We hardly realize how much our activities depend on the supply of electricity. There are the most varied types of equipment that only work with electricity. Cells and batteries are the energy sources that are closest to us.
For some people, certain equipment causes nervousness, for others it is their work tool. We are talking about the microphone, which is seen in many different places, such as, for example, on radio shows, freshman shows, etc. Based on a principle of physics called magnetic induction, it was possible to build this equipment that allows us to speak in public with greater intensity of voice.
We can say then that the microphone is an electromechanical device that transforms mechanical vibrations into electrical current.. There is a diaphragm on the microphone, protected by a perforated cover that allows sound to pass through and receives longitudinal sound vibrations.
When some vibration from the air hits this diaphragm, it transmits it to an electrical system, which can be a moving coil, a capacitor or coal grains, depending on the type of microphone used.
At the moving coil microphone, for example, the coil is fixed to the internal part of the diaphragm and is located next to a permanent magnet, whose function is to produce a magnetic field in the region where the coil is. The vibration of the diaphragm moves the coil according to the intensity of the sound waves. Due to the field of the magnet and this movement of the coil, an electrical current is induced, following the pattern of the sound waves that hit the diaphragm.
For this reason, the oscillations of the electrical current in the coil correspond to the vibrations of the sound that produced its movement: the sound was converted into electrical current.
At the capacitor microphone, one of its plates, which is mobile, is connected to the diaphragm so that sound vibrations can be transmitted to it. The capacitor is kept charged by a battery.
Vibrating jointly with the diaphragm, the plate responds to sound impulses, varies its distance with the other plate and, therefore, alters the capacitance of the capacitor. Changing capacitance produces an electrical current in the circuit, which again varies according to the pattern of the original sound vibrations.
At the microphone using coal grains, the sound vibrations captured by the diaphragm reach these grains, causing the average distance between them to vary. This set of coal grains is part of an electrical circuit, powered by a source. When the average distance between the grains varies, the electrical resistance of the circuit changes, causing a variable electrical current.
As with other types of microphone, the end result is the transformation of the mechanical vibrations of sound into oscillations of an electrical current.
By Domitiano Marques
Graduated in Physics
