Microphone Technology 101—Basic Terminology and Techniques for Stage and Studio

May 18, 2004 12:00 PM, By George Petersen

BASIC TERMINOLOGY AND TECHNIQUES FOR STAGE AND STUDIO

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Microphones are a key factor in achieving high quality recordings and sound reproduction. Having access to top-notch equipment helps, but more importantly, some knowledge about mic basics and a few simple tips and tricks can make a substantial difference on your next project.

There are two common categories of mics used in professional audio. Dynamic types operate when sound waves strike a diaphragm attached to a coil of wire. When the coil moves within the magnetic structure of the microphone, this creates an output voltage. The process is exactly the reverse of the way a speaker operates. One variation of the dynamic approach is the ribbon mic, which uses a thin ribbon of metal that is placed between the poles of a magnet. Note that most ribbon mics are bidirectional, meaning they pick sounds equally well from either side of the mic.

Condenser mics use an electrically-charged, metallized diaphragm, placed very close to a conductive back plate and separated by a thin air layer. Sound waves striking the diaphragm cause a very small voltage change, which is increased by a tiny amplifier circuit within the mic body. As power is required by both the mic capsule and the amplifier, condenser mics must have a power source, which can be a battery inside the mic body or "phantom" power coming from either the mixing console or an external power supply.

Dynamic mics tend to be extremely rugged, making them especially well-suited for sound reinforcement applications. However, the extremely thin, low-mass diaphragms used in condenser mics provide improved high-frequency response, with better reproduction of fast transient signals. Therefore, condenser mics are usually the best choice for instruments such as piano, cymbals, and stringed instruments.

With many instruments, two mics arranged as a stereo pair offer optimal reproduction. One popular arrangement is the "X-Y" configuration, where the capsules of two mics are spaced a few centimeters apart, with the left mic pointed to the right and vice-versa. Another stereo technique uses a "spaced pair", with the two mics placed parallel to each other and at least 50cm apart. The X-Y method offers a stereo image that retains consistent level when the recording is played in mono. The spaced pair technique provides a more dramatic stereo effect, but extreme left and right sounds may be emphasized more than sounds coming from the center. Both of these stereo techniques are frequently used on stage and in the studio, particularly for piano, orchestra, and above drum sets, large percussion setups, vocal choirs and horn or string ensembles.

Acoustic guitars can be miked in a number of ways, and condenser mics are best in this application. While a stereo X-Y pair works well, some unconventional methods can offer good results. A miniature condenser lavalier microphone can be taped or clipped inside the guitar and mixed with a second mic placed outside the guitar, pointing toward the soundhole. In a quiet studio, mics can be placed one or two meters away, offering a full, mellow sound. If more string or pick noise is desired, the mics can be placed closer.

Electric guitar speakers usually require no more than placing a single dynamic mic pointed toward the cabinet. However, the sound of a speaker varies widely when the mic is placed in different locations on the speaker. Moving the mic even a few centimeters can make a major difference, so it pays to experiment to find the "right" sound.

Generally, electric bass is not miked, but is usually connected to a direct box. Sometimes, this direct sound can be combined with a mic placed in front of the bass amp. In this case, the direct signal must be delayed—usually 1-5 milliseconds—to align the phase (timing) of the two signals.

As with electric bass, synths and electronic keyboards are usually connected to direct boxes or routed from the submixer in the keyboardist's rack. One exception is the Leslie speaker connected to a Hammond organ. Since the Leslie speaker consists of a bass speaker with an rotating horn above, one upper and one lower mic are required. The output of these can be mixed to provide a variety of effects, ranging from subtle to dramatic, especially when the upper rotor is miked in stereo, using two mics!

Acoustic grand piano can be a difficult instrument to reproduce, where the microphone method depends largely on the other instruments in the room. When recording solo piano, a stereo pair of mics is often placed at a distance, to capture the room ambience. Unfortunately, this approach doesn't work well on stage with a rock and roll band—here, close-miking is required. Depending on the piano, two condenser mics in a variation of either spaced pair or X-Y generally work well, with the lid raised open and the mic capsules placed 10-20cm above the strings. A brighter sound results when the mics are placed close to the strings. If sound leakage from other instruments is a problem, then the lid can be (carefully) lowered, or a heavy blanket can be placed over the piano top.

Most horn and reed instruments have a slightly harsh character and sound better through ribbon or large-diaphragm dynamic or condenser mics, especially at close distances. However, small diaphragm condenser mics offer a little high frequency "edge" that can help solo instruments stand out. Several companies—such as AKG, Audix and Shure—manufacture miniature condenser mics with clamps for mounting on saxophone or trumpet bells, and these are ideally suited for wireless mic applications.

Drum and percussion miking presents a major challenge to any sound engineer. These are extremely loud instruments that cover an exceedingly wide frequency range, no to mention the fact that placing mics within a maze of drum stands can be a difficult task, indeed. Another problem is placing mics out of range of flailing sticks! As mentioned earlier, an overhead stereo pair can be used to pick up cymbals or to cover a large percussion setup. Condenser mics are the best choice here, since they are placed out of harm's way and offer excellent high-frequency response.

Dynamic mics are most often used for snares, tom-toms, conga, timbales, bongos and other drums—one secret to getting a good sound here is to make sure that the mics are placed above the drum and pointed downwards. When the microphone is placed parallel (or at a slight angle) to the drum, then most of the sound energy strikes the side of the mic, resulting in a thin, weak sound. If you do not have enough mics (or console inputs) for each individual drum, then a single mic can be placed between two adjacent drums.

Getting a solid bass drum sound can be tricky. Removing the front head of the drum and placing a blanket or pillow against the beater head may be the first step. Whatever you do, keep your condenser and ribbon mics away from the bass drum, whose sound output can permanently deform a delicate diaphragm! A large-diaphragm dynamic mic offers the right combination of low-frequency response and the ability to reproduce high sound pressure levels to do the job. A little EQ also helps—a boost at 1.5kHz adds more attack, while a cut at 400-600Hz can reduce excessive boominess for a tighter sound.

Years ago, I became hooked on using electronic kick drum to trigger samples on rock, pop and R&B dates—even when I was recording an otherwise all-acoustic kit. This approach offer many advantages: As the kick is silent in the studio, sympathetic snare buzz is greatly reduced, and the overall signal in the overheads is cleaner, with less kick bleeding into the other mics. Also, with no bleed from the original kick, the kick patttern can be changed or altered later if necessary—a highly desirable option. On any session (sampled or otherwise), the goal of recording kick is to get a solid, consistent sound—and here the sampled kick adds—rather than detracts—from the drumkit sound.

Whether onstage or in the studio, getting a great sound requires good equipment and the application of fundamental techniques. Remember that there is no single correct method that works in all cases, and sometimes a bit of experimenting with angles and microphone placement really pays off. Be creative!

Note: This article first appeared in the Spanish language edition of Mix magazine and has been translated to English from the original.






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