Meet Microphone. The art of audio

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Meet the Microphone

The art of audio

The simplest microphone constantly used in television production is essentially a miniature electric generator. Like a dynamo in a power plant, a dynamic microphone has a coil of wire suspended in a magnetic field. The coil is mounted on a plastic or foil membrane. Sound waves cause the membrane to vibrate, the coil moves in the field and (remember the school physics course) causes electricity.

Electricity is obtained by direct conversion of sound pressure, and in order to provide an acceptable signal, this pressure must be quite high. Dynamic microphones are not as sensitive as condenser microphones (the second type of microphone widely used in TV production) and the voltage they produce is not enough to connect directly to a camcorder. But due to the simplicity of the mechanical design and rather high strength, they are less affected by moisture and temperature fluctuations, and direct sound conversion means no distortion and electronic noise.

Typically, such microphones are insensitive to extraneous sounds that occur when they are moved, which is especially important for interview recording. A dynamic microphone in its design is very similar to a regular headphone (as an electric generator is similar to an electric motor). In fact, a headset can be connected to the sensitive mixer input and used as an emergency carbon microphone. While working as an engineer at a radio station, I sometimes played a joke on the announcer, using the reverse effect: I whispered something into the intercom, which was previously connected to the studio microphone. And the microphone, acting in this case as a telephone, suddenly said: "Fu! You ate garlic!".

Condenser microphones

Condenser microphones are more commonly used in film and video production. They do not need large magnets, which means they can be light and compact, which is important for a pendant microphone or devices suspended from a crane. Compared to dynamic microphones, high-quality condenser microphones are more sensitive to weak sounds and are less expensive to manufacture, so they are usually used in low-cost complex systems. In a condenser microphone, the sound is not converted into electricity, but modulates the voltage generated by a separate source.

A metallized electrically charged plastic membrane is placed close to a rigid plate. When the membrane vibrates under the influence of sound, a microscopic but variable flow of electrons flows from it to the plate. Since this signal is too weak to be sent to the recording (it is attenuated when passing through even a short cable), a small preamplifier is installed in the microphone itself.

The diaphragm from the power supply of the preamplifier is charged to a couple of hundred volts. This results in high microphone sensitivity (or, equivalently, very low self-noise) and is considered the preferred design for high quality studio and boom microphones. But it is possible to charge a small object not only electrically, but also chemically - just as statically charged tampons are made to remove dust. Thus was created the ubiquitous electret condenser microphone, which can be found almost anywhere, from a cell phone to a movie camera.

Miniature electret pendant microphones are so small that they are almost impossible to see with the naked eye. Full-size electret microphones ($75 - $500 depending on design and polar pattern) are well-suited for hanging video applications. All condenser microphones need preamp power, but the way that power is supplied is different and incompatible with each other. If the camera has a mini-jack mic input, a low DC voltage can be applied directly to the sound membrane.

This may cause distortion of the microphone-level signal coming from the mixer or radio. In this case, an adapter is needed - with a blocking capacitance or transformer (the latter also provides a balanced connection). By the way, the terms "capacitor" and "capacitance" in electronics mean the same thing, but the first more often refers to microphones, and the second to circuit elements. Full-sized studio microphones are powered either by a built-in battery or by the "phantom power" method, when a low DC voltage is applied to both wires of a balanced audio cable at once, and the second pole is connected to its screen braid.

A dynamic microphone that does not need power will not sense phantom power at all (of course, if it is connected properly in a balanced circuit; if connected incorrectly, voltage can damage the microphone, so it is better to turn phantom power off unless it is absolutely necessary ). Miniature electret lavalier microphones have a built-in battery in the connecting plug or use phantom power. When connected in wireless systems, they are powered by the transmitter battery. A condenser microphone may make a loud pop when you first apply power, so it is recommended that you turn the volume control off when you change the battery and connect the microphone to phantom power.

Microphones of other types

A ribbon microphone can sometimes be used to record narration. It is based on the same principle as dynamic, but does not have a membrane. Instead, a small strip of foil is used, suspended in a strong field and driven directly by the sound wave. This design makes the ribbon microphone more accurate in transmitting sounds, especially those that are not too weak, such as speech, and are therefore well suited for speech broadcasting. But these devices are almost never used for video filming: they are too heavy for a hand rod and not very sensitive.

And besides, a strong wind can knock the tape off the correct position, which will require expensive repairs. In a piezoelectric microphone, a crystalline or ceramic plate is fixed on the membrane, which generates an electrical voltage when bent, and the principle of operation of a carbon microphone is based on the electrically conductive device of coal powder under pressure. These two types of microphones are characterized by low sound fidelity and high noise levels, so they have almost fallen into disuse; cheap electret microphones are used instead.

Author: Jay Rose, translated by "Digital Video"

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