Audio Basics: Dynamics, Condensers & Phantoms
Getting into the design of microphones

By Scott Foulkrod

Why are there so many microphones? Which is best? These are among the most common questions asked by my audio students.

First, there is no single mic best suited for all tasks, and this statement is also probably the best answer as to why there are so many mics on the market. Our ears are very sophisticated transducers capable of changing sound into electrical impulses that our brain can interpret as sound.

Sound frequencies are measured in Hertz (Hz). One Hz is one sound wave passing a certain point at the rate of one time per second. Our ears are capable of “transducing” sound pitches ranging from 20 Hz to 20,000 Hz (20 kHz). Unlike our ears, mics can only accurately “transduce” (and reproduce) a particular range of pitches within the normal “20 to 20k” range.

If a mic can reproduce frequencies without boosting or cutting volume in a certain range, it is said to be “flat” in that range. This measurement of mic accuracy is known as frequency response, and is one characteristic that is a “must know” for any engineer when choosing a mic for a particular application.


Frequency response chart for several microphones.

A mic’s frequency response can potentially change the timbre of a sound because of its inability to reproduce all of the frequencies present in an audio signal. Another characteristic to consider when choosing a mic is its signal-to-noise ratio (S/N) ­ the amount of usable audio from the instrument or vocalist as compared to the inherent noise that the mic generates by itself. (Every piece of analog and digital audio equipment has a S/N ratio and a dynamic range, which is S/N plus headroom.)

But possibly the biggest factor to consider when choosing a mic is its design type. There are two basic types of microphones, dynamic and condenser. Although dynamic mics have long been the choice of live audio engineers, condenser mics are making their way to the stage more of late, for reasons that we’ll address later. And both are worth examination. Let’s take a look under the hood.

LINES OF FLUX

There are two classifications of dynamic mics, “moving coil” and “ribbon.” Dynamic mics work by the principle of magnetic induction. Most of us have experimented with magnets. They are bi-polar, with one side we’ll refer to as “north” and another side we’ll call “south.” When two magnets are held “north to north” or “south to south” they tend to repel (or push away) one another. Oriented “north to south,” magnets tend to attract one another. If magnets are held in this orientation close enough to create an attraction, but not touching, a magnetic field is created between the two.


The inner workings of the two primary types of microphones.

This magnetic field contains invisible lines of “flux.” All dynamic microphones have this magnetic field. In ribbon mics, a thin corrugated strip of metal is suspended between the magnets in the magnetic field. As the sound waves strike the ribbon, it vibrates. This vibration breaks the lines of flux, which induces an electrical voltage. This voltage is conducted by the ribbon and is identical to the frequency of the vibrations of the sound waves.

In dynamic moving coil mics, there is a thin diaphragm that is attached to a coil of wire. This diaphragm/coil assembly vibrates in the magnetic field, which breaks lines of flux and induces a voltage in the coil. Again, the vibration is identical to the frequency of the sound waves.

The sensitivity of dynamic moving coil mics is determined by the size of the diaphragm, strength of the magnets and the amount of wraps of wire in the coil.

Although ribbon mics are more sensitive than moving coil mics, they are far more fragile. Ribbon mics also exhibit a bi-directional pickup (or polar) pattern ­ they are sensitive to sound in front of, as well as behind, the mic. Couple this pickup pattern with inherent frailty and most agree this severely limits uses for ribbon mics on stage.

MOVING ROBUSTLY

Conversely, moving coil mics are inherently robust. This design is an overwhelming success story for the working band or sound reinforcement crew. It is also inexpensive. Most moving coil mics are unidirectional ­ they only pick up what is in front of them. This is useful because sound entering the mic from the monitor system can cause that squealing sound called feedback that is an unmistakable beacon of failure. Sound engineers work hard to avoid it, and everyone in the house knows when they don’t.


Phantom power can be activated on individual channels of some consoles such as the Allen & Heath GL4000 (left), while others like the much smaller Mackie 406M powered mixer have one switch to enable phantom power for all channels.

The strength of the magnets and the number of wraps around the coil matters, as does the size of the diaphragm. Large diaphragm moving coil mics are more sensitive than their smaller diaphragm counterparts. They’re generally good mics for drums, with the exception of snares, which produce a high-end rattle (from their wire snares) that small diaphragms (found in mics like the Shure SM57) seem to do better on. Snare drums also produce very high sound pressure levels and can distort large diaphragm moving coil mics.

Large diaphragm moving coil mics like the AKG D112, D550 or the Shure Beta 52 make very good kick drum mics. Sennheiser also makes some nice mics for drums, such as their E602 for kick, as well as the E604 for toms. The latter also includes a mount that grips the top rim of the drum, eliminating bulky mic booms that clutter the stage and can be knocked over. (I’ve even used these in the studio.)

If you’re running sound using onstage wedges for monitors, dynamic moving coil mics are a good choice, providing a high amount of gain before feedback while also being relatively easy on the pocket book. If you’re using in-ear monitoring systems, you may opt for condenser mics for vocals and some instruments. There is quite possibly some additional expense, but the rewards can be considerable.


Technology evolves, but physics remains the same. Circa the mid-’60s, EV’s legendary Lou Burroughs demonstrating what happens when an open mic passes directly in front of a live loudspeaker.

CONDENSERS FOR STAGE

Condenser mics are finding their way into more and more live sound applications. Condensers are very sensitive and have a flat frequency response over much of the 20 Hz to 20 kHz audio range, in part due to their design.

These mics work by the principle of variable capacitance, with a fixed plate and a moveable plate. These plates function as the polarized magnetic source. Sound pressure enters the mic and causes the moveable plate to vibrate in proximity to the fixed plate. This vibration is identical to the original frequency of the sound vibration.

As the plates move closer, then farther apart, they perform the function of an electronic component known as a capacitor. The capacitance varies and a small electronic circuit in the mic produces a current flow that mimics the sound signal. Dynamic mics’ magnets are charged at the manufacturer so that they retain up to 20 percent of the voltage applied to them permanently.


The AKG D 112 (left) for kick drum, and the Sennheiser E604 for toms, including handy mount.

Unlike dynamics, condensers don’t have magnets that are charged permanently at the time of their manufacture. As a result, their diaphragms must be charged every time they’re used. This is accomplished with the use of phantom power. Supplied to the mic through the mic cable, phantom power is normally 48 volts DC for large diaphragm “air” condensers.

The variable capacitance design of condenser mics makes them more sensitive to incoming sound pressure. They’re very “hot” mics. Guitarists can relate this to active circuitry in guitar pickups, and for much the same reason.

Condenser mics also generally exhibit a very flat frequency response. One specific design of the genre is the electret condenser. This design can use a small DC battery or phantom power of considerably less voltage than the 48 volts required by air condensers. Don’t confuse this function with a battery-powered transmitter for wireless mic systems.

PHANTOM CASES

Not only does phantom power provide the necessary voltage for the plates, it also provides power to the onboard impedance transformer. Without phantom power, the large-diaphragm air-condenser mics will simply not work. So where does it come from? Normally, the mixing console supplies phantom power.

Many consoles offer a separate button or switch on each channel that will enable phantom power. Some consoles have a switch that enable several channels at once, or one switch that enables all of the channels. Fewer buttons saves cost but sacrifices function. And certain consoles don’t offer phantom power capability or only have 18-volt phantom power for electret condenser mics, making external power supplies necessary for 48-volt air condensers.


The Crown GLM-100 is an example of an electret condenser microphone.

With a crisp clean sound, it’s tempting to use condensers on vocals. You’ll find numerous models designed for just that purpose. But be aware of some precautions. Condenser mics are more fragile than dynamic moving coil mics, so it’s a good idea to have solid road cases for them when used on tour.

Also note that because condensers are so sensitive, they tend to be more susceptible to feedback. Care must be taken to manage stage volume. In-ear monitors can be useful in this scenario. Also be sure to make your vocalists are aware of these facts as well, so that they can take some precautions.

What if you phantom power a dynamic mic? It doesn’t damage the mic, but it’s unnecessary. Here’s a tip ­ if you’re using a condenser mic and notice that you’re not phantom powering it (because there’s no sound!), mute the channel before engaging the phantom power. Doing this while a channel is live can result in a loud pop.

There can never be too many mics to choose from, and inventive engineers and microphone manufacturers are discovering new techniques on a regular basis. Experiment, evaluate and make your own live sound a little better.

 

Scott Foulkrod has a degree in audio engineering and currently teaches audio engineering at Houston Community College in Houston, Texas. He can be reached at miditeach@yahoo.com

July 2003 Live Sound International

 

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