10.14.2009

How to Mic a Band - Types of Microphones

Getting a good live sound can be difficult. You need to take a lot of things into consideration when micing your band. A few key things to think about are, the area where you will be performing, types of speakers, microphones,  pickup and polar patterns, mixer and signal processors.

Today I would like to talk about some of the microphones that can be used to mic your band.

Dynamic Microphones

By far the most common type of microphone in contemporary sound work is the dynamic. The dynamic microphone is like a miniature loudspeaker -in fact, some dynamic elements serve dual functions as both loudspeaker and microphone (for example, in intercoms). A flexibly-mounted diaphragm, Figure 10-1 (a), is couple to a coil of fine wire (b). The coil is mounted in the air gap of a magnet (c) such that it is free to move back and forth within the gap.   

When sound strikes the diaphragm, the diaphragm surface vibrates in response. The motion of the diaphragm couples directly to the coil, which moves back and forth in the field of the magnet. As the coil cuts through the lines of magnetic force in the gap, a small electrical current is induced in the wire. The magnitude and direction of that current is directly related to the motion of the coil, and the current thus is an electrical representation of the incident sound wave.

Dynamic microphones are highly dependable, rugged and reliable. For this reason, they are extremely common in stage use, where physical strength is very important. They are also reasonably insensitive to environmental factors, and thus find extensive use in outdoor paging applications. Finally, because moving-coil technology is fairly refined and is capable of very good sonic characteristics, dynamic microphones also are widely used in recording studios.

Condenser Microphones

Next to the dynamic, the most common microphone type is the condenser. A gold-coated plastic diaphragm (a), is mounted above a conductive back plate (b), which is often made of gold-plated ceramic. The diaphragm and back plate, separated by a small volume of air (c), form an electrical component called a capacitor (or condenser).

A polarizing voltage of between 9 and 48 volts is applied to the diaphragm by an external power supply, charging it with a fixed, static voltage. When the diaphragm vibrates in response to sound, it moves closer to and farther from the back plate. As it does so, the electrical charge that it induces in the back plate is therefore an electrical representation of the diaphragm in motion.  

Condenser microphone elements produce a signal voltage with almost no power. Thus they represent a very high impedance. For these reasons, all condenser microphones incorporate an amplifier, which drives the microphone line. Its function is both to boost the signal level and to isolate the element from the lower impedance of the input to which the microphone is connected. Early condenser microphones employed tube amplifiers and thus were physically quite large. Modern condensers use transistor amplifiers, and can be made very small.

Because the diaphragm of a condenser is not loaded down with the mass of coil, it can respond very quickly and accurately to an incident sound. Condensers therefore generally have excellent sonic characteristics, and are widely used in recording. Being somewhat more sensitive to physical shocks and environmental factors (humidity), however, classic condensers are less often used in sound reinforcement.

Phantom Power

Condenser microphones require a polarizing voltage and power for their built-in amplifiers. Sometimes provision is made to supply this voltage directly through the microphone cable. The procedure is called phantom powering, and the most common phantom supply voltage available in mixing consoles is 48 VDC, although 24 V supplies are widely used. Most phantom powered mics can operate on a wide range of supply voltages from as little as 1.5 or 9 volts up to 50 volts.

In a phantom power system, the polarizing supply voltage is placed on both of the signal lines in a balanced connection, with the same polarity on each line. Dynamic microphones connected in a balanced system with a phantom power input are then protected from damage, theoretically, since the system results in a net zero DC potential across the coil. A dynamic mic connected unbalanced to a phantom power input may be destroyed however!

It is therefore very important to be aware of whether a mixing console input is wired for phantom power. Most such inputs provide a switch to disable the phantom power when it is not needed. Always be sure that this switch is set to off when dynamics, or electret condensers with internal batteries, are connected to the input.

Electret Condenser Microphones

The Electret is a special class of condenser microphone. Electrets incorporate diaphragms made of a unique plastic material that retains a static charge indefinitely. The manufacturer charges the diaphragm when element is made (usually by irradiating it with an electron beam), and no external polarizing voltage is required.

Electrets still require a built-in amplifier, however, and this is normally a transistor unit. The amplifier often is powered by a battery -between 1.5 and 9 volts- housed in the microphone case. (In some designs, the amplifier and battery are housed in a small case that is connected to the element by a cable. Increasingly, phantom power is being used instead of a built-in battery on electret condenser models.) The purpose of the amplifier here is primarily to buffer the high impedance condenser capsule output from the relatively lower impedance of the mic input.

Electrets are increasingly common in both recording and reinforcement. Because they may be made very small, electrets make possible some unique close-miking techniques. The technology is also relatively inexpensive, so electret elements are often used in consumer products. Electrets can be of professional recording and laboratory applications.

Next week I'll be posting about pick ups and polar patterns. Hit the subscribe button to make sure you get notified when it publishes.

©2009 Fred Russell Publishing, All Rights Reserved. This article can not be used without permission from the Author. To Contact the Author email curt@RockHouseMethod.com


2 comments:

Anton said...

Hey nice overview of the different types of mics. Alot of times i see folks wanting to buy a mic, and they just go out and get something without having researched it first. This is a good overview.


Anton

sarge1875 said...

Research is key, thanks for the good words.