Unsigned Podcast Network
Sunday, October 02, 2005
  Impedence matching for microphones:Is it necessary?
Part of the Audio Podcast Series, Shure Inc. was generous enough to let me use some of there excellant educational materials in order to lighten my load. So read, learn, enjoy!

From Shure Incorporated. http://shure.com/

What is impedance?

Impedance is how much a device resists the flow of an AC signal, such as audio. Impedance is similar to resistance which is how much a device resists the flow of a DC signal. Both impedance and resistance are measured in ohms.

What is the difference between low, medium, and high impedance?

When referring to microphones, low impedance is less than 600 ohms medium impedance is 600 ohms to 10,000 ohms, high impedance is greater than 10,000 ohms.

For audio circuits, is it important to match impedance?

Not any more. In the early part of the 20th century, it was important to match impedance. Bell Laboratories found that to achieve maximum power transfer in long distance telephone circuits, the impedances of different devices should be matched. Impedance matching reduced the number of vacuum tube amplifiers needed which were expensive, bulky, and heat producing. In 1948, Bell Laboratories invented the transistor - a cheap, small, efficient amplifier. The transistor utilizes maximum voltage transfer more efficiently than maximum power transfer. For maximum voltage transfer, the destination device (called the "load") should have an impedance of at least ten times that of the sending device (called the "source"). This is known as BRIDGING. Bridging is the most common circuit configuration when connecting audio devices. With modern audio circuits, matching impedance can actually degrade audio performance.

Are low impedance mixer inputs really low impedance?

Audio mixers often have inputs labeled as "low impedance". Actually, these inputs have impedances between 1000 ohms and 2000 ohms in order to properly bridge the low impedance microphone. A low impedance microphone may always be connected to an input with a higher impedance. However, the microphone may not always be able to provide enough signal strength to properly drive the mixer's audio input. Always compare the microphone's output level (or sensitivity) to the required mixer input level.
What happens when a high impedance microphone is connected to a low impedance input?
When a microphone is connected to a mixer input with lower impedance, there will be some loss of the microphone signal. To determine the amount of signal loss (or "load loss") in dB, use the following equation:

LoadLoss = 20 * log (------------------ )
Rload + RSource

RSource is the actual microphone impedance
RLoad is the actual mixer input impedance
As a rule of thumb, a loss of 6dB or less is acceptable.

© 2005 Shure Incorporated, All Rights Reserved.
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