Capturing Big Sounds

All about large-diaphragm dynamic microphones.
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Dynamic microphones, the blue-collar workers of the recording and live-sound world, have a well-deserved reputation for reliability and tough construction. Within the broader category of dynamic transducers, and the focus of this article, are those with large diaphragms. Although there is no strict industry standard for what constitutes a large diaphragm as it relates to dynamic mics, typically any diaphragm that is ¾ inch or more in diameter qualifies as large.

Large-diaphragm dynamic mics are known for delivering superior bass response, and many major microphone companies market such products specifically as bass drum mics. (For a comprehensive comparison of seven popular bass-drum mics, see “Kickin' It” in the February 1999 issue of EM, available online at www.emusician.com.) It is commonly believed that a larger diaphragm automatically picks up a greater proportion of low-end frequencies or a warmer sound; however, this is not quite true. In fact, it is technically possible for a sophisticated small-diaphragm condenser mic to provide a flat response down to 20 Hz. But the most cost-effective method of getting better lows out of a dynamic mic is to increase the diaphragm size and then boost low-end coloration through design principles. (In general, dynamic mics are affordable due to their simple mechanical design and relative ease of mass production.)

Although there are a few exceptions, large-diaphragm dynamic mics are usually front address, have a cardioid pattern, are bigger in size than a typical dynamic vocal mic, and are meant to be stand mounted for use with stationary instruments, rather than held in the hand. Because of the increased size and advanced design features, they also tend to be the most expensive type of dynamic transducer.

Dynamic mics function as a loudspeaker in reverse. The diaphragm is a moving induction coil — basically a flat disc or a shallow cuplike assembly with wire wrapped around its circumference. In response to sound pressure arriving at its surface, the diaphragm moves relative to a permanent magnet situated underneath it (see Fig. 1). The movement of the coil within a magnetic field generates a small electrical current.

Due to the relatively high mass of the diaphragm, dynamic mics tend to be less responsive to subtle sounds when compared with condenser or ribbon microphones. Many dynamic mics have tight cardioid patterns, which, combined with lowered sensitivity, make this kind of transducer ideal for reducing off-axis bleed in loud environments and in small or crowded studios. Overall, dynamic mics are rugged, maintenance-free, and ideally suited to the abuses of location recording and live sound. (For more information on dynamic mics, see “Square One: Vive la Différence” in the March 2006 issue.)

Added Color

Dynamic mics tend to be less flat and more colored in their frequency response than condenser mics. And compared with ribbon and condenser mics, the mass of a dynamic diaphragm simply can't offer the fastest possible transient response. These qualities make the dynamic mic less than ideal for situations where sonic fidelity is a priority, such as in classical or audiophile recording.

Dynamic mics are rarely my first choice for tracking acoustic guitar, percussion, piano, and the like. These delicate sonic sources usually pair best with the high resolution, sensitivity, and accuracy of a condenser mic.

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FIG. 1: In a moving-coil dynamic mic, the diaphragm is attached to a coil of wire that vibrates within a magnetic field. That induces an electrical signal in the wire that corresponds to the incoming acoustic waveform. Credit: Chuck Dahmer

However, it is important to consider that the most expensive mic, such as a condenser or ribbon, may not be the best-sounding choice for a particular style of music. Consequently, dynamic mics make an interesting creative tool for miking amplifiers or other sources in situations where an ear-catching sonic signature takes precedence over hi-fi realism. Top engineers who have every kind of mic at their disposal will often use a Shure SM57 (www.shure.com) on snare or a Sennheiser MD 421 (www.sennheiserusa.com) on toms, simply because it is a sound they prefer and can dial in quickly. Sometimes the slower transient response of these mics can be an advantage as well, providing a type of peak compression and a vintage vibe on drums, vocals, and guitar amps.

Dynamic mics are also well suited for use in high-humidity environments or where there is a lot of moisture, such as when close-miking harmonica, flute, or vocals. The ability to handle high sound-pressure levels (SPLs in excess of 150 dB), plosives (the blasts of air associated with vocalizing the letters p, b, t, and d), and wind is another advantage dynamic mics have over other kinds of transducers.

Additional Features

Some large-diaphragm dynamic mics have enhancements that are beneficial for studio recording. For example, the Sennheiser MD 421 has a 5-position adjustable low-end rolloff switch located near the XLR connector. This filter is designed to reduce the proximity effect when doing vocal work in studios and in radio broadcasting. But it also is effective when close-miking instruments. The Electro-Voice RE20 (www.electrovoice.com) also has a switchable single-position low-cut filter built in.

Stedman (www.stedmancorp.com) makes a few interesting variations on large-diaphragm designs. Its LD 23 and LD 50 models are both conventional-looking large-diaphragm mics that can be handheld. The Stedman N90 is a side-address dynamic that not only looks like a large-diaphragm condenser mic, but also offers condenser-like specifications.

Heil Sound (www.heilsound.com) also has a large-diaphragm dynamic with the appearance and flat frequency response of a side-address condenser. In addition, Heil makes two mics, the Commemorative Classic and the Classic Pro, that resemble vintage ribbon mics on the outside but have large-diaphragm dynamic elements on the inside.

Audio-Technica (www.audio-technica.com) makes a unique kick-drum mic, the ATM 250DE, that includes dynamic and condenser elements with two separate outputs. (The AE2500, a similar mic, was reviewed in the November 2003 issue.)

Preamp Makes a Difference

In any discussion of miking, it is always important to consider the role of the microphone preamplifier. At times in my testing of preamps for EM reviews, I have been very surprised by the way a humble dynamic mic can open up when connected to a high-quality tube or solid-state preamp, resulting in markedly improved sound quality. Typical results in these cases include an expansion in the pickup of the high- and low-frequency range, richer midrange harmonics, and a more immediate presence on percussive transients.

Proper loading — the complex impedance interaction between a mic and a preamp — can also make a big difference in the sound of a mic. A deluxe preamp with switchable input impedance will offer two or more loading options that may affect the timbre, as well as the output gain, of the connected transducer.

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FIG. 2: Miking the hole in a bass drum''s front head results in plenty of character, but success lies in how well the drum is dampened and tuned. Credit: Myles Boisen

The Obvious Choice: Drums

The powerful low-end response and superior isolation of many large-diaphragm dynamic mics make them a favorite for miking drums, the bass drum in particular.

Inside the bass drum. There are a number of variations in placement on a kick drum. Positioning the mic inside the drum shell — pointed at the beater and close to the beater head — yields a sound that will consist primarily of punchy lows and a well-defined, often clicky, beater attack. When soloed, this type of sound can appear to be deceptively thin and not at all representational of a real bass drum in a room. But when added to a full complement of drum mics, such a close-miked kick tone is often perfect for a range of rock, dance music, and contemporary roots-music styles.

Moving the mic further from the beater head increases the proportion of shell resonance. This generates a more realistic tone, with a fuller midrange and less attack. Adding tone in this manner often makes it more of a challenge to get the kick drum track to cut through a dense mix. But this approach may be more sonically and musically appropriate for a project where the bass drum is not necessarily a central or prominent feature.

Positioning the mic in the center of a hole cut in the drum's front head delivers a fairly realistic tone with a lot of character, but the resulting sound is often lacking in defined attack unless the drum is well tuned and somewhat dampened with a pillow or blanket inside (see Fig. 2). However, with proper tuning and the right beater choice, a drum can have a focused sound at the hole, making this a viable recording method for rock and other styles (see Web Clip 1 and “Capturing the Kit” in the July 2004 issue).

Outside the kick drum. For bass drums without a hole in the front head, close-miking the center of the front head gives the greatest amount of resonance and low end. Moving the mic toward the edge of the head increases the amount of attack and higher harmonics while downplaying the inherent boom of a drum. I often use an intermediate position, at a distance of 2 to 6 inches from the center, to balance low end and attack. Typically, I will roll off some of the low end from a kick drum track recorded in this manner when it comes time to mix.

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FIG. 3: In this photo, an Electro-Voice RE20 points between the center and the edge of the bass speaker. Note how it''s angled slightly toward the middle to increase high-end definition and achieve a balanced sound. Credit: Myles Boisen

For most of the bass drum recording I do, the Sennheiser 602 is a fantastic tool. The 602's handling of highs and lows gives me exactly what I need in most circumstances to get a sharp, punchy kick sound with little or no repositioning. This mic also has the ability to extract a usable tone from substandard or poorly tuned bass drums. (For more information on recording bass drum, check out “How to Record a Kick Drum” in the July 2002 issue.)

Toms. The principles mentioned for recording a solid-head kick drum apply directly to the use of large-diaphragm dynamics on rack and floor toms. Positioning the mic closer to the center of the head yields a resonant, bassy tone. Moving the mic toward the edge of the head picks up more of the complex harmonics of the drum and a higher proportion of stick sound.

Two additional issues come into play for this application. The first is obvious: keep your mics out of harm's way by not putting them too close to the center of the head, where the drummer strikes. Unless the drumheads are brand-new, there will be an obvious wear pattern on the head that will let you know immediately where the drummer tends to hit the tom.

The second issue is isolating the toms from cymbal and snare bleed. Dynamic mics tend to be less sensitive to off-axis pickup, but the leakage that does get through can be highly colored, creating problems in the mix. Gating the tom mics is one solution to the problem. Another is careful placement that maximizes the rejection of other components of the kit. When possible, avoid placing tom mics directly underneath cymbals. Position the mics so that the cymbals or other drums are oriented toward the rear of the mic, 180 degrees off-axis.

Beyond the Drum Kit

Any source with significant low-end content is a good candidate for a large-diaphragm dynamic mic. Let's look at some of the more common applications.

Bass cabinet. A large-diaphragm dynamic can be an excellent choice for miking an electric bass cabinet, especially when excessive volume raises concerns about using a condenser mic. Pointing the mic directly at the center of the speaker cone tends to emphasize treble response, upper harmonics, and pick and string sound. A rounder and bassier tone results from moving the mic toward the outside edge of the cone. A balanced tone can often be achieved simply by placing the mic between the center and the edge of the cone, and then angling it slightly toward the middle to increase high-end definition (see Fig. 3).

Bear in mind that many bass speakers these days have low-frequency ports, and often high-frequency tweeters as well. These features need to be taken into account and can either be emphasized or de-emphasized with mic placement.

Acoustic bass. I have found that a condenser mic is generally the best choice for recording acoustic bass, due to its faster transient response and better high end. However, a large-diaphragm dynamic can also deliver a satisfactory stand-up bass sound. And due to its lower susceptibility to feedback, this type of mic may be the best choice for a bass in live-sound reinforcement.

Miking the instrument in front of the bridge typically offers the most balanced and representational sound. But usually this kind of placement has to be at a distance of one foot or so to accommodate bowing. Miking the bass in front of the f holes provides more gain and better isolation but tends to favor a boomy and uneven timbre, with some notes coming out much louder than others.

Electric guitar. One of my favorite guitar mics is the large-diaphragm Sennheiser MD 421. The tonality of this mic, which blends high-end cutting power with low-end punch, seems perfect for many of my guitar recording needs. All of the aforementioned tips for recording a bass cabinet are also applicable here.

A great guitar-amp recording trick is to use the 421 and a ribbon mic side by side, directly in front of the speaker cone and about four to six inches back from the amplifier's grille cloth. To reduce the effects of phase cancellation, the diaphragms of the two mics should be as close together as physically possible.

Angling each of the mics slightly toward the outside edge of the speaker and then panning the two guitar tracks apart in the mix produces a stereoized sound that is bigger, airier, and more commanding than the conventional single-point guitar sound. This mic technique can also add excitement to any source run through a conventional amp: organ, synth, electric piano, amplified acoustic instruments, and samples.

Brass and sax. Condenser mics often have an exaggerated high-end response, which can make them less than ideal for recording brass instruments such as tuba and trombone. Dynamic mics (and ribbon mics as well) tend to soften the high-end sizzle on these instruments, making them easier on the ear when close-miking.

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FIG. 4: Position the mic at the edge of the bell of a brass instrument to capture low end while mitigating harshness. Credit: Myles Boisen

Positioning the mic at the edge of the bell, rather than aiming it down the center of the bore, captures a more complex sound with superior low end and less harsh highs (see Fig. 4). Low brass instruments can also put out deafening SPLs, which a dynamic mic can easily handle.

The Sennheiser MD 421 and Electro-Voice RE20 are large-diaphragm mics commonly used on saxophones and brass. When a suitable condenser mic is not available, this type of microphone makes a good alternate choice for studio work.

The aforementioned transducers are also staples of any decently equipped live-sound setup. A large-diaphragm dynamic provides needed low-end heft, due to increased proximity effect as well as the boosted bass response of the mic itself. These mics also work well for less common wind instruments such as bass flute and bass clarinet.

Vocals. It's no secret that the huge, booming tone of a classic radio announcer is created by working a large-diaphragm dynamic mic as close as possible to deliver maximum intelligibility and proximity effect. Most condenser mics are too susceptible to popping for this kind of trick, and ribbon mics may well be damaged or destroyed by plosives this way.

In the studio, rock and rap vocalists in particular may be most comfortable with the freedom of movement and familiarity that a handheld dynamic mic provides. And for beatboxing, a dynamic mic is the only way to go. The resulting sound, though it may lack the transparency and air of a condenser mic, is often punchy, well defined, and easy to place in the foreground of a mix.

Unconventional Uses

I have put large-diaphragm dynamics to work on a number of unusual jobs. These include miking the lower rotating bass speaker of a Leslie cabinet, tracking a didgeridoo, and using one as an experimental drum-room mic.

I obtained unusual coloration using a large-diaphragm dynamic as an ambient drum-room mic: I placed it inside the shell of a floor tom, which was laid on its side on the floor a few feet away from the drum kit. This technique tends to emphasize a few boomy low frequencies. But with some attention to placement and drum tuning, it can add a unique and hugely resonant timbre to the sound of the kit. I have also gotten some wild sounds by placing drum-room mics inside metal tins, canisters, and garbage cans.

Big and Bad

Although it's tempting to think of large-diaphragm dynamic mics as being for bass drum and other limited uses only, they form a versatile class of transducer with numerous potential applications for recording. Some models offer superior sonics and deluxe features applicable to instrumental and vocal use. And while dynamic mics have a reputation for being rugged and affordable rather than sonically sophisticated, in many cases they offer distinct advantages over the delicacy and flat response of condenser and ribbon mics.

Myles Boisen runs Guerrilla Recording and the Headless Buddha Mastering Lab in Oakland, California. He can be reached through his Web site atwww.mylesboisen.com.