The EM Poll

Capturing Big Sounds

Nov 1, 2007 12:00 PM, By Myles Boisen

           

All about large-diaphragm dynamic microphones.

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.

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.