With their small size and light weight, they can be problem solvers in a wide range of applications...
January 13, 2014, by Bruce Bartlett
For decades, dynamic microphones were the only choice for live applications due to their ruggedness. Live engineers didn’t want to take delicate, expensive condenser mics on the road.
All of that’s changed now that condensers have been made more robust and roadworthy, and they’re quite capable of handling a wide range of live applications.
First let’s explore the inner design. A condenser (or capacitor) capsule has a very thin, light, conductive diaphragm and a metal backplate mounted a few thousandths of an inch apart (Figure 1).
They’re charged with static electricity to form two plates of a capacitor. When sound waves strike the diaphragm, it vibrates. This varies the spacing between the plates, and in turn, this varies the capacitance and generates a signal similar to the incoming sound wave.
Because of its lower diaphragm mass and higher damping, a condenser responds faster than a dynamic to rapidly changing sound waves (transients).
Two types of condenser mics are true and electret. In a true condenser (externally biased), the diaphragm and backplate are charged with a voltage from a circuit built into the mic. In an electret condenser, the diaphragm and backplate are charged by an electret material that’s in the diaphragm or on the backplate. Both types can sound equally good, although some engineers prefer true condensers, which tend to cost more.
Figure 1: A look at the design of a condenser capsule.
Because the capsule impedance is very high, its output goes directly to an impedance converter (usually an FET) to reduce the impedance to a usable value. A power supply is required to operate the impedance converter and to bias the capsule in true condenser types.
The power supply can be an internal battery or phantom power. Phantom power is 12 to 48 volts DC applied to pins 2 and 3 of the mic connector through two equal resistors. The microphone receives phantom power and sends audio signals on the same two conductors.
Ground for the phantom power supply is through the cable shield. Nearly all mixing consoles, mic preamps and audio interfaces supply phantom power at their mic input connectors – simply plug the mic into the mixer and turn on phantom power.
Of course, different mic designs – condenser, moving coil, and ribbon – differ in the way they convert sound into electricity. Of the three types, the condenser transducer generally has the widest, smoothest frequency response and the highest sensitivity. It tends to provide a natural, detailed sound, and it produces a strong signal that overrides mic-preamp noise.
Miniaturization is a key benefit of condensers. Try to shrink a dynamic or ribbon design and the magnet gets too small to produce enough flux for a strong output signal. Compared to a dynamic with its moving coil and multiple air cavities, a condenser has a relatively simple construction and higher sensitivity, so it can be miniaturized to 1/4-inch and even 1/8-inch in diameter and still be useful.
Self-noise tends to be higher with small-diameter mics, but they’re typically placed very close to their sound source so they still produce a high signal-to-noise ratio.
Since condensers can be made very small, they can be inconspicuous and light in weight, so they’re ideal for numerous live applications.
Probably the first use of condenser mics in live concerts was for cymbal overheads. Cardioid condensers such as the AKG 451 or Shure SM81 have an extended high-frequency response up to 20 kHz and a sharp transient response that makes these mics a natural for picking up cymbals.
Figure 2: A Shure Beta 98AMP clamp-on mic.
Over the years, engineers have added numerous dynamic mics to the drum kit to capture the toms, snare, and kick separately, resulting in a forest of boom stands surrounding the kit.
Fortunately, in 1987 Shure introduced a tiny clip-on cardioid condenser mic – the SM98 – that could be clipped to individual drums. Although cardioid condenser mics of small size tend to roll off in the lows, an EQ circuit in the mic’s preamp boosts the bass, resulting in a full sound from a tiny mic. Proximity effect helps as well. The latest incarnation of that mic is the Shure Beta 98AMP, which has an integral gooseneck and preamp (Figure 2). A special clamp holds the mic onto a drum rim.
Figure 3: A single-mic technique for a drum kit.
Some live engineers hate the hassle, complexity and potential phase interference of multiple drum mics and are getting back to simpler methods, such as the one-mic technique invented by engineer/producer Tchad Blake: take a large-diaphragm cardioid condenser mic and mount it over the kick drum top, aiming at the snare drum (Figure 3).
The single mic picks up a decent balance of the snare, toms, kick and cymbals all around it. The balance can be tweaked by moving or rotating the mic, and raising/lowering the cymbals. There may be some off-axis coloration of the cymbals depending on the mic model and position, but in my experience it’s not too serious.
Attached & Hanging
The clamp-on technique for drums soon spilled over to other instruments. Now we have attachable mini mics for horns, acoustic guitar, upright bass, fiddle, and so on.
What are the advantages of attaching compared to stand-mounting?
—Less clutter on stage. There’s nothing between the musician and the audience.
—More consistent volume and tone. With a stand mic, the volume and tone changes when the player moves.
—Consistent sound from gig to gig. The musician always uses the same mic in the same position.
—Freedom of movement on stage. The player is not locked into a single position in front of a mic.
DPA Microphones is noted for its effective clamp-on mics, such as those found in the d:vote Series. They include a shock mount and windscreen (Figure 4). Another supplier is Applied Microphone Technology (AMT), which offers specialized models for horns, strings, woodwinds, drums and percussion, piano, and even accordion.
Figure 4: A DPA 4099 on a sax.
Some condensers mount right next to the instrument’s surface (Figure 5), outfitted with internal EQ to compensate for the tonal effects of close placement. For example, a flat-response mic in a guitar’s sound hole tends to sound boomy because of the sound hole’s resonance around 80-100 Hz. Mics designed for sound-hole placement are rolled off in the bass to compensate, resulting in a natural, non-boomy sound.
Another inconspicuous condenser for live sound is the hanging mic or choir mic. Suspended from the ceiling or mounted on thin stands, these mics nearly disappear in use. Thin boom stands are made by a variety of sources who usually also make choir mics.
Figure 5: A clip-on guitar mic.
So far we’ve covered mini models, but how about large-diaphragm condensers (LDCs)? Side-addressed designs with large diaphragms are the usual choice in the studio for vocals, and this type of mic is now affordable enough to take on the road. They’re a good choice on saxophone, trumpet, strings, and drum overheads.
There’s also been a return to the single-mic technique for bluegrass and “old-time” artists. The band members huddle around one or two LDCs and mix themselves acoustically by weaving toward and away from the mic(s). This application requires a low-noise mic with a good low-frequency response and a wide angle of pickup, and a large-diaphragm cardioid can do the job.
Figure 6: An AKG C535 EB cardioid condenser handheld microphone.
Road-tough condensers for vocalists are built with rugged steel grilles and thick handles to withstand drops and rough handling. Some examples are the Shure KSM9HS, Neumann KMS 104 and KMS 105, and AKG C535 EB (Figure 6).
In some models, the circuitry is encapsulated in waterproof conformal coating so that humidity and mouth spray are not serious problems. Especially for folk or jazz vocals, the uncolored, detailed sound of condenser mics is a treat to the sound mixer’s ears.
Condenser vocal mics come in wired and wireless formats, and analog or digital outputs. Many have an internal shock mount to reduce handling noise, and all have a large grille that acts as a pop filter. Some can handle 135-150 dB SPL with under 3 percent THD (check the mic’s data sheet). A capacitive pad can be used to attenuate the capsule’s signal so that it does not overload the electronics.
Staying with vocals but looping back to miniature designs, there are now a wealth of condensers available for headworn applications. The capsule is mounted on a light boom just off the edge of the mouth or directly in front of the mouth, very close (Figure 7).
The latter type provides exceptional gain-before-feedback and isolation, as well as studio-quality sound. Mini headworn condensers with an omnidirectional pattern are nearly invisible in use yet offer exceptional fidelity for quieter musical acts, and for actors in plays and musicals.
A surface-mounted omni condenser is an example of a boundary mic. These can be gaffer-taped to the side walls of a theater for audience pickup or to the underside of a piano lid for discreet miking.
If you place a conventional microphone on a stage floor to pick up actors, the high end rolls off due to phase interference from floor sound reflections (Figure 8). The delayed reflections combine with the direct sound from the actor, cancelling high frequencies.
Figure 7: Wisconsin Singers director/producer Robin Whitty-Novotny outfitting a vocalist with a Countryman ISOMAX headset mic (with windscreen).
Specially designed floor mics with tiny mic capsules prevent that phase interference because the reflected sound has such a short delay compared to the direct sound. The result is a natural sound without comb filtering. Stage floor mics with a cardioid or supercardioid pattern offer good clarity and gain-before-feedback for area pickup of drama and musicals.
Lectern & Lavalier
Remember the clunky dynamic mics on creaky goosenecks that used to pick up worship leaders at a lectern? Now we have slim, elegant lectern mics thanks to miniaturized condenser mic capsules. These mics adjust silently and come with a cardioid or supercardioid polar pattern.
Clip-on lavalier mics work great for lecturers and worship leaders who like to wander as they talk. They offer a response that rises at high frequencies to compensate for being off-axis to the mouth. A typical placement is 8 inches under the chin.
Figure 8: A stage floor mic (supercardioid boundary design).
Lavaliers have a tiny electret-condenser capsule connected by a thin cable to an XLR connector or wireless transmitter connector. The cable can be hidden under clothing, secured by adhesive bandages to prevent cable-rubbing noise. Actors often hang a lavalier mic just below the hairline. The cable routes through the hair and down to a hidden transmitter. Most mics intended for that application are moisture-resistant.
Lavs are available with omnidirectional or unidirectional polar patterns. A uni (cardioid) pattern reduces feedback and background noise, but is sensitive to cable noise, wind, and breath pops. It’s a good choice if the ambient noise level is high. An omni generally sounds more natural and is smaller. It also allows more head movement without level variations, and is less sensitive to cable noise, wind noise and pops.
If a lavalier must be hidden under clothing, its noise pickup can be minimized by wrapping the mic in a foam cylinder. Some high-frequency EQ boost may be needed to compensate for the muffling effect of clothing. Rycote offers Undercovers, Overcovers and Stickies, which are disposable adhesive pads that reduce clothing rustles and wind noise.
Figure 9: An Audio-Technica AE2500 dual-element cardioid.
You can also utilize two mic types on a single instrument to get a variety of tones without resorting to EQ. For example, the Audio-Technica AE2500 (Figure 9) dual-element cardioid combines both in a single unit. According to A-T, “the dynamic element delivers the aggressive attack of the beater; the condenser captures the round tonalities of the shell.” Several top mix engineers have come to utilize the AE2500.
Live sound is now a great arena to enjoy the advantages provided by condenser mics. Particularly in light of features such as small size and light weight, they can be problem solvers in a wide range of applications.
Bruce Bartlett is a live sound and recording engineer, audio journalist, and microphone engineer (www.bartlettaudio.com).