Vive la Différence

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We've all heard it: “If you want to do a job right, use the right tool for the job.” If your job is to record something, your main tool is the microphone. Choose which one to use and where to put it, and 80 percent of the job is done.

But how do you choose the right tool, given the range of microphone designs and prices? Though mics have been covered extensively in these (and other) pages (see “More than the Sum” in the June 2003 issue of EM, “Smokin' Condensers” in the March 2004 issue, and “Ribbon Revival” in the November 2005 issue), I'll explore a few topics that should help you get a better sense of what mic will work for you.

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FIG. 1: This image is a cross-section view of a dynamic transducer. As the diaphragm moves back and forth due to air-pressure changes, the coils (conductors) connected to it move back and forth within the magnetic field, producing an alternating current.

At the Heart of It

Recording technology involves taking acoustic energy — air molecules oscillating back and forth — and mapping it to alternating-current electrical energy — electrons moving back and forth at a corresponding frequency and amplitude. Microphones accomplish this with a diaphragm, which is typically an extremely thin sheet of Mylar coated with gold, aluminum, or nickel. The diaphragm vibrates in response to air-pressure changes, and this motion is converted to electrical current with a transducer.

Transducers come in two varieties: dynamic (also called electrodynamic, electromagnetic, ribbon, and moving coil) and condenser (also called capacitor). Dynamic microphones operate by magnetic induction. This principle states that if you poke a conductor (such as a wire) into a magnetic field, you'll get a jolt of current in it. Pull the wire out, and you'll get a jolt of current in the opposite direction. Move the wire in and out continuously, and the result is an alternating current flow. In a dynamic mic, metal coils attached to a diaphragm extend into a magnetic field. As the diaphragm moves back and forth, alternating current is produced in the coils (see Fig. 1). Dynamic mics are sturdy, inexpensive, simple to design, and a good choice for onstage amplification.

Condenser mics are more complex in design than dynamic mics. Unlike dynamic microphones, which generate their own current, condenser microphones require an external source of electrical current because they operate by means of a capacitor and therefore need to be charged. The current can come from a battery or from an external power supply in the form of phantom power.

Condenser mics are good at picking up sharp transients, such as those from pianos or drums, and they tend to have a wider frequency response than dynamic mics, making them the preferred type for use in recording studios. Historically, condensers were more expensive than dynamic mics, but high-quality, lower-priced models have emerged in recent years.

Point Me in the Right Direction

A microphone's pickup pattern, or directionality, can be seen in a polar chart. Fig. 2 shows two common polar patterns: omnidirectional (left) and cardioid (right). Omnidirectional microphones, or omnis, can receive signals at equal magnitudes from all directions. They can be effective when used in groups, especially when recording an ensemble in a large room. The mics are often arranged in an arc, spaced a few feet apart. The instruments' signals reach the microphones at different times, and the phase differences among them create a nice diffuse spaciousness. (For more about phase, see “Square One: About Phase” in the May 2004 issue of EM.)

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FIG. 2: Microphone directionality is shown using polar plots that illustrate the mic''s relative sensitivity to signals from all -directions. Here are plots for an omnidirectional microphone (left), which is equally sensitive in all directions, and for a cardioid microphone (right), which is oriented to the front with some signal from the sides, and very little from the rear.

Cardioid condensers are often used in a coincident pair, or XY, configuration, in which two mics are placed at the same location, one pointing 45 degrees to the right and the other pointing 45 degrees to the left. The fine pressure differences received by each of the mic capsules produce precise spatial imaging. The XY configuration is effective for recording small groups of instruments or for collecting location sounds. Stereo mics, such as the Røde NT4, make the process convenient by putting a coincident pair of diaphragm units on a single body.

Stereo recordings using coincident configurations, however, can lack the spaciousness found in recordings using spaced omnis. As a compromise, many engineers use an ORTF configuration, a near-coincident technique where the mics are separated by about 7 inches, each angled about 55 degrees away from center.

Getting Low Down

The frequency response of a directional mic depends on its construction. The output level depends on differences in pressure (called a pressure gradient) hitting the front and rear sides of the diaphragm. (In contrast, omnis have diaphragms that receive pressure changes on one side only.) The greater the pressure difference, the greater the microphone's output. The difference depends on two things: phase and intensity drop-off.

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FIG. 3: Because the far side of a microphone diaphragm represents a longer travel distance for the wave front, the front and rear respond to the same signal, but with a difference in phase. A high-frequency signal (lower left) produces a big difference in pressure levels, while a low-frequency signal (lower right) produces minimal difference in pressure at the two sides of the diaphragm.

Phase differences are due to the time difference between when the wave first reaches the front of the diaphragm and when it refracts around and hits the diaphragm from the back. They are much more significant with high frequencies than with low frequencies (see Fig. 3). High frequencies, with their short wavelengths, may be at very different levels in their cycle on either side of the diaphragm. Note the difference in pressure shown between points x and y in the lower left of Fig. 3. This creates a big difference in pressure levels, which means the microphone has high output. But low-frequency pressure waves, with long wavelengths, create nearly equal pressure levels at both sides of the diaphragm, as shown at points x and y in the bottom right of Fig. 3. With minimal pressure differences, the output signal is not as strong.Thus, a cardioid or figure-8 has a natural tendency to roll off low frequencies and emphasize the highs. Mic manufacturers compensate for this by adding an output transformer that has a frequency response curve mirroring the mic's curve. The sum of the two produces a flatter response.

Intensity drop-off results from the longer distance to the far side of the diaphragm, and the drop-off is significant at close range. This is because acoustic intensity drops according to the inverse square law: basically, a change of an inch means a lot when you are close to the sound source but means next to nothing at a distance.

At close ranges, the added distance from the front to the rear of the diaphragm adds a significant percentage to the travel path. As a result, there is a stronger gradient because the intensity drops significantly by the time the wave front reaches the far side. This intensity difference affects all frequencies, including the low ones, meaning that the lows have greater presence at close range. When combined with the corrective curve of the output transformer, the result is a bass boost at close range. This is known as the proximity effect, or bass tip-up. It was exploited by crooners and announcers in the 1930s and 1940s with ribbon mics, a classic bidirectional dynamic design. They learned to make subtle adjustments in their distance from the microphone to give a husky sound to their voices.

Different types of cardioid mics have different frequency response curves built in. Most handheld dynamic cardioids, such as the Shure SM58, are designed to give a flat response at 4 to 6 inches from the source. If it's closer, there can be too much of a bass boost, resulting in a muffled sound and popping plosives. If it's positioned farther away, there can be a roll off of lows and a thin, AM-radio-like sound. On the other hand, small-diaphragm condensers, such as the Shure KSM137, Røde NT5, and Neumann KM 184, have a response that is designed more for distant positioning.

Are You Experienced?

Mic choice and placement is an art learned over time — like playing an instrument — and there's no substitute for experience. But with the basics presented here, you can get started on your road to recording virtuosity.

Mark Ballora teaches music technology at Penn State University.