the physics behind capturing a sound


The microphone has been in use since its invention in 1876 as a tool for capturing sound waves and transmitting them through electronic signals. The first microphone was created by Emile Berliner for use in Alexander Graham Bell's telephone. The microphone soon was adapted for the recording and amplification of music both live and in-studio. Over the years, microphone design has shifted and audio engineering itself has changed to accommodate the new ways in which people listen to media. The three most widely used types of microphones today are the condenser, the ribbon, and the dynamic microphone - each of them unique in their design and reproduction of sound waves.

The condenser microphone, invented in 1916 by E. C. Wente is also known as the capacitor microphone, or electrostatic microphone. In condenser microphones the the plate of a capacitor acts as a diaphragm, and the vibrations from sound waves changes the distance between the plates. This pair of a movable and fixed plate is called a "capsule." A nearly constant charge is maintained on the capacitor. As the capacitance changes, the charge across the capacitor changes slightly, but at audible frequencies it is unnoticeable. The audio signal is filtered out from other charge movements through a high-pass filter.

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The development of the condenser microphone and the electronic vacuum in the 1920s tube amplifier enabled the advance into sound recording for films. Condensers rapidly overtook the carbon microphone, which was the previously popular microphone used at the time. The condenser microphone was vastly superior to the carbon microphone in both signal-to-noise ratio as well as overall output volume. The only disadvantage to condensers was their need for a power supply. In later years, this problem was conquered through the use of “phantom power”, where a DC current could be supplied to the microphone through the grounding wire of the common three-pin XLR microphone cable directly from the mixer without need for batteries or an outboard power supply.

Condenser microphones are considered to be the most accurate type of microphones because of their “flat” frequency response and their clarity. The condenser's frequency response is very regular across the audible spectrum and also extends from around 50 Hz all the way to 18 kHz before cutting off in dB. This extended response range makes them ideal for studio use, especially for the recording of vocals, cymbals, guitars, and other brass and acoustic instruments. Small diaphragm condensers are also used to capture cymbals live because they are the only type of microphone that can accurately reproduce the high-end frequencies. Condensers are also occasionally used for capturing vocals during live performances because of their definition.

The ribbon microphone gains its name from an extremely thin, ultra light ribbon of aluminum or another conductive metal that is suspended at its ends within the microphone. This ribbon is vibrated by sound waves and moves back and forth within a strong magnetic field. The ribbon's bidirectional movement and its insensitivity to vibration at its clamped sides produce a figure-8 pickup pattern, capturing sound from in-front and behind equally. In older models, magnet assemblies tended to be bulky and made the microphone inconvenient for many non-studio situations. Also, the inner ribbon is extremely delicate and can not handle loud sound pressure levels. The ribbon microphone also emits a very quiet output signal. The only way to increase volume is unfortunately to make the ribbon even thinner which makes the microphone increasingly delicate and fragile.

Many consider the ribbon microphone to be the most natural sounding of all types of microphones. After their invention, they were immediately accepted by the broadcast and recording industries of the early 1930s. They were more convenient because they did not need any awkward power supply or batteries to operate. More recently, with the advent of condenser microphones in the past 30 years, ribbon microphones have slowly disappeared off the radar. Smaller clip-on dynamic or condenser microphones have almost entirely replaced the on-stand microphones of the past. Despite their falloff from commercial media, the huge growth in home recording has drastically increased their demand, with new high quality ribbon microphones being produced for a widespread consumer market instead of industry. Many home and professional studios keep a few ribbon microphones in their shelves in case their recordings crave the ribbon microphone's distinct sound.

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Ribbon microphones are delicate, and can only be used on the softest of instruments. They are generally reserved for vocals, a soft guitar, or woodwinds. The ribbon microphone's frequency response remains ample at frequencies below 200 Hz, but cuts off at higher frequencies around 10 kHz. This response gives off a much deeper tone than that of a dynamic or condenser. This type of sound response is often interpreted as “retro” sounding because many classic blues, jazz, and rock recordings were captured with only a single ribbon microphone placed within a live studio room.

Dynamic, or moving-coil microphones are the most simple and straightforward type of microphone. Utilizing electromagnetic induction by the movement of a coil attached to a diaphragm, they are durable, fairly inexpensive, and resistant to moisture. This together with their potential for high gain before feeding-back makes them ideal for on-stage use. The design of the moving-coil microphone resembles that of a speaker, only reversed. A small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to a diaphragm. When the diaphragm is vibrated by sound waves, the attached coil is moved across the magnetic field. This produces a varying current in the coil through electromagnetic induction.

The durability of dynamic microphones as well as their ability to withstand high sound-pressure levels makes them the perfect match for loud instruments as well as general use during live performances. The Shure SM-57, a common dynamic microphone, is often noted as an “indestructible” microphone, for its reliability even after years of abuse. Dynamic microphones are commonly used to capture the kick and snare drum, as well as the toms of a drum kit. Loud guitar amplifiers are another match for the durable dynamic, as the amplifiers may be turned up to full blast so that the guitar may achieve full distortion. Dynamics are also often used above condensers for capturing vocals during live performances because of their high-gain before feedback.

Not uncommonly in audio engineering it is the combined use of several kinds of microphones on a single instrument that achieves the fullest-sounding result. The phase alignment of the multiple audio signals is critical in multi-track recording to ensure that the resulting mix is clear, and that the addition of the multiple microphones does not dirty the sound but instead adds definition and fullness to the audio wave. The positioning and choice of microphone to achieve the best sounding signal is an art form in and of itself.

The various types of microphones each work in their own unique way and color the reproduced sound wave a certain way depending upon their design. Condenser microphones reproduce sound waves through a diaphragm that acts as one plate of a capacitor, and the vibrations from sound waves produce changes in the distance between the plates. Condensers are the most accurate microphone and also boast the widest frequency response. Ribbon microphones emit signal from the vibration of a thin metal ribbon between two magnets. Ribbons provide a vintage and “bassy” frequency response, but they are also extremely delicate. Dynamic microphones are the simplest and operate through a diaphragm that moves a coil through a permanent magnetic field. They are nearly indestructible and can handle the loudest of sound-pressure levels during live performances. Each type of microphone has its advantages and disadvantages. It is up to a well-trained audio engineer to decide which microphone is appropriate depending upon what the situation requires.