I read electret microphone changes its resistance according to sound applied. the resistance is proportional to loudness of sound.
Where did you read this? Please provide a link to the Internet reference because it is, at best, inaccurate.
An electret microphone behaves as a variable capacitor with virtually infinite leakage resistance. Sound waves cause the capacitance
of the electret element to change, not
the electret resistance
. A change in capacitance causes a voltage to be developed across the electret plates, louder sounds producing more voltage. The voltage is applied between the gate and source terminals of an FET, causing the conductance of the FET from drain to source to vary with sound level. The FET is usually an integral part of the electret microphone package, which is a two-terminal device called an electret microphone capsule.
The big difference between an electret element and a capacitor is the electret has a permanent
dielectric polarization that creates an electrical field between the electret front and back plates. This polarization is applied during manufacture by heating the electret crystal above its Curie temperature while applying a strong electrical potential across the two plates. The temperature is then lowered back to ambient with the potential still applied. When the temperature falls below the Curie threshold temperature, the polarization become "locked in" to the electret dielectric and the external potential is then removed.
The dielectric polarization of the electret element represents a trapped fixed charge Q = CV, where C is the capacitance in farads between the two electret plates, and V is the applied polarization voltage in volts. This "charge" is immobile because it is caused by a permanent stress in the electret crystal structure, created when the crystal was heated above the Curie temperature, a voltage applied, and then the crystal allowed to cool with the voltage still applied. However, even though it is trapped inside the electret crystal, the charge will still cause a voltage change between the two electret plates when the plates move relative to each other, thereby changing the electret capacitance.
The series resistance of the electret element is very high, virtually an open circuit. And because the trapped charge is very small and the electret capacitance is also very small, the voltage generated between the electret plates (as a result of sound waves moving one plate relative to the other plate) is from a very high impedance source. Any attempt to conduct this voltage to an external amplifier via a shielded coaxial cable is doomed to failure because the capacitance between the center conductor of the cable and the cable shield is much, much, greater than the electret capacitance. In effect, the cable capacitance "shorts out" the electret element. The solution to this problem is to integrate a small field-effect transistor with the electret microphone element, connecting the plates of the electret between the gate and source terminals of the FET. The FET drain and source terminals are the only two terminals brought out of the integrated package, now called an electret microphone capsule.
When sound is captured by the electret microphone capsule, it is converted into a variable resistance by the integrated FET amplifier. To do this requires an external voltage, typically about 3 to 5 V DC, and a current-limiting resistor, typically about 1 to 10 kilo-ohms, in series with the FET drain. The now relatively high-level audio signal is taken between the FET drain and FET source, coupled through a fairly large, 1 to 10 μFd, capacitor to preserve the low-frequency response of the microphone.
Another big difference is one of the two plates (the electret diaphragm) is movable with respect to the other plate under the influence of sound waves striking the movable plate. This is basically how a condenser microphone works, except the electric field between the plates is permanent in the electret microphone and must be applied by an external voltage source (phantom power) for the condenser microphone.
is there any information on net that I can read?
Sure. Try reading some of the links on this page
. However, the information above and the advice that @Audioguru
gave in his post #3, plus the information you can read about at some of the links, is probably more information than you will ever need to successfully use an electret microphone capsule. BTW, except for the moving diaphragm plate, an electret microphone is virtually identical to one half of an electret passive infra-red (PIR) sensor.