Understanding Microphone Specifications

While microphones may not have the same glitz factor as some of the new digital audio recording devices to be unveiled at NAB2006, they are first in line in capturing a quality acoustic audio signal, with appropriate frequency response, polar pattern, level, dynamic range, and low noise.
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While microphones may not have the same glitz factor as some of the new digital audio recording devices to be unveiled at NAB2006, they are first in line in capturing a quality acoustic audio signal, with appropriate frequency response, polar pattern, level, dynamic range, and low noise.(Here, I'm using "acoustic' for pickup of any sound through the air, as opposed to a direct electronic connection, like a guitar pickup.)

Using good quality mics and good microphone techniques helps ensure that the recording on that fancy new hard disk recorder will be both technically useable and artistically pleasing.

Choosing a mic can be a daunting task because there are so many models to choose from. NAB is a good forum to see the various models up close and personal, and at least talk through them and listen on headphones at most of the mic booths.

While this isn't necessarily a good test of how the microphone will actually be used (like for singers or instruments), you should be able to narrow down your choices for further investigation.

A big help in choosing a mic for a particular application is the specification sheet. Another good source of information is the users guide that many mic manufacturers provide.

Table 1 lists some key specifications for two types of mics from the same manufacturer, to be used for different applications. Example 1 is a vocal mic designed to be used extremely close up. Example 2 is a mic more suited for the close micing of acoustic instruments, overhead micing of drums and other percussion instruments, and also ensemble pickup. This mic is meant to be used both close up and at some distance from the sound source.

On the spec sheet, let's focus on the transducer type and polar pattern this month, and frequency response and sensitivity next time.

TYPES OF TRANSDUCERS

Two common types of microphone transducers are dynamic and condenser.

(click thumbnail)Table 1: A list of some key specifications for two microphones -- Example 1 is a vocal mic, while Example 2 is primarily used as an instrument mic.
The dynamic mic is also called a moving coil mic since it is constructed with a diaphragm connected to a coil of wire that moves through gaps in a magnet. When the voice coil moves through the magnetic field in response to impinging sound waves on the diaphragm, it produces an electrical current proportional to the sound pressure level.

Condenser is an old term for capacitor, and that's what forms the pickup for this type of mic. The backplate of the capacitor is supplied with an electrical charge, either permanently in the case of electrets, or otherwise through an external power supply.

The front "plate" is actually the diaphragm of the mic, and when it moves in response to sound pressure, it changes the size of the spacing between itself and the backplate, thus changing the electric field between the two, and producing an electrical signal.

Unlike a dynamic mic, a condenser mic requires a pre-amp that is usually installed internally in the body of the mic. This active circuitry needs to be powered either with an internal battery or externally through phantom power from the console or external power supply. Phantom power is actually a DC voltage sent over the same mic line as the audio signal.

Because there are electronics involved in condenser mics, there are more things to spec, such as the maximum sound pressure level, maximum clipping level, noise figure, and optimum voltage for phantom power.

PICKUP OR POLAR

There are two main types of microphone pickup patterns--omnidirectional and directional.

The omnidirectional mic, as the name implies, has a uniform response to sound coming in from all directions. At higher frequencies, even omnidirectional mics can exhibit some directionality, with response stronger at the front of the mic than at the sides or rear. The frequency where directionality starts depends on the size of the microphone capsule and the body design.

Directional microphones can be subdivided into two categories: unidirectional and bidirectional. Unidirectional mics have the highest response to sound on-axis to the mic with response falling off to zero at the rear of the mic (180 degrees off-axis).

A cardioid mic is an example of a unidirectional mic. Its polar pattern is rather heart-shaped, with maximum response in the front of the mic, minimum response at the rear, and a gradual decrease in response along the sides, from the front to the rear.

Helpful hint--when you look at published graphs on spec sheets, note the scale of the axes on the graphs. The wider the increment, the less detail will be evident in the response curves.

A bidirectional mic has a response to sound pressure at both the front and rear of the mic. One type of mic in this category exhibits equal response in both the front and the back with no response at the sides (90 degrees to the front).

While some people consider that a mic with this "figure 8" type of response is the only candidate for this category, other mics with narrow pickup patterns, like supercardioids and hypercardioids, also have some response to sound coming into the back of the mic.

This rear "lobe," as it's called, has a response that is lower in level and narrower in coverage angle than the response at the front of the mic, but it does need to be considered in mic placement. The minimum response on these types of mics occurs at some angle to the side, but towards the rear.

Next time we'll look at microphone frequency response and sensitivity specifications.