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busknuckle

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Microphone Primer
« on: May 11, 2003, 04:33:38 PM »
http://electronics.howstuffworks.com/framed.htm?parent=question309.htm&url=http://home1.pacific.net.sg/~firehzrd/audio/mics.html

MICROPHONES
There's a lot more about this topic than most people realise. Here I will discuss about various types of microphones, as well as my favourite microphones. Of course, there are better microphones out there, but I haven't had a chance to play with them yet.

How microphones work
Microphones just convert a real sound wave into an electrical audio signal. In order to do so, they have a small, light material in them called the diaphragm. When the sound vibrations through the air reach the diaphragm, they cause the diaphragm to vibrate. This in turns will somehow cause an electrical current in the microphone to vary, whereupon it is sent out to a mixer, preamplifier or amplifier for use.
Microphones are typically classified according to how the diaphragms produce sound.

Dynamic Microphones
Dynamic microphones typically use moving-coil technology. This consists of a diaphragm of usually thin plastic being attached directly to a dense coil of wire. The coil has a magnet either surrounding it or at the centre. As the diaphragm vibrates, the coil vibrates, and its changing position relative to the magnet causes a varying current to flow through the coil. This current is your audio signal.

Dynamic microphones have hardy diaphragms, but are typically damaged when the suspension wires break, due to dropping or rough handling. These are the little things that hold the coil and diaphragm in a floating position relative to the magnets.

Dynamic microphones have to cause a whole coil of wires to move, a mountain when you're talking about little movements of air. This makes them not as sensitive as higher-grade microphones, especially to very soft sounds or high-pitched sounds, such as sibilants in speech or harmonics in music. They have a reputation of being very hardy, and some higher grade dynamic microphones can give a very clean and sensitive sound. Dynamic microphones are also known to give a 'fat' sound, which is flattering to those frequencies that the human ear can hear. They are usually not expected to reproduce the sounds they pick up very faithfully.

Because of their hardiness, they are usually used in live performances, where mic droppages and rough handling are the norm. As such the dynamic sound has also been identified as a 'live performance' sound that sounds cruder and more powerful at the expense of pin-sharp clarity. Performers such as Bono from U2 favour simple dynamic microphones over the more standard studio condensers for precisely that reason.

Condenser Microphones
The technology for condenser microphones has improved greatly in recent years, bringing costs down, increasing its hardiness, and making them even better. In condenser microphones, a static charge is impressed on the diaphragm or on a back-plate to the diaphragm. As the diaphragm vibrates, the distance from the back-plate to the diaphragm vibrates, altering the capacitance of the diaphragm and the back-plate. This fluctuating capacitance results in a fluctuating electric current. Voila! You have an audio signal!

Instead of moving a whole coil of wires, condenser microphones only have a thin diaphragm and solid back-plate making up a capacitor. Condenser microphones are therefore sometimes known as capacitor microphones. The quality difference of impressing a charge on the back plate or on the diaphragm is debateable, it is accepted that the smaller and lighter the diaphragm, the more accurate and pin-sharp the sound will be.

The developments in the clarity of condenser microphones has be partially spurred by the development of digital recording. As standards for recording improved, the quality of recording microphones had to improve to keep pace. However, just as digital recording has its detractors, the anal-ness and transparency of condenser microphones are also not universally popular. Most damning is the fact that condenser microphones tends to record sound as it really is. This means that vocalists must be really good, as any imperfections stand out clear as day.

Large-Diaphragm Condenser Mics
This has lead to the creation of large-diaphragm condenser microphones, which use the same basic technology, but have special larger high-quality diaphragms to produce a more flattering sound. Studios usually have an array of expensive large-diaphragm condenser microphones, as each microphone would reproduce the sound in its own special way. When the correct microphone is chosen for a vocalist, the results can be extremely flattering, making the singer much better than he or she really is.

The cost of a large-diaphragm microphone is usually attributed to the R&D cost of creating a microphone that records sound the way you want it, instead of the way it really sounds. Large-diaphragm mics can also maintain most of the high-frequency sensitivity of regular condenser microphones, making them sharp enough for digital recording to do the sound justice.

Phantom Power
Condenser microphones need a source of power to impress the charge on the capacitor. One of three methods is used. Either a battery will be inserted inside the microphone, a permanent charge is retained on the diaphragm or backplate thanks to some clever material scientist, or phantom power is used.

Phantom power is the supply of power through the ground cable of an XLR cable. Ranging from 9 volts to 52 volts, typically 48 volts, this power can be put into the cable either from a mixer, a phantom power box or a battery pack. A mixer might have a button that allows phantom power through the ground cable. A phantom power box is like an intermediate component between a mixer and a mic. It is a box (duh) connected to the mains that essentially just puts a charge on the ground cable. A battery pack does exactly the same thing, but works only with batteries instead of mains power.

Electret Microphones
Note: information in this section might not be very accurate.

Electret microphones are a variant of condenser microphones that mostly utilise a permanently charged diaphragm over a conductive metal back-plate. They somehow tend to be small, even minuscule, cheap and light. They are especially responsive to the range of sounds from the lower mids to the highest frequencies, i.e. they aren't very good for bass. Back-electret microphones use a charged back-plate instead of a charged diaphragm. These may or may not be phantom powered. Electret and back-electret microphones have special preference for voice communication, where clarity of speech is essential at the sacrifice of perfect sound reproduction.

Plaintalk Microphones
The microphones that are designed specially for use with Macintosh sound-in jacks are called Plaintalk Microphones. They have a special mini-phone jack that is slightly longer than the standard mini-phone jack, and has 4 contact points instead of 3. The last contact point at the tip is meant to carry phantom power up to the cable in order to power the back-electret microphone. The other three contact points are in the same arrangement and have the same purpose and standard stereo mini-phone jacks.

The phantom power provided by Macintoshes is only enough for the microphones that Apple provides or recommends, and is thus not as universal as true 48V phantom power. However, since standard mini phone jacks will not be long enough to reach the last contact point, they can be used with Macintosh sound-in jacks easily as long as they do not require phantom power.

Ribbon Microphones
Note: information in this section might not be very accurate.

These are very rare, and only used in the best recording studios. They are very sensitive to shock and large sound volumes, and tend to be damaged easily. They consist of a thin ribbon of a metallic foil suspended in front of a metal plate. Sound waves cause the foil to vibrate, causing fluctuations in the electrical current. Thus, an electrical audio signal is created.

Now that condenser microphones are sensitive enough for studio use, they have mostly replaced ribbon microphones as the mics of choice in professional situations. Ribbon microphones are still reserved for very specialised applications. Condenser microphones are also hardier than ribbon mics and cheaper to replace.

Carbon Granule Microphones

These are the microphones found in many older telephones. They consist of a heavy diaphragm placed within a 'bath' of minuscule carbon granules, sealed to prevent the granules from falling out. As with graphite cables, the carbon is there to conduct electricity. The vibration of the diaphragm alters the resistance of current passing through the microphone, creating an audio signal. Note that this type of microphone requires an electrical current to pass through its circuitry, while dynamic microphones create their own current. Condenser microphones sometimes have batteries within them, so they might be considered as being able to create their own current.

These microphones are more responsive to middle frequencies and do not reproduce sibilants clearly. However, telephone speakers in the handsets are usually equally dreadful, so there isn't much point in having a super microphone. They are cheap, hardy and in common use.

Pickup Patterns
Microphones are made with certain applications in mind. For example, stage use, studio use or field recording use. Microphones are not always expected to pick up sound universally and from all directions. The way that a microphone picks up sound from various directions is known as its pickup pattern. There are a few standard pickup patterns: Omnidirectional, Unidirectional, Bidirectional and Cardioid. Pickup patterns are usually depicted as polar diagrams, a circular graph of sensitivity of a microphone from various directions

Omnidirectional
Literally, from all directions. Omnidirectional microphones pick up sound well from all directions, and are frequently used for recording ambient and background sound. Omnidirectional microphones are also used for vocals, because of their lack of proximity effect. They could be used for recording of a group of vocalists, although the preferred method would to split the group into individual singers and each one having their own microphone.

Unidirectional
Literally, from one direction. Although cardioid microphones could arguably be unidirectional, the term 'unidirectional' is now usually reserved for 'gun' microphones. These microphones are long and rod shaped. Grooves on the side of the microphone allow sound coming from the sides to either pass through without reaching the pickup or cancel each other out. As a result, only a thin, cone-shaped area in front of the microphone is picked up satisfactorily. Gun microphones are good for recording individual voices in noisy locations, such as interviews, as well as picking up sound from a long distance.

Bidirectional
Literally...well, guess what it means. Correct! It picks up sound from two opposite directions! Also known as the figure-8 pickup, as the pickup pattern resembles a figure-8 when viewed from above. It used to be popular in the old days when two people would stand around a microphone and sing a duet, but now it is usually preferred to mic each person up separately.

Cardioid
Cardioid microphones have a heart-shaped pickup pattern. Probably the most common microphones in use today, They reject sound coming from the back of a microphone and are progressively more sensitive to sounds as the direction approaches the front of the microphone. They are favoured for stage use as they do not pick up the sound from on stage speakers or monitors so readily, thus preventing feedback. There are versions of the Cardioid pattern called Supercardioid and Hypercardioid, which represent increasingly limited ranges of pickup. As the pattern narrows, feedback rejection improves even more, but due to limitations of construction, a narrowing of the pattern does add a little sensitivity directly behind the microphone. The narrowing does cause the sound recorded to be more pinched and less flattering, but in a stage situation with many speakers placed in unsuitable places, a hypercardioid microphone can be a highly effective feedback prevention measure.

Proximity Effect
Cardioid microphones have a funny phenomenon called the Proximity Effect. This describes the increase in bass as the microphone moves nearer the sound source. Similarly, the further a cardioid microphone is from a source of sound, the more pinched it will sound. Experienced vocalists and producers have used this phenomenon to great effect, especially in simulating a punchy, 'live' effect by almost eating the microphone while singing. The proximity effect can also cause problems, especially when dealing with inexperienced speakers, as the tonal qualities of his or her voice will change as he or she moves his head.

Speciality Mics
A number microphones are considered unique not because of the technology involved, but more importantly, they are designed for certain unique purposes. Even so, considerable success has been reported in utilising these mics in situations they were not designed for.

Wireless Mics
Very visible in Karaoke bars and stage performances, wireless mics can be both convenient (no cables) and a pain in the ass (batteries needed, interference from outside sources). These mics are essentially the same as ordinary microphones with a transmitter. The transmitter can be in the body of a handheld mic (which accounts for the larger size of a wireless) or in a separate belt-worn pack (for lavaliers and instrument pickups (popular for electric guitarists). Wireless microphones typically transmit on only one unique frequency per mic. A 'true diversity' wireless system will have two antennae on the receiver end (which, incidentally, usually puts out a line-level signal for the mixer instead of a mic signal). When the signal strength between the two antennae varies, the receiver will opt to receive the signal from the stronger antennae. This switching can be very rapid and is usually unnoticeable. True diversity wireless systems are usually far less sensitive to radio interference and blockage than single-antennae systems.

Lavalier Mics
The familiar 'interviewer's collar pin', which consists of a small, usually electret microphone worn at the chest, clipped to clothing. This can either be corded or wireless, though the latter is usually preferred. The wireless version runs into a transmitter, usually worn on the belt. Lavalier mics can be powered by batteries or phantom power, depending on the make of the mic. They have also been good for miking up wind instruments, clipped to the edge of the 'bell'.

Bass Mics
These are very large diaphragm, usually dynamic microphones. Since they are usually employed in situations that produce very loud sound pressure levels, they are very sturdy and have lousy high-frequency response. Typically found inside bass drums of drum sets at rock concerts.

Pressure Zone Microphones
A general purpose microphone for amplifying a large source of sound, like a choir or a stage performance. These typically use condensor pickups, mounted a few millimeters over a flat surface, usually a metal plate integrated into the microphone. The concept is the sound reaching the metal plate will bounce back into the pickup, adding to the sound directly going into the pickup. They are also known as Boundary Microphones or Phase-Coherent Cardioids . They have remarkably good pickup, and when placed properly, give very little feedback for sound reinforcement. They are not very discerning in what they pick up, so they aren't that good for recording.

My Favourite Mics
My personal all time favourite is the AKG C1000S condenser microphone. It can be powered by an internal battery or phantom power, and it's quite hardy. It can be converted from an omnidirectional microphone to a cardioid by unscrewing part of the microphone off and changing the pickups inside, a relatively simple operation. It has remarkable pickup from a distance, and when used from a reasonable distance it has a flat, clean and bright sound. Its disadvantages are a cold, clinical sound and the necessity for a power source. It can be used in both stage and studio use.

Another favourite, this time a dynamic mic, is the ATM61HE by Audio Technica. Granted, there are better microphones out there, but for emergency use, the 61HE rises above all the others. It is a hypercardioid and is very difficult to create feedback with. It has a bright top end although it is sometimes lacking in the bass regions. It is sometimes a little too sensitive to pops from the speaker's mouth, though.

Nice general purpose mics include the SM57 and SM58 microphones from Shure. The SM57 looks like a small brother of the C1000S, and is good for stuff that don't put out a lot of wind, especially off-centre miking. The SM57 is very sensitive to pops. I usually use the SM57 as an instrument or even drum microphone.

The SM58 is a 'classic' ice-cream cone shaped microphone that is meant for vocal use. I find it's durability questionable and its pickup quite low, but it does produce a warm, flattering sound and is preferred among many experienced singers.

The AKG D112 is an interesting oddity, as it is designed strictly for bass drum use. It is big and blocky, and it's construction leans more towards a brick than a microphone. It is very hardy, the diaphragm is designed for high volumes, and there's nothing like a dedicated bass drum mic for big concerts. It is a dynamic microphone.

For gun microphones, I like the AKG C568EB. It performs better than the longer ATM815R from Audio Technica, though it's pickup is less narrow. Phantom power is a must. Distance pickup is good, although you might require a mixer with the capability to supply large amounts of gain. I actually use gun microphones pointed at speakers, as they can be located at a safe distance to avoid damage, while preventing most crowd noises from creeping in.

The AKG C747 is a favourite among many, being a minuscule, pencil sized gun microphone. I personally hate the mic, because the small size requires special adaptor cables, the pickup is too narrow and unpredictable for a close mic while being too wide and weak for a distance mic.

There are many 'legendary' microphones, such as the AKG C414 and the Neumann U76, but I haven't had the fortune to play with them. They have been highly recommended by the true professionals in the field, so I guess they must be pretty good.

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« Last Edit: February 18, 2005, 12:41:12 PM by dmonterisi »

 

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