How loud can you shout to make yourself heard at a distance? How large is that distance? A hundred meters, a kilometer, is that it? What if you were to talk to people separated by miles, could you shout that loud and kill your lungs? Or you’d rather whisper into an adjacent ear, and be heard at some other location, no matter the distance, and perhaps irrespective of time? An electronic ear that couldhelp store and transmit your voice and sound at light speeds and even ramp it up if you are too frail? Yes, such is the microphone!
But how does a microphone achieve such a feat? How does it follow our rhythmic and unpredictable sounds and change them into electrical signals?Let us find out how by peering into one such device, the dynamic microphone.
Shown in the image above is a handheld wired dynamic microphone used for speech purposes. It is just a sample from an ensemble of microphone types like collar, hand held, wired, wireless mic etc. The word dynamic signifies the operation principle of this mic out of various methods that have been developed. Basically an acoustic transducer, microphone converts sound/voice to its corresponding electrical signal which is carried out to the speaker or amplifier unit with the help of a wire or wireless means.
Grille and Anti-Roll Guard Band
The grille is the top most part of the microphone. A mesh of aluminum or plastic wires, the semi spherical grille protects the diaphragm of the microphone that does the actual transduction. It is made spherical in shape so as to receive maximum voice signals from nearby sources in all directions. This makes an omnidirectional sound pattern for this microphone. Using a totally opaque shell would reduce the intensity of signal conveyed to the diaphragm which is the actual electric ear; hence a grill or a mesh structure is used so that the diaphragm can collect nearby sound energy.
Lying in the lower hemisphere of the grille is the anti-roll guard band. This structure is based on the anti-rolling mechanism which stabilizes the microphone to be used when the user is moving so that no undesirable sounds due to the movement are detected.
The grille usually cannot be separated from the body as it will then expose the diaphragm to the open conditions which might harm it. Hence, from its periphery, it is embedded into the conical sleeve of the microphone.
Pop Filter
The Pop filter is placed immediately under the grille of the microphone.A thin synthetic foam film attached to the head, the pop filter reduces the popping-sound produced while speaking certain alphabets like ‘B’. It also significantly attenuates the effects of wind on the microphone grille. It is thin foam film acting as a windshield and also protects the sensitive diaphragm from dust.
Internal Structure
The main part of the mic is housed inside the outside shell in yet another plastic frame. The mage above shows this plastic frame which holds the mic part. On removing the upper cap, the diaphragm is revealed. It is a thin plastic membrane just beneath the cover as shown in the image below.It is placed over the magnet and a coil of thin copper (called the voice coil) is attached to it firmly, insulated by a fine layer of varnish.
A permanent circular magnet is required to produce the necessary magnetic field which will help in the conversion process. This magnet is placed concentric to the voice coil and a conducting bobbin, and is shown below.
Working
Unlike light waves, sound waves require a medium to propagate and in this case, air serves as the propagation medium. Sound waves propagate longitudinally through a medium in the form of compressions and rarefactions of the medium depending on the frequency and loudness of sound. Thus, sound approaches the diaphragm like a pack of arrows hitting a common elastic target. Whenever the incoming waves reach the diaphragm they cause it to vibrate. Compression of medium reaching the diaphragm pushes the diaphragm inwards while a rarefaction releases it.
The voice coil is placed such that it is almost at the verge of entering a region surrounded by magnets when the diaphragm is in rest position.This assembly forms the transducer part of the microphone and transfers electric signal to the amplifier/speaker which can then convert it back to acoustic format. The diaphragm senses the sound signal and is compressed and depressed corresponding to the sound signal that it receives. This vibration of the diaphragm causes the coil to move inside the magnetic field of the magnet surrounding it.
Whenever, a conductor is made to move in a magnetic field, a voltage is induced across its terminals the magnitude of which is proportional to the amount of flux linked with it. Known as the Faraday’s Law of Electromagnetic Induction, this principle comes into action here as the voice coil makes brief incursions into the magnetic field due to the movement of the attached diaphragm. This voltage so induced is harnessed out of the assembly through a pair of wires connected to the rear part of the bobbin as shown below.
Terminals
The rest of the wiring is pretty straightforward thereafter. As shown in the image above,beginning at the magnet-voice coil assembly, this wiring is connected to the ON/OFF slider switch and then the terminal. A combination of two wires is used to have a balanced output. In this output configuration, both the wires carry same magnitude of the signal. When two wires are configured to get a balanced output, they are made to be precise copies of each other i.e. made of same material, having the same length, exhibiting the same features of impedance; balanced output mode has reduced interference in the output and gives maximum output at significantly longer lengths.
A microphone can have various types of connecting pins and terminals such as XLR, TRS or Z connector. Shown in the image above is the male part of XLR connector. It has three pins, out of which one denotes ground while the other two are soldered to the balanced wiring. This type of connector is specifically for audio related applications. The number of pins in XLR can vary from three to seven.
Shown in the image above is the connector section of the wired microphone. It takes the input from the female part of the XLR connector and gives the output from the 3.5mm or the 6.3mm jack connectors, whichever is compatible with the receiver. Usually, in smaller devices such as a computer or a home audio system, 3.5mm jack connectors are used. On the other hand, in larger systems such as turn tables, 6.3mm jack connectors are used.
Dynamic Microphones are structurally similar to a speaker in terms of construction; however their working is just the opposite.
Filed Under: Insight
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