Technology

Smart Microphone Selection: What to Grab and Why

There’s good reason why the putative majority of top tracking engineers choose a Shure SM57—a $99 Everyman’s mic—to record snare drum. These pros have access to the world’s priciest transdu

Photo 1: Small diaphragm condensers such as the DPA 4011 excel at making detailed recordings of plucked and struck instruments.

There’s good reason why the putative majority of top tracking engineers choose a Shure SM57—a $99 Everyman’s mic—to record snare drum. These pros have access to the world’s priciest transducers, but they know expensive isn’t always better. Creative whims aside, a mic’s inherent physical, acoustic and electrical properties are what determine whether it’s the best choice for a given application.

This month, Mix takes a look at the advantages and disadvantages of using different types of microphones for various recording applications in music production. To be sure, there are no absolutes; a superbly designed dynamic mic might outperform a second-rate condenser in an application where the latter should’ve ruled by virtue of its breed. To avoid repeating “all other things being equal” ad nauseam, we’ll assume it’s understood our story is presented here using broad strokes. We’ll begin with recording percussive instruments.

HOW MUCH DETAIL DO YOU WANT?

When you want to bag gobs of high-frequency detail in a recording, think small. Small-diaphragm condensers excel at recording plucked and struck instruments. Their relatively tiny diaphragms have so little mass, they respond faster to transients than the membranes used in large-diaphragm condensers, assuming equal thickness. While ribbon mics also feature an element with very low mass, other factors typically make its transient response less prominent than that for a condenser, whether SDC or LDC. Moving-coil dynamic mics are far less responsive to transients than any type of condenser or ribbon mic.

To fully understand why mass is such a big factor in determining transient response, consider the physics of sound. Transients are essentially made up of high frequencies, which have very short wavelengths and are highly vulnerable to air-transmission loss. They’re practically over and done with by the time a heavy mic diaphragm—such as that used by a moving-coil dynamic, with its attached wire—overcomes its inertia to respond to them and move. The mass of a condenser mic’s diaphragm is only about one-thousandth that of a moving-coil dynamic mic’s, which is why condensers generally offer far greater transient response and more extended high-frequency response.

The element in ribbon mics responds quickly to transients. However, a ribbon mic’s inherently bi-directional polar pattern tends to soften transients slightly due to phase cancellation of very high frequencies arriving at both the front and rear of the mic. (Bass frequencies, with much longer wavelengths, are far less prone to cancellation.) The figure-8 (bi-directional) mode for a condenser mic exhibits the same phenomenon and virtually always produces a less-extended high-frequency response than its other modes, such as omni and cardioid.

Aficionados argue that, despite their inherent attenuation of very high frequencies, ribbon mics offer a smoother, more natural-sounding transient response than condensers. They point out that condensers actually hype transient response due to a phenomenon called overshoot that’s inherent in the design of their diaphragms. A condenser mic’s circular diaphragm acts much like a drum. (The edges of the membrane are secured to what’s essentially a hoop.) When responding to a transient spike in signal level, it tends to briefly produce higher output before its diaphragm settles down. This overshoot tends to cause ringing somewhere in the range between 3 and 15 kHz. The exaggerated but ephemeral bump in high frequencies can sometimes make a recording made with condensers sound a bit glassy.

Photo 2: Royer R-121 ribbon and Shure SM57 dynamic mics are used to record an electric guitar amp (along with a Sennheiser MD421, not visible). Engineers often mark a studio’s resident cab with tape to designate the sweet spots for each mic.

Photo 2: Royer R-121 ribbon and Shure SM57 dynamic mics are used to record an electric guitar amp (along with a Sennheiser MD421, not visible). Engineers often mark a studio’s resident cab with tape to designate the sweet spots for each mic.

Ribbons exhibit negligible overshoot. But that doesn’t make them the best mic for all recording applications. It depends on how much detail you want to capture for a particular track; more isn’t always better. Theory aside, recordings made using condenser mics usually sound more detailed than those using ribbons. When capturing detail is paramount, reach for an SDC. The superior transient response makes it a great choice for recording plucked and strummed string instruments, hand percussion, cymbals, snare drum and rack toms—especially when these instruments will need to cut through a dense mix (see Photo 1).

All that said, ultra-highlighted detail is not always a good thing. For recording an exposed solo performance or simply where a slightly softer high end and less pronounced transients are desired, an LDC or ribbon mic might be a better tool. Many engineers prefer to use LDCs or ribbon mics on cymbals. Ribbons provide excellent tonal balance—especially when paired with a Shure SM57—for recording electric guitar cabinets: just enough detail, without edginess or glare (see Photo 2). And while most condenser mics sound way too edgy and brittle on brass instruments, ribbon mics deliver a relatively soft and more balanced tone on trumpets, trombones and the like (see Photo 3).

CAPTURING LOW BASS

A high-quality LDC such as the Neumann U 87A is an excellent tool when recording a percussive instrument that produces low bass frequencies, such as floor tom (see Photo 4). Any type of quality condenser, LDC or SDC, will reproduce a drum’s batter-head strike beautifully, but the LDC’s larger diaphragm offers a bigger net for capturing the long wavelengths of very low bass frequencies. The larger diaphragm also captures more signal overall; for this reason, the best LDCs are inherently less noisy than SDCs. This is one of the reasons why you’ll usually see an LDC used for ADR, narration (for books on CDs and the like) and quietly sung lead vocals.

But an LDC is often the best-sounding mic for any kind of vocal track. The lightweight diaphragm captures enough detail for vocals to sound articulate, yet the sound will usually be less edgy than what an SDC would produce. And unless a highly directional polar pattern is selected, an LDC will exhibit less proximity effect than a ribbon mic, allowing the singer to stand very close to the mic without blowing up the low end. It may not be the appropriate setup for all productions, but there’s nothing quite like the intimate sound of a close-miked singer (see Photo 5).

Again, there are no absolutes, and breaking the rules can lead to stellar results. The English pop group Right Said Fred used an SDC to record the lead vocal for their 1991 hit, “I’m Too Sexy.” The SDC’s hypersensitivity to transients no doubt contributed heavily to the highly articulate sound of that track, which was arguably critical to the record’s success.

ENSEMBLE RECORDING

When tracking basics for a group or recording a large instrument with multiple mics, the mics’ off-axis frequency responses take on added importance. That’s because bleed into a mic with poor off-axis response will skew the timbre of the overall recording.

Photo 3: Renowned jazz trumpeter Arturo Sandoval is recorded using a Royer R-121 ribbon mic. Ribbons typically lend a softer, more balanced tone on brass instruments.

Photo 3: Renowned jazz trumpeter Arturo Sandoval is recorded using a Royer R-121 ribbon mic. Ribbons typically lend a softer, more balanced tone on brass instruments.

In theory at least, ribbon mics have an inherently superior figure-8 polar pattern—offering better rejection at its null points and smoother response off-axis—compared to that for a multi-directional condenser mic set to produce the same pattern. The reason is that a condenser mic produces its bi-directional pattern by using two diaphragms and subtracting the electrical output of one from the other; the subtraction is essentially accomplished by flipping the phase of one diaphragm’s output 180 degrees. Any imprecision in this process will cause a somewhat frequency-dependent polar pattern, wherein cancellation at the null points (90 and 270 degrees off-axis) isn’t perfect across the spectrum. A ribbon’s construction, on the other hand, creates a figure-8 pattern mechanically. Sound originating at either of the mic’s null points wraps around to front and back faces of the same element and cancels out perfectly. Its excellent off-axis response makes ribbon mics a great choice for tracking multiple instruments in the same room.

For mics that produce polar responses other than figure-8, SDCs—pencil-shaped mics in particular—generally offer the best off-axis response. The SDC’s smaller diaphragm, usually mounted in a slimline head capsule and mic body, imposes a smaller obstacle to high frequencies arriving off-axis. That is, the high frequencies’ short wavelengths have less difficulty wrapping around an SDC’s slim mic body and small head capsule to reach the diaphragm. An LDC, on the other hand, creates a relatively large acoustic shadow for off-axis signals, typically resulting in progressively reduced response as frequency and angle of incidence increase.

You might think this superior off-axis response makes an SDC the unquestionable choice for recording ensembles and large instruments. However, in many cases you’ll want to use an LDC’s poorer off-axis response to your advantage. When miking toms with an LDC, for example, cymbal bleed is reduced by the mic’s relatively greater attenuation of high frequencies arriving off-axis (see Photo 4). That allows you to apply more high-frequency EQ boost to the toms (to emphasize the stick attack) without cymbal bleed ripping your head off.

SDCs are a great choice, however, for recording instruments such as acoustic guitar, harp and timbales—percussive instruments that produce dissimilar timbres from different parts of their bodies and with varying playing techniques. The SDC’s superior off-axis response tends to accurately capture all the frequencies emanating from multiple vibrating surfaces, smoothly integrating the composite sound into a coherent whole.

PROXIMITY EFFECT AND PLOSIVE CONTROL

A mic’s polar response largely determines the strength of its bass-proximity effect. As the directionality of the polar pattern increases, so does this effect. The figure-8 pattern produces the strongest bass-proximity effect. This is the reason why close-miking with a ribbon mic or an LDC in bi-directional mode can result in a sound that’s too bass-heavy. (Some ribbon mics are electronically altered to produce a less-directional response than figure-8.)

Photo 4: A Neumann U 87A is placed on a floor tom and set to bi-directional mode. The mic’s null point is aimed, as much as is practical, toward the ride cymbal to reduce cymbal bleed into the mic.

Photo 4: A Neumann U 87A is placed on a floor tom and set to bi-directional mode. The mic’s null point is aimed, as much as is practical, toward the ride cymbal to reduce cymbal bleed into the mic.

An omni mic exhibits no bass-proximity effect; you can place an omni mic virtually right on the sound source without any increase in bass response. Position an omni only an inch or two away from a singer, and the sound will be completely devoid of the blurriness and boomy bass a cardioid or more directional pattern would impart at that distance to the source.

That said, bass-proximity effect is sometimes a very good thing. Some female singers, for example, sound too thin when using an omni mic. By gradually progressing from omni through cardioid to hypercardioid or bi-directional mode with a multi-pattern LDC that offers continuously variable polar patterns, you can dial in the perfect amount of bass and lower-midrange boost to make a paper-thin vocalist sound fuller and bigger. A multi-pattern LDC gives you the most options for shaping the timbre of a track.

Unfortunately, as a mic’s directionality increases, so does its vulnerability to popping when subjected to plosives (wind turbulence from a singer’s mouth). The bi-directional polar pattern is the worst in this regard. Similarly, the lower the mass of a mic’s diaphragm, the more likely it is to pop when subjected to plosives. SDC’s are the most vulnerable, generally making them a poor choice for recording vocals unless from a considerable distance. That said, you can sometimes get incredible results recording a singer with an omni SDC by pointing the capsule at the ceiling and having the vocalist sing over the top of the head grille.

KEEPING NOISE AT BAY

A mic’s sensitivity and noise specifications are important to consider together when choosing a mic to record a quiet source. A low inherent- or self-noise spec is not enough to guarantee the track will have a low noise floor. If the mic’s sensitivity (output voltage for a given SPL) is very low, you’ll need to apply a lot of preamp gain to its output signal in order to attain a healthy recording level—inordinately boosting the mic and preamp’s self-noise in the process. The best mics for recording quiet sources exhibit both low noise and high sensitivity—hallmarks of high-end LDCs.

Self-noise specs for LDCs range from a mouse-quiet 1.5 dBA (for the Marek Design RS1 tube condenser) to a relatively noisy 25 dBA or higher. SDCs and active ribbon mics generally exhibit self-noise in the middle of this range. Passive ribbon and moving-coil dynamic mics contain no active circuitry, so they produce negligible noise. However, their sensitivity is so low that a very high amount of noise-inducing preamp gain must sometimes be applied in order to achieve an acceptable recording level.

LDCs range in sensitivity from about 10 mV/Pa to an earth-shattering 653 mV/Pa (for the RS1), with the vast majority of models producing less than 40 mV/Pa. (1 Pa, or pascal, equals 94 dB SPL.) SDCs offer roughly 6 to 25 mV/Pa sensitivity—generally a little less than LDCs as a group. Active ribbon mics produce around 10 to 20 mV/Pa, about the same as the average SDC. Passive ribbon mics produce very weak output: typically between 1 and 4 mV/Pa. The Shure SM57’s sensitivity, at 2 mV/Pa, is fairly representative of that for moving-coil dynamic mics.

Photo 5: A Korby Audio Technologies KAT Blue HRS (a multipattern large diaphragm tube condenser) is set to omni mode to close-mic lead vocals for an intimate, highly detailed sound free of proximity effect.

Photo 5: A Korby Audio Technologies KAT Blue HRS (a multipattern large diaphragm tube condenser) is set to omni mode to close-mic lead vocals for an intimate, highly detailed sound free of proximity effect.

High sensitivity is a good thing when recording quiet sources, but it can be a drawback when tracking drums, guitar amps and brass instruments. Unless it’s fitted with a mic pad, a high-sensitivity mic can easily overload your DAW’s input when placed on a loud source. Moving-coil dynamic and modern (durable) ribbon mics are great choices for recording these sources. Moving-coil dynamics are the sturdiest of all professional mics. They can often survive a stick hit, making them a good selection for placement on traps when recording undisciplined drummers.

CHOOSE WISELY

Smart mic selection begins with deciding what you want to capture in your recording: more or less detail, a bigger or leaner bottom end, and brighter or softer highs. Your choice may be modified by imperatives such as avoiding mic bleed, noise and clipping. Each situation suggests using a particular type of mic. Suggests, but doesn’t require. Break the rules, but know why you’re breaking them.

Mix contributing editor Michael Cooper is a recording, mix, mastering and post-production engineer and the owner of Michael Cooper Recording (www.myspace.com/michaelcooperrecording) in Sisters, Ore.

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