The orientation of the diaphragm within the head of the microphone determines if the microphone is addressed from the top or the side. While not an absolute rule, medium diaphragms are typically top-address while large diaphragms are usually side-address. As you might surmise from the previous discussion about design considerations in attaining various polar patterns, top-address mics typically have single pattern (at least without physically changing the capsule) while side- address mics lend themselves to the possibility of back-to- back capsules for switchable patterns. Note that on side- address mics, the side with the logo is usually the primary or cardioid side.
top address
side address
Side address and top address
microphone designs
Polar patterns aside, the practicality of side-address versus top-address designs also has to do with logistics. Top-address microphones can usually fit into tighter spots than can side-address mics (between drums, for example).This is yet another reason why pro engineers keep a variety of mics
in their arsenal.
Microphone Electronics
As weve seen, the microphone capsule is responsible for translating sound waves into electrical signals. The other important part of the microphone is the head amp that conditions the sound coming from the capsule so that it can be transmitted through a length of cable to an external preamp or console.
Part of a head amps job is impedance conversion. (See A Word About Impedance for more information on impedance.) The average line-matching transformer found in dynamic or ribbon microphones has to convert on the order of several thousand ohms down to around 200 ohms (or from half an ohm up to about 200 ohms). The condenser microphone presents a challenge of a different magnitudeconverting a signal in the range of two billion ohms down to 200 ohms.This incredible leap is beyond the scope of most output transformers, requiring the addition of a specialized amplifier.
A Word About Impedance
Impedance essentially describes the resistance in a circuit. Water flowing through a pipe is a good analogy to electrons flowing through a wire. Lets say youve got a pump designed to put 100 pounds of pressure into an eight-inch pipe. If you double the size of the pipe to 16 inches,you get half the pressure.While the pressure is now only 50 lbs,there is no damage to the system. Halving the size of the pipe, on the other hand, yields twice the pressure that the system was designed for. As a result, back-pressure affects the pump, further reducing its efficiency and increasing the potential of an explosion.
In terms of audio electronics, the pipe scenario is analagous to inputting the output from a 100-watt amp into 8-ohm speakers. While using 16-ohm speakers is safe (though it reduces output power), switching to 4-ohm speakers will almost certainly blow up the amp. Thats why guitar amps designed to run into different speaker ratings often have output transformers with 4-, 8- and 16-ohm taps which appropriately condition the output signal.
Guitar pickups and most dynamic mics are considered to be high impedance, meaning that they exhibit an impedance of many thousands of ohms. Low-impedance signals are generally around 200 ohms or less. While the high-impedance signals typically exhibit greater voltage, they can only be run through about 20 feet of cable before they begin to lose high frequencies (or require additonal amplification in order not to). Low-impedance signals can typically be run much further without detriment.
Choosing & Using Microphones | 8 |
