karma101
因緣
Some General Mic Information. Hope it helps out some of the beginners.
A microphone is a transducer. It changes one kind of energy into another. As a comparison, the human body has two transducers: the ears and the voice box. What you need to know about microphones can be broken down into the following word: TRAP. Each letter of "TRAP" stands for a mic attribute or a fact that you should know about mics in general.
T — Type - Studio mics break down into two basic types: Dynamic and Condenser. Dynamic mics work on the principle of magnetic induction. They need no external power to run, are very simple to make and relatively cheap to buy. Dynamic mics come in two varieties: Moving coil and Ribbon. Moving coil mics use a magnet, a coil wrapped with wire and a diaphragm that sits over the top of both. Sound pressure hits the diaphragm and moves the coil across the magnet. This creates the voltage that travels out and along the mic cable on the way to the Mic Pre. A ribbon mic is a bit different. Instead of a coil and diaphragm, a thin metal corrugated ribbon is stretched across the magnetic field. Sound pressure hits the ribbon and moves it across the magnet. Both of these mics have their own particular characteristics dealing with Frequency and Transient Response.
Condenser mics are generally more expensive and have a flatter frequency response than dynamic mics. They also operate in an entirely different fashion. For one thing, these mics need power to run. This is called PHANTOM POWER and is +48Vdc. Phantom power comes down the mic cable from the console, a battery inside the mic or standalone power pack. The phantom power charges a capacitor, which holds a charge in the mics' fixed back plate. In front of the back plate resides a thin diaphragm. When the diaphragm moves in relation to the fixed back plate, a charge is developed in relation to how much movement the diaphragm makes. Unlike the signal created by the dynamic mic, a condenser's signal is very weak and must be amplified before it gets to the console. In order to do this, the mic contains a small amplifier that boosts the signal before it leaves the mic.
R — Recognition - To run a session successfully in a studio environment, you need to know your tools, just like a carpenter could tell you what his/her tools are. To achieve this, you must study mics in general, ask questions and do a lot of comparison.
A — Application - Which mic for what task? Do you know the best mic for recording kick drum? How about vocals or horns? Once again, study is the key. Do your comparisons, ask engineers; everyone has their favorite. Soon you will have a bag of tricks that will serve well a studio.
P — Patterns - The Directional Response of a mic has to do with how the mic picks up sound around its polar axis. The front of a mic is called on-axis; all other directions into the mic are called off-axis. When you talk about the on-axis and off-axis response, you are talking about how a mic picks up sound in reference to those directions. There are five basic polar patterns that mics offer: Omni, Cardioid, Hyper-cardioid, Super-cardioid and Figure eight.
• Phantom Power
Phantom power is a DC voltage (typically 48V) applied to pins 2 and 3 of a condenser microphone's XLR connector to power the microphone's electronics. Pin 1 (shield) is the ground for the phantom voltage.
All condenser microphones require power for their impedance-matching electronics. Externally biased condenser mics also require a polarizing voltage between the diaphragm and back plate. Electric condenser mics get their polarizing voltage from the static-electricity charge on the electric material, which is in the diaphragm or on the back plate.
In professional microphones, a provision is made to supply this voltage directly through the microphone cable. This procedure is called phantom powering, and the most common phantom supply voltage available in mixing consoles is 48 VDC, although other supplies of between 18-24 VDC are widely used.
How does it work? In a balanced microphone cable are two inner wires surrounded by a shield. It is on these two signal lines that the polarizing voltage is placed, with the same regulated positive voltage on each line. With the positive voltage on the two inner wires, the shield wire is used as the negative return to ground. The audio signal uses the same cable on its way to the mixer, and is not affected by the phantom DC voltage since the signal is AC.
Dynamic or ribbon microphones connected to a phantom-power input are protected from damage, theoretically, since the system results in a net zero DC potential across the coil or ribbon. However, if the dynamic or ribbon microphone is unbalanced internally (one side of the coil or ribbon accidentally tied to ground), damage is sure to occur.
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