WHEN STAGING AN EVENT WITH music, you can't underestimate the important role microphones play in achieving a high-quality sound. As you are probably already aware, omnidirectional microphones are rarely used in sound reinforcement due to problems caused by acoustical feedback. Directional (cardioid and hypercardioid) mics are preferred in live sound applications.
 The Sennheiser K6/ME64 is a small-diaphragm condenser mic with a cardioid pickup pattern, a style of microphone frequently used on guitars and acoustic stringed instruments. |
But to obtain optimal reproduction from any instrument, there are a number of other important things to keep in mind. What follows is a rundown on a few tips and tricks that can help you choose just the right microphone for your next event and place it correctly in relationship to the instruments with which you are working.
There are two common categories of microphones 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, it 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 placed between the poles of a magnet. Most ribbon mics are bidirectional, meaning they pick sounds equally well from either side of the mic.
Condenser microphones use an electrically-charged, metallized diaphragm that is 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. Since power is required by both the microphone capsule and the amplifier, condenser microphones 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. The “phantom” part of the name comes from the fact that the voltage is carried over the same standard 3-pin XLR cable that carries the audio signal, so the power connection is “invisible.”
Dynamic microphones tend to be extremely rugged, making them especially well-suited for most sound reinforcement applications. However, the extremely thin, low-mass diaphragms used in condenser microphones provide improved high-frequency response, with better reproduction of fast transient signals. Therefore, condenser microphones are usually the best choice for instruments such as piano, cymbals and stringed instruments.
Mic Configurations
With many instruments, two microphones arranged as a stereo pair offer optimal reproduction. One popular arrangement is the “X-Y” configuration. In this configuration, the capsules of two microphones are spaced a few inches apart, with the left mic pointed to the right and vice-versa. Another configuration called the “spaced pair,” places the two microphones parallel to each other and at least two feet apart. The X-Y method offers a stereo image that retains a 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, 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 microphones are best in this application. Whereas a stereo X-Y pair works well, some unconventional methods also offer satisfactory 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.
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. The center of the speaker cone provides a smooth sound, with the effect becoming more harsh as the mic is moved toward the outside edge of the cone. Moving the mic even a few inches can make a major difference, so it pays to experiment to find the best 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 amplifier. In this case, the direct signal must be delayed — usually 1 to 5 milliseconds — to align the phase, or timing, of the two signals.
As is the case with electric bass, synthesizers 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 a 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 — in that case, 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 lowered to half- or quarter-stick 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 microphones, especially at close distances. However, small diaphragm condenser mics offer a high-frequency edge that can help solo instruments stand out. Several companies — such as AKG and Shure — manufacture miniature condenser microphones with clamps for mounting on saxophone or trumpet bells, and these are ideally suited for wireless microphone applications.
Miking the Drums
Drum and percussion miking presents a major challenge to any sound engineer. These are extremely loud instruments that cover an exceedingly wide frequency range, not to mention the fact that placing microphones within a maze of drum stands can be a difficult task. Another problem is placing microphones out of range of flailing sticks, which is one reason that rugged dynamic mics are frequently used when close-miking drums. 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 microphones away from the bass drum, whose sound output can permanently deform a delicate diaphragm. A large-diaphragm dynamic microphone, such as an Electro-Voice RE20, Sennheiser MD-421 or AKG D112, offers the right combination of low-frequency response and the ability to reproduce high sound-pressure levels to do the job. A little bit of equalization also helps — a frequency boost at 1.5kHz adds more attack, while a cut at 400-600Hz can reduce excessive boominess for a tighter sound.
Whether on stage 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!
George Petersen is both the editorial director of Mix magazine as well as an active producer whose recent projects include The Aquaphobics (www.mp3.com/aquaphobics).