Mics in the Mix
Aug 1, 2000 12:00 PM, Myles Boisen
Whether you have a closet full of mics or are contemplating your very first microphone purchase, selecting the proper mic for a recording application is crucial. Based on the e-mail I receive from EM readers, and on recurring discussions at audio newsgroups, it would also seem that deciding which mics to buy (and what to do with them once you have them) can be an agonizing process.
This article will help beginning recordists understand the three main classes of microphones-dynamic, ribbon, and condenser-and how they differ from one another, and explain why one type may be preferable over another for different recording applications. Before getting into specifics, though, let's identify some features that most microphones-whether for studio or stage-share. Perhaps the most important of these features is the diaphragm, a lightweight, delicate membrane that responds to changes in sound-pressure level. Protection for the diaphragm is provided by a wire-mesh or other type of grille, which typically also serves to identify the address, or active, surface of the mic.
The physical energy of the diaphragm's movement is transduced (that is, changed to electrical energy) by a variety of methods, each of which belongs to a distinct category of microphone electronics. Subsequently, the internal electronics provide an output signal via a male 3-pin XLR or multipin jack.
MICS YOU CAN EAT The most common type of microphone is the dynamic. Most dynamic microphones employ a relatively massive, moving-coil diaphragm that is impervious to damage from rough handling or extreme sound pressure. The low cost and rugged construction of dynamic mics make them ideal for live sound, and certain models made by Shure, Sennheiser, Audix, Beyerdynamic, AKG, and Electro-Voice often do double duty in the recording studio.
With very few exceptions, modern dynamic mics are front address and designed for unidirectional pickup in a cardioid polar pattern. This means that sound arriving on-axis (that is, in front of the mic at the end of the cylindrical body) is emphasized, while sound arriving off-axis (that is, at the sides and rear of the mic) is suppressed or rejected.
The term cardioid derives from the heart-shaped pattern of the microphone's "hot zone," or on-axis area. However, I find it easier to think of the pattern as being in the shape of an apple, with the microphone as the stem (see Fig. 1). This pattern has obvious benefits for live sound, where monitor speakers may be placed in front of a performer-and therefore are pointed at the rear of the microphone. But dynamic mics are also an asset in the studio, where their focused pattern and limited sensitivity to distant sounds is useful for "tuning out" unwanted sound sources.
The dynamic mic is always the best candidate for controlling leakage (when excess room sound or the sound of other instruments "leaks into" the mic) and, likewise, for maintaining the maximum separation of sounds when close-miking. This makes it a natural choice for miking a drum kit, where, for instance, the snare-drum mic may be within two to three inches of both a tom-tom and the hi-hat cymbals. Models such as the Shure SM57 and Sennheiser MD 421 are preferred by many engineers for snare and toms, respectively, as well as for some hand drums. Dynamic mics are generally not used for high-frequency sources such as cymbals and metallic percussion and are not optimal for distant miking of room sound or a group of instruments.
Some dynamic mics are specially designed for recording bass drum, and they feature an extended low- frequency response as well as a high-end emphasis to bring out the attack of the bass-drum pedal. These mics can also be put to good use on acoustic bass, tuba, and speaker cabinets for electric bass and organ.
Though leakage is rarely a problem when miking guitar amps, dynamics are also widely favored for this application. Because most dynamic microphones have a midrange frequency response that is optimized for vocals, they will enhance the equivalent range of amplified guitar tones as well as that of brass-band instruments. And, in contrast to other types of microphones, the dynamic is unfazed by high sound-pressure levels and can further aid the engineer by filtering out boomy lows and grating highs at the extremes of the frequency spectrum.
Vocalists accustomed to using handheld dynamic mics on stage may feel more at home using them in the studio and, in some cases, will prefer the sound of a dynamic over a more expensive condenser microphone. There is no reason not to try a blue-collar dynamic as a main vocal mic; George Benson's "This Masquerade" is a good example of a hit song in which the vocal was recorded with a dynamic microphone. For singers who like to "eat the mic" but don't want to produce a disproportionate amount of bass response (due to the proximity effect), some deluxe models provide low-cut filters. This feature enables the user to trim the low-frequency output of the mic via single or multiple switch positions, and typically it's the only "extra" found on dynamic mics.
MICS YOU SHOULD BABY The ribbon, or velocity, microphone is an early form of dynamic mic that was ubiquitous in broadcasting, film sound, and music recording from the 1930s through the 1960s. Vintage RCA ribbons can still be found in studios today, and three companies-Coles, Royer, and AEA-are continuing to make microphones based on classic figure-8 (also known as bidirectional) designs.
Because its "diaphragm" is a thin rectangle of metal foil suspended vertically, the traditional side-address ribbon mic is extremely responsive to sound arriving at either the front or the back of the grille, while sounds from the side of the mic are rejected because the suspended ribbon can't move from side to side. Therefore, the pickup pattern, if visualized from above the mic, resembles the numeral 8 (see Fig. 2).
Classic ribbons are known for having a flat frequency response with a smooth, softened high end that can be magical on brass instruments, bowed strings, and some saxophones and reed instruments. Ribbon mics can also be very effective for medium-distance miking (4 inches or more from the grille cloth) of electric- guitar cabinets as well as for distant room miking of single instruments or ensembles. And for re-creating the warm, roomy qualities of a vintage jazz or pop recording, the ribbon mic is an obvious choice, as evidenced by its popularity with the swing-revival set. As an alternative to the bulky and costly ribbon designs of yesteryear, Beyerdynamic makes a number of hybrid, short-ribbon mics with both figure-8 and unidirectional pickup patterns.
There are some important caveats to consider when choosing ribbon mics for a job. Foremost is the fragility of the ribbon, which must be protected from plosives and other strong blasts of air. Extreme sound pressure, such as that from a bass drum, a loud amplifier, a vocalist's popped "P"-or even from slamming the lid on the mic case-can stretch or destroy a ribbon. Phantom power can also ruin a ribbon and therefore should not be used. In addition, ribbon mics do not have a high output level, so it is important to pair them with a quiet microphone preamplifier with lots of gain.
THE SENSITIVE TYPE Condenser (also known as capacitor) microphones come in two basic flavors: "true" condensers, for which the capacitor requires an externally applied electrical charge, and electret styles, which have a permanent charge already applied to the capacitor. Either way, the heart of a condenser mic is a complex, precision- manufactured diaphragm assembly (referred to as the capsule) that is an integral part of the microphone's electronics.
The sophistication of the condenser capsule and related circuitry offers numerous sonic advantages. These include hotter output level, increased dynamic range, extended and more accurate frequency response, lower noise, and richer sonic character. In addition, condensers are optimized to pick up very quiet and distant sounds, and they typically exhibit superb transient response (that is, a fast reaction time to the spiky, initial attacks of sounds, particularly percussive ones).
Premium condenser models typically offer multiple polar patterns (usually omnidirectional, figure-8, and one or more varieties of cardioid) as well as highpass filters and attenuation pads. A highpass filter (also known as a low- cut) decreases the low-bass response, and an attenuation pad reduces the output level. These various options, which can produce timbral changes ranging from drastic to barely perceptible, figure into the reputation that top condenser mics enjoy as versatile, high-performance recording tools.
Condenser microphones are usually more fragile and expensive than their dynamic cousins and can be very sensitive to popping and proximity effect when used on vocalists. Also, the increased sensitivity of a condenser mic, though advantageous in many situations, can be a drawback when having to contend with excess leakage, say in multiple-mic setups for ensemble recording. In addition, condenser microphones aren't just "plug and play"; rather, all types require a power source for the internal amplifier circuitry. Most contemporary solid-state condenser mics run on 48-volt phantom power, which is commonly supplied by mixing consoles or outboard gear. Tube condenser mics, on the other hand, have dedicated power supplies, and some electret condensers run on batteries.
Condenser microphones are generally favored by engineers for recording vocals, acoustic guitar, piano, woodwinds, strings, cymbals, percussion, mallet instruments, sound effects, ambience, and any source requiring lots of detail. Condensers are thus an essential ingredient in any classical and modern-jazz recording, and they are relied upon in all manner of pop music to put a bright, high-end sheen on sources. Professional stereo and live-concert recording is made possible by the use of carefully matched condenser pairs or dedicated stereo microphones.
There are many different kinds of condenser mics, each with its own uses and idiosyncrasies. We'll examine three more closely: small-diaphragm condensers, large-diaphragm condensers, and tube condenser mics.
SMALL, SMALL WORLD All condenser mics use ultrathin, low-mass diaphragms, but those measuring 11/42-inch or less in diameter are in a class all their own. Highly prized for their superlative high-end and transient response, small- diaphragm condensers are often the first choice of engineers worldwide for use on drum sets (as overheads), hi-hat, percussion, acoustic guitar, mandolin, banjo, piano, classical ensembles, and more. However, the honesty and sharp treble emphasis of this design may be unflattering or lacking in warmth on some sources, and for this reason it is not a common choice for low saxophones, bowed bass and cello, male vocals, or distorted electric guitar.
The majority of small-diaphragm condenser mics are front address and single pattern, usually cardioid. But many of the major manufacturers offer modular systems with an assortment of interchangeable, single- pattern capsules with differing polar patterns. Other companies, such as Earthworks, have made their mark by producing very-small-diaphragm condenser mics that have the same accuracy and extended frequency response as scientific measurement microphones.
FEELING LARGE Nothing gets the engineer's juices flowing quite like a large-diaphragm condenser. These are the fetish items of the recording world, renowned for their rich midrange, thunderous low-end response, creamy highs, and larger-than-life quality.
With the exception of a few models, all large-diaphragm condensers are side address, and their elegantly rounded grilles often afford a clear view of the 1-inch or larger capsule mounted inside. Since the advent of the home-recording boom, many no-frills models have come on the market, most offering only a fixed cardioid pattern, a basic mount, and (sometimes) a carrying case. For bigger budgets, the sky's the limit- your Rolls Royce large-diaphragm mic may be handmade and finished in 24-carat gold, with elaborate suspension mounting, custom-made screws and cables, and more (see Fig. 3). Whatever your tastes, the large-diaphragm mic is a must-have for recording lead and background vocals, saxophones and other reeds, and it's commonly used on low-end instruments such as acoustic bass, electric bass, jazz guitar, bass clarinet, hand drums, tom-toms, and bass drum.
TUBE JOB All condenser mics made before the mid-1960s used vacuum tubes in their internal amplification circuitry. Yet despite the lower cost, convenience, and reliability of newer, solid-state transducers, vintage tube mics are more popular now than ever. In fact, large-diaphragm tube mics made by Neumann, AKG, and Telefunken in the 1950s and 1960s are responsible for the lush vocal sounds on many of today's big-budget pop productions. Renewed interest in these classic mic designs (not to mention skyrocketing prices) has spawned a new crop of large-diaphragm tube transducers that is within reach of the personal-studio owner.
In the digital-recording age, tube mics have become a hot commodity for home recording, especially on vocals. If it's accuracy you're after, look elsewhere. However, once you've tracked with a great tube mic, you'll understand why many engineers prefer these microphones for vocals, lead instruments, guitars, amplified keyboards, acoustic bass, and many other sources. The secret is in the tube itself, which adds subtle coloration, compression, and harmonic distortion to the sound.
Although large-diaphragm tube mics get most of the attention, small-diaphragm models are still plentiful and relatively affordable on the used market. In recent years, some of these have been retrofitted with new, large-diaphragm capsules made by Baltic Latvian Universal Electronics (BLUE), expanding their usefulness beyond their traditional duties as microphones for drum overheads, percussion, and stringed instruments. In addition, a new "medium-diaphragm" (31/44-inch) tube microphone is being marketed by GT Electronics (a division of Alesis).
IT DOESN'T STOP THERE Though dynamic, ribbon, and condenser mics account for most of the music that gets recorded these days, other types are also available. One very useful (and popular) kind is the boundary effect microphone (the generic name for the PZM, or pressure zone microphone, trademarked by Crown), which is notable for capturing direct and reflected sound in phase. Other special types of mics include contact mics (the piezo electric, for example), which operate via direct vibrations from the instrument; crystal mics; and ceramic mics. These microphones are beyond the purview of this article, however.
AND THE WINNER IS... Hopefully, this article has answered some of your questions about the different microphone types and what each does best. If you're like most of the folks I talk to, though, I'll bet that one big question is still unanswered. For most people, it goes something like this: "Mirror, mirror, on the wall, what's the best mic of them all?" Now, repeat after me: there is no single "best" microphone for any recording job. Some microphones do one thing very well, whereas others work well for a variety of applications. Moreover, it's easy to convince yourself that a particular mic sounds good on almost anything-especially if it's all you can afford.
Every musician is singular, as is every instrument, room, song, and so on. The combination of these elements yields a sonic result that is as unique-and irreproducible-as a snowflake. In your studio, on a given day with your equipment and knowledge, a microphone may sound very different than it would in my studio, or in the hands of Steve Albini, or on a Beatles record. What counts is that you know how to get the best results from the microphone at hand-and that those results sound right to you.
Myles Boisen is a guitarist, producer, composer, and head engineer/instructor at Guerrilla Recording and The Headless Buddha Mastering Lab in Oakland, California. He can be reached by e-mail at firstname.lastname@example.org.