The microphones (mic or mike in short) and the speakers are very common audio equipment. You see them not only in public meetings and conferences, you come across them even when you use your phone. The work of a microphone and a speaker are opposite of each other. A microphone converts sound vibrations into electrical entity (voltage/current) while a speaker converts the voltage/ current into sound vibrations by moving the diaphragm of the speaker and producing vibrations in the air. Basically a microphone has a diaphragm which moves when sound pressure pushes it. This movement can be converted into proportional voltage using several possible transducers. Here, a transducer is a device which receives electrical, mechanical or acoustic waves from one medium and converts them into related waves for a similar or different medium. Thus, it can be said that a microphone (mic) is a transducer that converts acoustical sound energy into electrical energy. Its basic function is therefore to convert sound energy into electrical audio signals which can be used for further processing.
Dynamic vs. Condenser Microphones come in many different shapes and sizes, but all do the same thing: convert sound waves into voltage (measured in millivolts). The two types you are most likely to encounter are dynamic and condenser.
Dynamic microphones are passive—that means they do not require a power source. They’re rugged, robust and reliable microphones. Dynamic mics are often used on stage, but they can add depth and value in a recording environment as well. They’re perfect for drums and electric guitar cabinets. Since dynamic microphones have a weaker signal than condenser mics, they must be placed relatively close to the audio source.
It has a very thin diaphragm of thickness 1 to 10 micrometers. One micrometer (or micron) is one millionth of a meter or one thousandth of a millimeter. Close to this plate (metallic or metalised plastic) stands another metallic plate with holes. These 2 plates act as electrodes and are kept at opposite polarities by supplying D C to behave as a condenser, they should be insulated from each other. When sound wave pushes the diaphragm, it vibrates and the capacitance of the condenser (or capacitor) changes. This is because the capacitance is proportional to the potential difference and inversely proportional to the separation between the plates. Any change in the separation changes the capacitance. The capacitance is also dependent upon the medium but as the medium here remains the same, so we ignore this parameter. The values of the resistance and the capacitance are chosen such that the change in voltage is immediately reflected in the voltage across the resistance in series. Any change in sound leads to change of the capacitance and leads to voltage change. The voltage is fed to an amplifier to amplify the level of the signal. Condenser microphones were invented in Bell Labs in 1916.
Condenser microphones require a power source, which can come from a phantom power supply or a battery. Condenser microphones are generally more sensitive than dynamic microphones and have better high frequency response characteristics. Condenser mics are a good choice for choirs or stage performers who are some distance away from the microphone. They have higher fidelity than dynamic microphones, but they’re less durable. They’re usually more expensive, too.
This works on the principle of electro-mechanical induction. This type of microphone is called moving coil microphone also. Here a very small coil is used which is attached to a diaphragm and suspended in a magnetic field of a magnet as shown in the diagram: 6.2 below. When sound waves impinge on the diaphragm it vibrates and attached coil moves. This movement of the coil inside the magnetic field produces a emf across the terminals of the coil. The current so produced in the coil is in proportion to the sound.
