microphone .
Figure : “old school” circuit a) high pass RC filter b) low pass RC filter c) Negative feedback Amplifier
Figure above show the old school circuit used . Figure a) and b) is a filter circuit function to filter the unwanted noise from going through it . The most important parameter for filter in this thesis was it cut - off frequency that determine the point between pass frequency and stop frequency . The cut - off frequency equation for the circuit in figure a) and b) can be described as equation (1) below . Figure c) is an amplifier circuit function to amplify whatever signal that go through it . The most important parameter for amplifier in this tutorial was it signal gain that determine how large the output signal will be amplifier from the input signal . The signal gain equation for the circuit in figure c) can be described as equation (2) below .
Figure above show electret microphone will couple with Microphone Amplifier with AGC and Low-Noise Microphone Bias (MAX9814) breakout board
Then for the improved sound sensor module which is second revision , the electret microphone will couple with Microphone Amplifier with AGC and Low - Noise Microphone Bias ( MAX9814 ) as shown in the above . The MAX9814 features a low - noise preamplifier , variable gain amplifier (VGA) , output amplifier , microphone - bias - voltage generator and AGC control circuitry . This electret microphone and MAX9814 is available on a separated breakout board that will be implement in the sound sensor module main board .