27-11-2012, 01:03 PM
3b Microphone Circuit
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Introduction
In this second part of the lab#3 you will construct a microphone circuit using a compact electret condenser microphone cartridge. The audio amplifier and microphone are important building blocks of many audio communications systems, and will be used in our ultrasonic transceiver system.
Background information
Microphones
A speaker can be used in reverse to create a microphone. In the case the incoming sound wave leads to a mechanical deflection of the cone and voice-coil. According to Faraday’s law, a time-varying current will be induced because the coil is moving in through a magnetic field (produced by the permanent magnet). Although any speaker could be used for a microphone, most speakers are unnecessarily large for this purpose, except in simple intercom applications where it is common to use the same component to perform both the speaker and microphone functions.
Some comments on our choice of ICs
Why the LM353?
Almost any generic op-amp can be used for a microphone pre-amplifier. The signal levels are very low, and all we need is a simple gain stage with a high input impedance. If we were to operate the microphone circuit from a battery, the LM358 would be fine, but this microphone circuit will be used in our AM modulator system later, and that is powered from a bipolar +/-12V supply, so we went with a dual-supply op-amp. Like the LM358, the main virtue here is cost and availability!
Simple Intercom System
The last step is to hook up both circuit modules (microphone and audio amplifier/speaker) to make a complete audio system that detects/receives an acoustic signal, electronically amplifies it and transmits it over a cable, and converts it back to an acoustic signal. This is obviously the basis of most telephone, intercom, and public address systems.
Connect the microphone circuit output to the unused “aux” input on your amplifier circuit. Demonstrate the working circuit to the TA and answer the remaining questions in the LAB RECORD. For this step, C2 should be removed from the circuit, and the level-adjust potentiometer should be set to the middle of its range. It also helps to keep the microphone and audio amp circuits as far apart as possible, try using a long cable as suggested in Figure 1-8.
Possible Improvements
In contrast to the audio amplifier, potential improvements to the microphone circuit are more subtle, and largely involve the microphone itself, not the electrical circuit. The little omnidirectional electret component that we choose is very convenient because it is small, and cheap, but do we really want an omni-directional mic in our system? Probably not, since people are usually speaking directly into the mic, hence the omni-directionality just creates problems by picking up unwanted noise from the surroundings, or undesired feedback from the distant speaker. There are some electrets on the market that have a more unidirectional “cardoid” reception pattern, but these are quite expensive. One fairly simple way to make our mic directional is to put a little cone around it, which can be fashioned out of some heavy paper as shown in Figure 1-10.
Condenser microphones are very sensitive devices, and will generate a signal in response to any nearby air movement. In a “windy” environment, such signals can easily swamp out the desired audio signal.