By Barry Rivman, Senior Staff Writer, Musician's Friend
The art of recording starts with selecting the right tool for the job—the essence of the craft is microphone selection and placement. It can take volumes to describe everything you need to know. The goal here is not to blind you with science, just give you enough understanding of how certain elements of mic design will help you choose the right mic for any situation. So here's a crash course on mic selection.
There are three elements combined that most affect proper mic selection: transducer type, diaphragm size and mass, and polar (or pickup) pattern.
Before we get into it, there are a few terms you need to know:
Diaphragm: The diaphragm is a surface inside a mic that vibrates when struck by sound waves, which in turn generates a voltage that becomes an audio signal. A smaller diaphragm responds better to high frequencies, a larger diaphragm responds better to low frequencies. The lighter and thinner the diaphragm, the faster it will respond to transients.
Transient: A transient is the leading edge and initial attack of a sound envelope, such as the crack of a snare.
Compliance (transient response): How fast a diaphragm responds to transients is referred to as compliance. If a mic doesn't have good transient response, the sound will have less attack and percussiveness.
Tip: Boosting treble to bring out a dull high end will cause the instrument to lose its midrange in a mix and will make it sound thin and scratchy. Fix it at the sound source by recording with a more compliant mic.
Sensitivity: How far a mic's diaphragm travels in response to sound-pressure waves determines how great the output voltage will be. A high-sensitivity mic will have greater output voltage. If the output voltage is higher than a preamp can handle, distortion results.
SPL-handling: A mic's ability to withstand high-pressure waves is measured as SPL (Sound Pressure Level).
Signal-to-noise ratio: Put simply, a ratio of how much desired sound (signal) a mic picks up versus how much noise is picked up by the mic. Moving a mic closer to a sound source improves the signal-to-noise ratio.
Tip: Proximity to the source determines output voltage. A high-SPL source will produce a higher output. Low-sensitivity mics can be closer to high-SPL sources without distortion, which has the added bonus of higher signal-to-noise ratio. If you want to use a high-sensitivity mic in front of a high-SPL source, move it back, but watch your signal-to-noise.
Proximity effect: An increase in bass or low-frequency response when a sound source is close to a mic. Depending on the mic design, proximity effect may easily result in a boost of up to 16dB, usually focused below 100Hz. Vocalists tend to like proximity effect since it fattens their voice, but a constantly varying bass boost can wreak havoc on headroom and carefully set levels.
Three Basic Transducer Types:
There are three general types of microphones: The moving coil or dynamic mic, the condenser mic, and the ribbon mic. The physical components and operating principles of each type of transducer determine sound quality and use.
Dynamic (Moving Coil) - This approach employs a circular diaphragm attached to a coil that moves back and forth within a magnetic field, which induces a voltage that becomes the audio signal. The diaphragm of a dynamic mic has the most mass of all three types, which makes it the least compliant since it must overcome greater inertia. It also provides an advantage: The dynamic mic's diaphragm is the most durable of the three and therefore well suited to the demands of live sound reinforcement and high-SPL sound sources.
Condenser - Also called a capacitor mic, it has two oppositely charged thin metal plates placed a small distance from one another. The rear plate is fixed while the front plate moves back and forth. This configuration mimics function of a capacitor. The movement of the diaphragm in response to pressure waves produces an oscillating voltage from the capacitor, proportional to the original pressure oscillation. The diaphragm is very thin and lightweight, making condensers high-sensitivity, highly compliant mics.
Ribbon mics operate under the same principle as the moving coil, but the diaphragm is a long, thin, corrugated metal ribbon that fluctuates between two magnets of opposite charge. However, instead of creating current in a coil as it vibrates back and forth, current is generated in the ribbon diaphragm itself. The ribbon has much less mass than a moving coil and is therefore much more compliant. It responds better to percussive sounds and high frequencies depending on the length of the ribbon. Ribbons are very low sensitivity and require a clean preamp with around 65-70dB of gain. Though traditionally ribbon mics were fragile, today ribbons, such as the Royer R121, are more durable and can handle high-SPL sources.
Tip 1: Unless the manufacturer states otherwise, do not expose a ribbon mic to phantom power, since +48V will travel along the ribbon and burn it out.
Tip 2: Even though ribbons are tougher these days, never blow into a ribbon mic to see if it's working; when walking with a ribbon mic, cover the grille with your hand to protect the ribbon.
From the information we have so far, we can deduce that a compliant, small-diaphragm mic (condenser) is good for recording high-frequency percussive sounds such as a triangle, cymbals (drum overheads), or acoustic guitar. A compliant large-diaphragm mic (condenser) would be good for recording vocals and acoustic bass. A less-compliant, large-diaphragm mic (dynamic) would be useful for recording high SPL sources such as a kick drum and electric bass. A less-compliant small-diaphragm mic (dynamic) would be good for recording electric guitars and snare drums (also high-SPL sources). However, depending on the sound you're going for and how you envision the texture of guitars in a mix, any type of mic is fair game. Just keep their characteristics in mind and you can make an informed choice.
Polar or pickup patterns basically describe the physical area or directional characteristics of the mic's "hearing." Any mic capsule can be made to have any polar or pickup pattern. This is mainly achieved by the design of the housing. Most microphones have one polar pattern, however, there are also mics that have multiple patterns, which can be accessed either by switch or by interchangeable capsules.
Directional pickup patterns include cardioid, supercardioid, hypercardioid, and shotgun. Directional mics pick up mainly from the front and reject sounds that come from the rear and sides. Directional mics provide more gain before feedback and greater isolation, which makes them very popular for live sound reinforcement. Super and hypercardioid mics are more directional than cardioid, rejecting more sound from the sides. But they do pick up some sound from the rear (hyper more than super), so don't place your floor monitors directly behind them. Shotgun is a very directional pattern with extended reach.
Bidirectional or figure-8 mics pick up from the front and rear of the mic while completely rejecting sounds from the sides. The advantage of the figure-8 pattern is the ability to add some room tone to vocals or instruments without off-axis coloration. Ribbon mics are bidirectional, while some condensers have a switchable figure-8 pattern. Figure-8 mics are also an essential component of Mid-Side stereo recording.
Omnidirectional: Omnis pick up sound equally from all directions … well, mostly. Omnis give you the sound of "being there;" can handle extremely high SPL; do not suffer from proximity effect; and are not that sensitive to wind, pop, or handling noise. However, omnis are more prone to feedback, therefore not widely used in live sound.
Tip: Never wrap your hands around a vocal mic directly behind the capsule (as so many singers do at an emotional peak). This effectively blocks the ports in the capsule that enable rear rejection and turns a cardioid mic into an omni, which is more prone to feedback (squeal!!!, emotional peak ruined).
Dynamic, Ribbon, or Condenser?
You should have at least one of each, but if your preamp lacks sufficient gain for a ribbon mic, an L-D and S-D condenser and a dynamic should get you through all recording chores. However, as platinum producer Michael Wagener (Ozzie, Motley Crue, Alice Cooper) says, "Two SM57s and an SM58. I can make a record with those."