Microphone Types and Basic Use
As explained in the first section on sound, a microphone has a diaphragm that induces an electric current. There are, however, several types of diaphragms and many different pickup patterns to choose from.
A dynamic mic has a diaphragm attached to an induction coil. That coil is surrounded by a magnetic field. The sound wave’s compressions and rarefactions move the diaphragm, which moves the coil, which induces current (since it’s inside a magnetic field). The voltage then goes to a transformer, where it’s amplified, then heads through the cable out to the system.
A condenser mic is always charged with some voltage, usually in the form of +48VDC coming out of a mixer; we call this “phantom power,” (possibly since it slips through AC current (the sound coming back to the mixer) like it wasn’t even there. It has a super thin plate of metalized material (like gold on Teflon) for a diaphragm, which sits adjacent to another charged plate. When air pressure hits the diaphragm, it moves slightly, causing the capacitance (the ability of adjacent conducting bodies to store an electric charge) between the two plates to vary slightly. This variation is read as the audio signal.
Dynamic vs. Condenser
The larger, more robust diaphragm of a dynamic mic makes it ideal for certain uses. Generally, they handle very loud noises better without being damaged. They can usually survive more physical abuse (like a fall). Being less sensitive, dynamics aren’t quite the microscope that a condenser is; in some instances, they can hide some intricacies that a condenser would pick up (the good and the bad). Another result of decreased sensitivity is a dynamic usually picks up less of the surrounding noise (bleed) than a condenser. Dynamics are uses often in live situations and on the loudest of sources (guitar cabinets, kick/snare drum). Most of the top broadcaster/voiceover mics are dynamics as well.
Condensers are usually more complex, expensive, sensitive and flimsy than dynamics. They are better as overhead mics since they can pick up sounds that are far away. They are also great for the studio since you usually want every intricacy to work with. Studios often use condensers on every source, especially when recording one instrument at a time (a whole band in one room must be set up more like a live show).
There are exceptions to just about every generic principle when it comes to gear. There are flimsy dynamics and strong condensers. There are active dynamics that use phantom power to achieve higher sensitivity. There are good condensers that cost the same as a similar-quality dynamic. And any good mic is sufficient to learn on; entire songs have been recorded with a single, $100 dynamic and sounded fine.
Microphone Pickup Patterns
What audio a microphone picks up and its specific frequency response is what determines if a mic is well-suited for a particular application. The common pickup patterns are:
Cardioid: If there’s a “normal” pickup pattern,
Omnidirectional: This is the most natural-sounding pickup pattern, and also the most prone to feedback. It picks up from all directions equally well and usually with every nuance. It’s an ideal candidate when you want to capture the ambiance of a room or tune a system. They are also used in the studio when you want a very natural, open sound. The downside is bleed since they pick up everything, even stuff that’s far away; in live situations, an
Figure-eight: These are like taking two cardioids and putting them back to back, but is still only a single, mono signal. They pick up equally well from the front and rear while rejecting the sides. Often, they are paired with each other, but 90-degrees off in what’s called a “Blumlein Pair.” Ribbon microphones (that use a thin, metal ribbon as a diaphragm) always have a figure-eight pattern.
Hyper- and Super-Cardioid: Hypercardioid has a more narrow pattern than a regular cardioid at the expense of less rear rejection. Supercardioid is tighter still, with minimal loss in rear rejection.
Many mics are multi-pattern. They either have two cardioid capsules that can act as other patterns or even blends of patterns at the flip of a switch, or they have interchangeable capsules.
The frequency response of a mic is how well it picks up different frequencies. This information is usually conveyed via a response range and a frequency response graph/curve. In the case of the Oktava MK-319 picture above, it’s 20Hz-18kHz, with this curve:
Most often, this graph will only show what the frequency response is right in front of the mic. In this graph, it shows in front, in front
Auditioning microphones is the best way to know if it’s well-matched to a specific sound. Odds are the standard near-flat response won’t sound the best on a given instrument. There is usually a
Other Basic Principles
Proximity Effect: Mics picks up more low end the closer you get to the diaphragm. This can be helpful if you have a thin voice. I can be detrimental to a singer with poor mic technique, or to a system that feeds back at the affected frequencies. Omnidirectional microphones are the exception to the proximity effect; as such, their low end is almost always accurately represented.
The 3-to-1 Rule: To get adequate separation between sources, you should position mics three times farther away from each other than they are from their sources. So if a singer is a foot away from a mic, the next mic should be at leat 3 feet away from the first.
Phase Cancellation: When multiple mics are close to the same source, there is a good chance their signals will cancel each other out to some degree. This can be fixed with better mic placement or if in a recording, nudging one track into phase alignment with the other.
Cardioid Stereo Techniques: There are a few popular ways to mic in stereo with two-three mics close together and no phase cancellation, even after summing the audio to mono. An X/Y (also known as a “coincident”) pair is two cardioids set at 90-degrees from each other, with the recording elements (diaphragms) as close to touching as possible. A spaced pair is done with two mics that are usually 3-10 feet apart, angled as desired. An ORTF pair has two mics right next to each other, angled at 110-degrees away. An M/S (mid-side) setup uses a figure-eight mic for the sides, with a cardioid in the middle; it requires a special process called mid-side decoding to sound right.
Decca Tree: Out of the seven or eight variations of Decca Records’ stereo micing techniques, the modern Decca Tree uses three omnidirectional mics high above an orchestra—one dead center, and two out to the sides for a wide stereo image.