Chapter Two: Studio Gear
3. Microphones | page 2
There is an incredible variety of microphones availed on the commercial market, from a $5 Radio Shack special to microphones costing $20,000 and up. For all this variety, most fall into one of the following basic types:
Dynamic: Electrical generators create current by passing coils of conductive wire past magnets. This principle is known as induction. A dynamic microphone works on the same principle. Usually a coil of thin wire called a voice coil is attached to the diaphragm. The wire is coiled around a fixed magnet separated from it by a very small gap. As sound waves hit the diaphragm it causes the coil to move in the magnetic field of the magnet, thereby producing fluctuation in the current moving through the coil.
Dynamic mics are best used for loud sounds or sound that may contain sharp transients. The are not as good for minute detailed sounds and usually do not have the high frequency response of a condenser microphone. Also, they are usually cheaper and more robust than condenser mics.
Condenser: A condenser mic works on an electrostatic principle, not an electromagnetic one, as in the dynamic mic. Two plates separated by only a small distance are electrically charged, creating a capacitor or condenser. One of the plates, often called the backplate, is fixed; the other is part of the microphone diaphragm. As sound waves hit the diaphragm, the distance between the charged plates changes, producing equivalent fluctuations in the capacitance. An additional component, a resistor, is attached across the leads of the two plates. It is the voltage across the resistor that is sent out as the mic's signal. To charge the plates, condenser mics need a form of power called phantom power. This is supplied by most professional mixing consoles and is usually +48 volts DC — if you need phantom power from the board, look for a switch on individual channels or channel groups. Phantom power from a console requires a balanced line, since the shield carries part of the charge (pins 2&3). Some condenser microphones are self-powered with a battery, designed to go dead at the worst possible moment. Some mics, such as our Rode NT5's, can run on either phantom power or a battery. A variation of a condenser mic is called an electret or electret-condenser. This mic has a permanently charged element requiring no phantom power (although it may have an internal amp that requires power). It can however lose its charge in high temperature and humidity.
The studio AKG 414's are condenser mics, and they do require phantom power. They normally have superior frequency response to dynamic mics but are much more sensitive to high SPL transients. So, they are good for recording most instruments, and small bug noises — bad for close-mic'ing bass drums and Howitzers.
Ribbon: Ribbon mics work on yet another principle, which responds to the velocity of the air molecules, not just the SPL of sound passing over its element. An unbelievably thin corrugated ribbon of aluminum — or more recently a polyester film with aluminum printed on it — is placed over a magnet. As the film moves when hit by air molecules, the changes in induction produce a fluctuating current, much like a dynamic mic. The famous Johnny Carson mic was a ribbon mic and similar to the one used by Larry King. Newer versions, such as the Beyerdynamic M160, house the capsule in a much smaller ball.
PZM: Short for pressure zone microphone, PZM (which is a trademarked term) is a type of boundary microphone that uses a small electret capsule suspended over the backplate. The capsule is mounted to a large, flat surface that picks up the reflected waves from the surface, thereby doubling its output. While used for low quality recording of business meetings by putting a PZM mic in the center of a conference table, newer quality mics are now being used for recording pianos. One issue is that if the surface it is mounted on is too small, lower frequencies will not be reflected due to their larger wavelengths.
Contact: A contact mic, normally stuck to or clamped onto an instrument, receives most if its audio signal from the mechanical vibration of the object it is attached to. It is often used for mic'ing instruments where feedback or unwanted sounds from other instruments would be an issue. One of the drawbacks of using a contact microphone on acoustic instruments is that it emphasizes frequencies resonating at a particular spot on the instrument, thereby creating a misrepresentation of the overall timbre of the instrument. For this reason, many players, such as double bassists, have gone back to acoustic microphones. When using contact mics, experiment with different placements on the instrument, be it a string bridge or a clarinet barrel.