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Microphone Basics | |
Article from Recording Musician, November 1992 |
Why are microphones so different, why do some cost the earth — and which one should you use for which job? All is revealed.
The microphone could be considered the electrical equivalent of the human ear — but if that is indeed the case, shouldn't all microphones be identical? Paul White explains why they're not.
Every microphone performs the same basic task: converting sound energy into electrical energy. But there are many different types of microphone, and one of the recording engineer's many skills is knowing the best type for a particular job. If the right choice is to be made, it helps to have a basic knowledge of how microphones work and in what areas the various types differ.
There are several different mechanisms that can be used to turn sound energy into electrical energy, and can hence be used to design microphones, but only two are in common usage in music recording; these may loosely be termed dynamic and capacitor. These two basic types may be further subdivided, as we shall see later, but one factor they all have in common is that the air vibrations that constitute sound are used to move a lightweight diaphragm, the movement of which is then used to generate an equivalent electrical signal. To put it more directly, the rapid positive and negative air pressure changes which we perceive as sound are converted by the microphone into analagous positive and negative fluctuations in electrical current. The trick is in making the conversion as accurate and as efficient as possible — efficiency is very important because of the very small amounts of acoustic energy involved, even in the case of relatively loud sounds.
The most common type of microphone, used both in recording and on stage, is the dynamic, or moving coil, model. A light, rigid diaphragm is attached to a coil of very thin wire which is, in turn, suspended in a magnetic field created by a permanent magnet. Faraday's laws dictate that moving an electrical conductor in a magnetic field will cause an electrical current to flow in that conductor — which is exactly what happens when the diaphragm (and hence the coil) is set in motion by a sound.
Dynamic microphones are relatively inexpensive to build and are mechanically robust. Furthermore, they require no electrical power to operate — they function as a passive transducer, changing one form of energy to another. Because the sound energy reaching the microphone is very small, the electrical output is also very small, so it has to be amplified many times before the signal is large enough to be useful; this is easily accomplished by the microphone amplifier in any mixing console. So far the principle seems very simple and very effective, but there are drawbacks.
Not only does the acoustic sound energy have to move the microphone's diaphragm, it also has to move the wire coil attached to it. The higher the frequency of the sound, the faster the diaphragm has to vibrate and the more it is opposed by mechanical inertia. Even using the lightest diaphragm materials and the thinnest wire to make up the voice-coil, this still exhibits inertia which is proportional to its mass. Inertia resists acceleration, and a vibrating microphone diaphragm has to accelerate and decelerate many times each second, as the diaphragm moves first one way and then the other. This limits the rate at which the diaphragm can move, making it less effective at high frequencies. This inefficiency at high frequencies means that dynamic microphones invariably fail to do justice to sounds which have harmonics right at the top of the hearing range.
Another side-effect of the finite mass of the diaphragm/coil assembly is that the dynamic microphone is not particularly efficient — the signal needs to be amplified many times before it is large enough to be usable, which has the effect of increasing background noise. With loud or nearby sounds, this problem isn't significant, but when working with quiet or distant sound sources, the dynamic microphone simply cannot cope.
Another drawback of this type of design is that the output of a dynamic microphone usually starts to deteriorate at frequencies higher than 15 or 16kHz, yet the limit of human hearing can exceed 20kHz. While a good dynamic microphone can reproduce most of the human hearing spectrum with reasonable accuracy, the vital, higher harmonics tend to lose definition. For these reasons, dynamic microphones are most popular when working with drums, close-miked vocals and relatively loud instruments, but for quieter acoustic instruments, distant voices or sounds containing a lot of high frequency detail, they are less satisfactory.
Another type of microphone you may encounter is the ribbon microphone. This is a form of dynamic microphone, but the diaphragm and voice coil are replaced by a thin metal ribbon suspended in a magnetic field. In this type of microphone, the ribbon moves in response to sound energy and the resulting electrical signal is induced in the ribbon itself. The ribbon is lighter than the combined voice-coil and diaphragm assembly of a conventional dynamic microphone, which makes it slightly better able to cope with high frequencies (though electrical efficiency is not significantly improved). The main advantage of ribbon microphones is their smooth, detailed sound, and though they are now far less popular than they once were, many discerning engineers still swear by them in certain applications, such as in the recording of orchestral strings. The original ribbon microphones were very fragile, the ribbon component being susceptible to corrosion from breath moisture and physical damage from air blasts. Modern versions are rather more durable, but should still be handled with care.
"Choosing which microphone to use tends to be an artistic decision; the most accurate microphone will not always produce the most pleasing result."
To significantly improve on the dynamic microphone, it was necessary to find a method of converting acoustic energy into electrical energy that would work with a much lighter diaphragm. The solution was the capacitor microphone, and though these have been with us for several decades, they still represent the state of the art in microphone design. The underlying principle relies on the properties of an electrical capacitor, hence the name of the microphone, though you will also find them referred to as condenser mics, condenser being an older term for capacitor.
If a capacitor is given a constant electrical charge and then the value of capacitance is somehow changed, the voltage across the capacitor changes accordingly. A capacitor is, at its simplest, two conducting plates separated by an insulator such as air, and to change its capacitance, we only have to change the spacing between the two plates; bring the two plates closer together and the capacitance increases; move them further apart and it decreases. This simple fact is exploited in the capacitor microphone, where the capacitor is made up of one fixed metal plate and one very thin conductive plate which is light enough to pick up the vibrations of sound in air. As the thin plate vibrates, its distance from the stationary plate varies accordingly and, if a fixed electrical charge is applied to the capacitor, a corresponding change in electrical voltage is produced.
In practice, the thin plate is in the form of a plastic diaphragm just a few microns thick, and this is given a metal coating only a few atoms thick in order to make it electrically conductive. Holes are drilled in the fixed back-plate to prevent air being trapped between the plates — trapped air would tend to act as a cushion, pushing against the moving diaphragm and adversely affecting the microphone's performance. The result is a very sensitive microphone capable of resolving high frequency detail right up to the limits of human hearing and beyond. It might seem from this description that capacitor microphones are so good that there is no need to build any other type. This might be true but for a few critical factors, one of which is cost.
Capacitor microphones produce such a tiny electrical signal that they require a special type of preamplifier built into them in order to boost the signal to a manageable level. Furthermore, the capacitor assembly (or capsule, as the transducer section of a microphone is generally called) has to be electrically charged, which requires a polarising voltage. All capacitor microphones, therefore, need electrical power in order to operate, the most common source of this power being the 48v 'phantom' power source built into the microphone preamplifier of a suitable mixing console. The term 'phantom power' was coined because it is supplied via the signal leads of the microphone itself and so needs no additional cabling. Special circuitry separates the phantom power from the electrical signal, both at the microphone end and the mixer end. All phantom-powered microphones use the so-called balanced wiring system, which employs a screened lead with two cores plus a screen rather than just a single core. All mixing consoles and microphones fitted with XLR connectors are wired this way, though not all mixers provide phantom power. Cassette multitrackers tend to have unbalanced microphone inputs on jack sockets, which means these cannot be used with conventional capacitor microphones.
Because of their complexity, capacitor microphones are, in the main, rather more expensive than their dynamic counterparts and tend to be less robust. However, they are much more sensitive, making them suitable for recording both quiet and distant sources, and they can resolve high frequency detail much better than a dynamic microphone.
There is another type of capacitor microphone, known as the electret which, until recently, had a reputation for being cheap and cheerful — but not particularly good. The underlying idea is ingenious; instead of applying an electrical charge to the microphone capsule via an external power source, the diaphragm itself is built from an insulating material that contains a permanent electrical charge. A preamplifier is still needed to bring the tiny output signal up to a usable level, but this need not be elaborate and can be made to run from a small battery. The problem is that the diaphragm material has to be made much thicker than on a conventional capacitor microphone in order to carry the permanent charge. Ultimately, the microphone ends up with a diaphragm so heavy that all the problems of the dynamic microphone reappear — low sensitivity and limited high frequency response.
Just as it seemed that electret mics were fit only as low-cost accessories to domestic cassette machines, someone had the bright idea of fixing the permanently charged material to the stationary back-plate and replacing the moving diaphragm with the same metallised plastic material as used on a professional capacitor model. The result was the back-electret microphone, and the best of today's models can equal a conventional capacitor microphone in performance. Though the best back-electret models are just as expensive as top capacitor models, it is possible to build lower-cost versions by sacrificing some sound quality and sensitivity.
Currently, it is possible to buy a good back-electret mic for around the same price as a good dynamic model and, though these don't have the same sensitivity as top-end models, they out-perform their dynamic counterparts by a significant margin, both in sensitivity and frequency response. Most back-electret microphones in this price range offer a choice of battery or phantom power operation and are robust enough for stage use, as well as for recording.
LIVE VOCALS | Dynamic Cardioid |
STUDIO VOCALS | Dynamic Cardioid or Capacitor Cardioid |
DRUMS | Dynamic Cardioid (close mics) |
Capacitor Cardioid or Omni (overheads and hi-hats) | |
ELECTRIC GUITAR | Dynamic Cardioid (close mic) |
Dynamic or Omni Capacitor (ambient mic) | |
ACOUSTIC GUITAR | Capacitor - Cardioid or Omni |
BASS GUITAR | Dynamic Cardioid |
BRASS | Dynamic Cardioid (close miking) |
Capacitor Cardioid or Omni (ensemble miking) |
For most serious home recording applications, back-electret models are ideal, though dynamic microphones sometimes produce a more artistically pleasing sound in certain applications. For example, dynamic mics generally have a 'thicker' sound and are favoured by some rock vocalists. British engineers tend to prefer the sound of dynamic microphones on drums and electric guitars, whereas American engineers make more use of capacitor microphones in these applications. Choosing which microphone to use tends to be an artistic decision; the most accurate microphone will not always produce the most pleasing result. If the sound source is loud enough, then the engineer can pick any of the microphone types, based on subjective sound alone, but for quieter or more distant sources, dynamic microphones are invariably excluded on technical grounds.
So far I have not mentioned microphone pickup patterns, but it is a very important subject and must be covered. The most basic microphone pickup pattern is omnidirectional — generally abbreviated to omni — which means that the microphone picks up sound equally well from all angles and not just from in front of it. Physically, the microphone may look no different from microphones producing other pickup patterns — the difference is in the capsule design. Omni pattern microphones give, arguably, the most accurate representation of a sound, but because they pick up equally well from all directions, they may pick up some sounds that we'd rather exclude. For example, where several instruments are playing in close proximity and each one has its own microphone, using omni microphones may allow too much unwanted sound leakage from one instrument into a mic intended for another instrument. In this case, a directional microphone may be more suitable.
Directional microphones, as their name implies, tend to be more effective at picking up sounds from one direction. The common name for this type of microphone is cardioid, which means 'heart-shaped'. While the microphone may not be heart-shaped, the pickup pattern, if plotted on a circular graph, most certainly is. This shows that the microphone is most effective when picking up sounds from in front, though the pickup angle isn't narrow like a torch beam, but may be 90 degrees or more. Outside this angle, sounds are picked up less and less efficiently as the sound source moves towards the rear of the microphone. The least sensitive spot of a cardioid microphone is right behind it. Cardioids are used extensively in live performance because of the need to prevent sound spill and acoustic feedback, but they are also used in the studio for virtually all applications. Purists will argue that the omni mic gives a more natural sound and that the additional leakage problem is not that great, but in practice, most pop music engineers still use cardioid mics for most applications.
There is one more major microphone pattern, which is less commonly used than either the omni or the cardioid, but in certain applications is very valuable. This is the figure-of-eight pattern, so called because its sensitivity graph looks like the number eight. Translated into words, this means that the microphone picks up sound equally well from in front and behind, but is relatively immune to sound arriving from the sides. Ribbon microphones have a figure-of-eight pattern, due to their construction, whereas dynamic and capacitor microphones can be built to produce any desired pickup pattern. The figure-of-eight microphone is particularly valuable in certain stereo recording applications, and these will be covered in specialised articles in the near future. They were also popular in the '60s as backing vocal mics, where two singers could share the same microphone, one singing into the front and the other into the back.
A unique advantage enjoyed by capacitor microphones is that they can be built to provide several different pickup patterns at the flick of a switch. This requires the capsule to be built with two diaphragms, and by changing the level and polarity of the polarizing voltage on one of the diaphragms, every possible pickup pattern can be created from omni, through cardioid patterns of different widths to figure-of-eight. Many models provide three types of cardioid pickup pattern — wide cardioid, normal cardioid, and a narrower pattern known either as hypercardioid or supercardioid. The wider patterns give the most natural sound, but the narrower hypercardioid pattern is useful in situations where sound leakage is a real problem.
So far, we have mentioned that different types of microphone are better or worse at handling the top end of the audio spectrum, but what goes on below this limit is often far from flat. In theory, the perfect microphone should have a ruler-flat response right across the audio spectrum, but most mics have a deliberate low frequency roll-off, making them less sensitive to frequencies below 50Hz or so. If this roll-off were not present, low frequency vibrations and gusts of wind would produce large, unwanted output signals.
But apart from being 'rolled-off' at both the upper and lower extremes, many microphones are built with a deliberate boost in the upper mid frequency range to help make vocals more intelligible. This is known as the presence peak, and may be sited anywhere between 3kHz and 10kHz, depending on the model of microphone. This boost has the effect of brightening the sound slightly, and gives each microphone a unique tonal character.
Another factor to be aware of is the so-called proximity effect, which only applies to cardioid pattern microphones. This takes the form of a significant bass boost when the microphone is used with sound sources closer than a couple of centimetres. In the studio, it is unlikely that anything will get this close, but in live performance, where the singer's lips are often in physical contact with the microphone grille, the effect is much more significant. Used properly, the proximity effect can give the experienced vocalist a means to add tone and expression to a performance, but in the studio it is an unwelcome source of inconsistency.
When choosing a microphone for a particular task, we have first to determine whether the sound source is loud or quiet. If it is loud, we can choose a dynamic microphone to give a solid, punchy sound or a capacitor microphone, which will produce a more transparent, detailed sound. On the other hand, if the sound source is quiet or several feet away, then a capacitor microphone is the preferred choice because of its greater sensitivity — and I'm also including back-electret capacitor models here.
The directional characteristics of the microphone will depend on how close you can get to the sound source and how far away other sound sources are. If several loud sources are close together, than a cardioid-pattern microphone will help to reduce the spill, but in less difficult circumstances, an omni pattern microphone may produce a more natural sound. Finally, should we choose a mic with a relatively flat frequency response or one with a presence peak? The answer to this one is purely subjective and, particularly when working with vocalists, it is often best to try two or three different types of microphone and see which one suits the voice best. If the singer has a bright, sibilant voice, then a model with a strong presence peak might make this worse. On the other hand, a voice lacking in definition will benefit from the extra brightness of a model with a presence peak. Making the the best choice is all part of the engineer's skill and only comes with experience. Nevertheless, knowing the important differences between the different microphone types makes the choice much easier.
What Makes A Mike Right? - Microphone Tips |
When the Pressure's On |
Using Microphones - Recording the Bass |
Looking at Microphones |
Live Wire - Miking Up |
Hands On: Large Diaphragm Microphones |
Miking Music |
The Rough Guide To Choosing Microphones |
Where to Stick It - Miking Tips |
Studio Sound Techniques (Part 1) |
Room To Roam - Radio Microphones |
When Is A Microphone |
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Feature by Paul White
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