Choosing A Microphone: Types & Uses

Tips & Techniques

Published in SOS June 1995
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Technique : Theory + Technical

Every studio needs at least one mic, but which type and which model is best? PAUL WHITE provides some guidance.

 

All microphones convert sound energy into electrical energy, but there are many different ways of doing the job, using electrostatics, electromagnetism, piezo-electric effects or even the change in resistance of carbon granules. Fortunately for SOS readers pondering over which mics to buy, the field of choice is narrowed considerably when it comes to mics used in music recording or live performance. The vast majority of mics used in these applications are either capacitor (electrostatic) or dynamic (electromagnetic) models. Both types employ a moving diaphragm to capture the sound, but make use of a different electrical principle for converting the mechanical energy into an electrical signal. The efficiency of this conversion is very important, because the amounts of acoustic energy produced by voices and musical instruments are so small.

DYNAMIC MICROPHONES

Most of you will have used a dynamic mic at sometime or another -- if it looks like a mesh ball on a stick, then it's almost certainly a dynamic model. In live sound, nearly all the mics used are dynamics, and in the studio, instruments such as drums, electric guitars, and basses may also be recorded using dynamic mics. Dynamic microphones have the advantages of being relatively inexpensive and hard-wearing, and they don't need a power supply or batteries to make them operate. So, how do they work?

A lightweight diaphragm, usually made of plastic film, is attached to a very small coil of wire suspended in the field of a permanent magnet. When a sound causes the diaphragm to vibrate, the whole assembly works as a miniature electricity generator, and a minute electric current is produced. Because the electrical output is so very small, it has to be amplified using a mic preamp before it is large enough to be useful.

Given the stated advantages of dynamic mics, why bother with any of the other, more expensive types? The weakness of the dynamic mic lies in the fact that the sound energy has to move both the mic diaphragm and the wire coil attached to it. The mass of the coil adds to the inertia of the diaphragm assembly, which in turn restricts the frequency response of the microphone. In practical terms, the outcome is that dynamic microphones fail to reproduce very high frequencies accurately. In some applications, this isn't too serious, but if you're working with an instrument where a lot of tonal detail is contained in the upper harmonics, a dynamic mic is unlikely to bring out the best in that instrument.

Another side-effect of the finite mass of the diaphragm/coil assembly is that the dynamic microphone is not particularly efficient -- a lot of amplification has to be used to make the signal usefully large, and the more gain you use, the more noise you add to the signal. In the studio where the mic is used very close to the sound source, this lack of efficiency is not a major problem, but if you're trying to capture a quiet or very distant sound, then a dynamic mic isn't likely to produce good results.

To summarise; dynamic microphones are most effective when working with relatively loud sound sources that don't contain a lot of very high-frequency detail. They're also tough as old boots, which makes them good for live work, or for any application within six feet of a drummer!

Another type of dynamic microphone is the ribbon microphone, but these are only used in fairly esoteric recording applications by engineers who appreciate the subtleties of the ribbon sound. These mics are comprised of a thin metal ribbon suspended in a magnetic field, and when sound energy is encountered, the electrical signal generated is induced in the ribbon itself rather than in a voice coil. The main advantage of ribbon microphones is their smooth, detailed sound; the disadvantages are their higher cost and the fact that they are more fragile than conventional dynamic mics.

CAPACITOR MICROPHONES

Capacitor mics have been around for several decades, and although modern capacitor mics do incorporate a few small technical improvements, the sound character has actually changed very little -- some of the best-sounding models were designed over 20 years ago. Basically, the heart of any capacitor mic is a pair of conducting plates, one fixed and the other in the form of a moving diaphragm. When the spacing between the plates changes (as it does when the diaphragm vibrates) the capacitance varies, and if a fixed electrical charge is applied to the capacitor, an electrical signal is produced which faithfully represents the diaphragm vibration.

To keep the weight down, the diaphragm is often made from gold-coated plastic film. As a result, the diaphragm assembly is very light compared to that of a dynamic mic, so the system is much more efficient, and is capable of capturing harmonics right up to the range of human hearing and beyond. The size of the diaphragm also has an effect on the tonal quality of the mic -- large-diameter models are chosen for vocal work because of their warm, flattering sound. Small-diaphragm models tend to be chosen where high accuracy is required.

Even though they are relatively efficient, capacitor microphones still produce such a small electrical signal that they require a special type of built-in preamplifier to bring the signal up to usable levels, and this is one factor that contributes to the higher cost when compared to dynamic mics. Additionally, all capacitor mics need a polarizing voltage in order to work. The most common source of polarizing voltage is the 48V 'phantom' power source, which is why many mixing consoles have a phantom power supply built in. The term 'phantom power' came about because the polarizing voltage is supplied via the signal leads of the microphone -- no additional cabling is needed. Because of the way phantom power is supplied, all phantom-powered microphones must be balanced, and must employ the same wiring configuration. Budget mixers or cassette multitrackers with unbalanced mic inputs cannot be used with conventional capacitor microphones unless an external mic preamp (with phantom power) is used.

Broadly speaking, capacitor microphones are more expensive than their dynamic counterparts, but they are also much more sensitive, and can capture high-frequency detail much more accurately. Furthermore, the capacitor principle, unlike the dynamic principle, lends itself easily to the production of mics with switchable pickup patterns (see the box elsewhere in this article for more information on these), although the cheaper models tend to offer just a fixed cardioid pattern. Currently, there are some real bargains to be had in the capacitor mic market -- for example the AKG C3000 (currently retailing at £351) and the Russian-made Oktava MK219 (£311).

ELECTRET MICROPHONES

There is another type of capacitor microphone, known as the electret mic. Despite inauspicious beginnings, these have now been developed to the point where they can rival true capacitor quality for a much lower price. Instead of applying an electrical charge to the microphone capsule via an external power source, electret mics use a diaphragm made from an insulating material that has a permanent electrical charge. A preamplifier is still needed, but this can be built very cheaply, and will run from a battery in some cases.

Electret mics made in this way don't offer any real advantage over dynamic mics, because the diaphragms have to be quite heavy in order to carry the permanent electrical charge -- but what if the permanently-charged material was fixed not to the diaphragm, but instead to the stationary back-plate? This way, much thinner diaphragms can be employed, made of the same metal-coated plastic material as on a true capacitor model. What I've just described is the back-electret microphone, and the best of today's back-electret models can rival conventional capacitor models in every aspect of performance. The best back-electret models are actually just as expensive as top capacitor models (the famous Bruel and Kjaer mics are back-electrets), but there are some less costly models around which deliver studio quality at a bargain price. One of the most popular low-cost back-electret models in recent years is the AKG C1000 (yours for £281), but don't neglect the models from other manufacturers. Currently, it's possible to buy a good back-electret mic for around the same price as one of the better dynamic models -- the new Beyer back-electret, the MCE83, retails for just £234. Most back-electret microphones in this price range offer a choice of battery or phantom power operation, with a fixed cardioid pickup pattern.

FREQUENCY RESPONSE

You might expect the perfect microphone to have a perfectly flat response right across the audio spectrum, but there are various practical reasons why this isn't so. Virtually all mics have a deliberate low-frequency (or LF) 'roll-off' -- in other words, they are less sensitive to frequencies below about 50Hz. Without this roll-off, low-frequency vibrations, knocks, breath pops and other such problems would produce very large, low-frequency output signals, which would not only compromise the sound quality, but might even damage loudspeaker systems.

While LF roll-off is designed to exclude unwanted sounds, mics are also often designed to accentuate specific parts of the audio spectrum in order to create a flattering sound. For example, numerous models are designed with a 'presence' boost in the upper mid-frequency range, to help make vocals more intelligible. Presence boost (which is usually between 3 and 6 kHz), has the effect of making the sound more transparent or detailed.

PROXIMITY EFFECT

The proximity effect comes into play when vocalists sing very close to a mic, and the effect is that the level of bass in the recorded signal goes up enormously. The proximity effect is all down to the laws of physics, and may be a benefit or a problem depending on what you do with it -- experienced live performers can use the proximity effect as a type of dynamic EQ, allowing them to alter the tone of their voice as they sing, simply by varying the mic-to-mouth distance.

In the studio, mics tend to be used at a reasonable distance from the performer, usually with a pop shield in between, so the proximity effect doesn't affect the recording of vocals. When recording electric guitar or bass, the proximity effect is often deliberately brought into play, to help create a more punchy sound.

SUMMARY

A professional studio will accumulate a varied collection of mics, but in project studios, where finances are limited, it's usually a matter of buying whatever mic suits the regular vocalists the best. If you're working with a number of singers, then it may be better to go for a more tonally neutral mic, so that you can use EQ to fine-tune the sound.

If you're currently working with budget equipment, you might feel that paying a lot of money for a mic is pointless, because the ultimate quality of your recordings will be limited by the gear you're using. To some extent this is true, but if you don't capture a good sound at source, things can only go downhill from there. Microphones don't tend to go out of fashion in the same way that effects units and synths do, so there is an argument for buying only quality models that will always be useful to you, no matter how your recording setup evolves over the years. Quality dynamic mics aren't expensive, and even the top studios use mics such as Shure SM58s and 57s (currently going for £150 and £139 respectively), AKG D112s, or Sennheiser 421s (£288 each). Similarly, you can now buy a really good capacitor mic for under £300, and if you can stretch to buying two of the same model, you'll be well set up for any stereo work that might come your way. If you can't afford a capacitor mic straight off, a good dynamic will get you by for the time being, and when you eventually add a capacitor mic to your setup, the dynamic model will still be useful for instrument recording. It's no longer true that professional mics are only for professionals.

 

PICKUP PATTERNS

The most basic microphone pickup pattern is omnidirectional -- which means that the microphone picks up sound equally from all angles. Omni pattern mics tend to be the most technically accurate, but they may well pick up sounds not intended for them, especially where several instruments are playing close together. That's why cardioid (unidirectional) mics are more suitable for use in smaller studios.

Directional microphones are referred to as cardioids because a graph of their sensitivity at different angles looks 'heart-shaped'. The least sensitive spot of a cardioid microphone is right behind it, with the most sensitive spot being 'on-axis'. Cardioids (or the more tightly-focused hypercardioids -- see below) are used extensively in live performance, because of the need to prevent spill and acoustic feedback.

Hypercardioid mics, sometimes also known as supercardioids, are useful in situations where sound leakage is a real problem, but in the studio, they tend not to be used, as relatively small physical movements by the performer can cause the sound level to change significantly.

One mic pattern which now tends only to get used in specialist applications is the figure-of-eight -- so called because its sensitivity graph looks like a figure eight. This type of mic picks up sound equally well from in front and behind, but is relatively 'deaf' to sound arriving from the sides. Back in the early days of pop, it wasn't uncommon to see two backing vocalists singing into opposite sides of a figure-of-eight mic to save on the number of microphones, but now they tend to be used mainly for classical stereo recording or as part of a Middle and Side (M&S) stereo pair (the workings of M&S mics are beyond the scope of this article, but keep your eyes peeled for an detailed explanation of the subject in future issues of SOS).

As mentioned elsewhere in this article, capacitor microphones can be built to provide several different pickup patterns which may be changed with a simple switch. To achieve this, the capsule is made with two diaphragms, and by changing the level and polarity of the polarising voltage on one of the diaphragms, every possible pickup pattern can be created, starting with from omni, progressing through cardioid and hypercardioid to figure-of-eight.

 

WHAT TO BUY

It's a fact that different mics are better at different jobs, and in the studio, where there are lots of different instruments to record, it helps to have range of mics from which to choose. If you can afford it, buy at least one good capacitor mic (ideally a large-diaphragm model) or at least one of the better back-electret models for serious vocal work, and keep a few general-purpose dynamic mics around for drum and electric guitar work. In smaller studios, where the acoustics are seldom ideal, the cardioid pattern mic is the most useful, because it helps exclude unwanted room reflections -- so don't spend a lot more money buying a switchable pattern mic unless you have very clear reasons for needing those other pickup patterns.

When it comes to choosing a capacitor mic, you have to decide whether to opt for a large- or small-diaphragm model, but on top of that, you may have a choice of transformer or transformerless electronics, and maybe even a valve option to consider. As a very general rule, modern transformerless electronics offer the best paper specification, and the subjective sound is often brighter and more detailed than the transformer version of the same mic -- but that's no guarantee that the sound is better. Matching transformers can introduce benign coloration, and the mechanisms by which they do this are wrapped in almost as much mystique as the alleged sound of valves. For this very reason, some users will insist on the transformer version of a mic simply because they like the tonal character of the microphone. Subjectively, transformers seem to produce a warmer, more comfortable sound than transformerless circuits.

At one time, it looked as though valve mics would stay extinct, but the escalating prices of original models forced some manufacturers to reconsider, with the result that you can now buy re-issues of original valve mics alongside brand-new valve designs such as those built by Groove Tubes. Valve mics invariably cost more than their solid-state counterparts (prices range from around £700 upwards), but those who use them seem to feel that the enhanced sense of warmth and transparency is worth the extra cost. On the other hand, it can be argued that a good solid-state capacitor mic processed via a valve mic preamp (or via a mixer channel with a valve DI box in-line) should sound just as good, and will cost rather less.

The problem arises when you want to try out a mic before buying it, because there's no simple way to judge a mic without actually working with it for a period of time. That's one reason people tend to buy tried and trusted mics instead of risking an unfamiliar brand. Ultimately, most people either buy something that they've used before in another studio, or they buy what the professionals use. This is a pity, really, as there are some excellent new mics on the market too. If you're interested in an unfamiliar mic, try to arrange to hire one for a few days, to see how it works in your studio. Most dealers should be happy to offer an arrangement whereby you get the hire money back if you make a purchase.

 

WHAT TO USE: QUICK GUIDES

• VOCALS
When it comes to recording vocals, there are no hard and fast rules about the type of mic to choose, because all that matters is the end result. For this reason, some top pop singers record using relatively inexpensive dynamic mics, rather than capacitor models, because the dynamic mic gives them a warmer, thicker, more punchy sound. On the other hand, a breathy, intimate voice can benefit from the detailed high-end of a capacitor mic. Not only do the different types of mic sound quite different, but you'll also notice that even similarly-specified mics of the same type but from different manufacturers will also sound noticeably different. For example, the classic Neumann U87 (retailing at £2028) provides a warm, confident sound, but with plenty of detail and 'air', while something like the Audio Technica 4033 (£622) has a distinctively silky sound. By the same token, AKG's 414 (£1035) and Beyer's MC740 (£1173) have a very bright, open sound, because of their presence characteristics.

• DRUMS
For drum work, the most common setup is to use dynamic mics for all the close mics (on the individual drums) and then augment these with capacitor mics for the overhead mics and the hi-hat. Dedicated kick drum mics are available (such as the AKG D112, currently retailing at £216), and these will produce a more powerful bass drum sound than general-purpose dynamic models. Some engineers also prefer to use capacitor mics on the snare drum for a crisper sound.

• GUITAR
Electric guitar is less demanding on mics, because a typical guitar cab has a very limited high-frequency output, and in most instances, a dynamic vocal or drum mic will do fine. As an alternative, a capacitor mic can be used for a more American sound.

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