[SOLVED] Why microphone impedance is so high and how does it relate to sensitivity?

[soham_2342]

Hi

I have a question on Mic impedance.

1. Why the mic impedances are so high. Like from 600 Ω to 10000 Ω. where speaker impedance ranges from 4 to 64 ohms etc...
2. How does the sensitivity of microphones are related to their impedance.

I am using very small mics on the pcb of phone sizes. the mic i am using has the 2.6kΩ of impedance. so is it good.

Regards

 

[yadavvlsi]

Re: Microphone Impedance

Electric Impedence is a measurement of the ac resistance-to-current flow that would be observed looking back into the microphone. Source impedance determines the size of the load that the microphone can comfortably drive. It is important to recognize that the impedance of a microphone should not be matched to the impedance of the device to which it is connected. Doing so will cause a significant loss in signal level. Ideally, a microphone should be connected to a mixer's input whose input impedance is higher than the output impedance of the microphone.
Microphones are usually divided into two basic classes: low impedance 50-1,000 ohms (also called Low-Z) and high impedance 10,000+ ohms (also called High-Z). Most professional microphones designed for long cable runs are low impedance devices. This means their source impedance is below 600 ohms. Properly connected, they are far less susceptible to extraneous noise pickup in the cable and can be used with long cable runs (over 1,000 feet) with very little loss in sound quality. High impedance mics are limited to about twenty feet before degradation.
High impedance microphones require a buffer amplifier or transformer when using low impedance inputs and/or long mic cables.
A microphone's impedance is not necessarily an indicator of quality or performance. it is simply a factor that must be weighed for any given application and the characteristics of the input to which it will be connected.

Speaker Impedance Explained - Ohms

 

[FvM]

Re: Microphone Impedance

The question can't be discussed without clarifying the microphone type.

Classical microphones are moving coil designs similar to a speaker with low impedances of 200 to max. 600 ohm. In the tube amplifier era, they sometimes had built-in transformers to achieve a higher sensitivity, raising the impedance up to several 10k. Professional sound recording and stage entertainment equipment used low impedance microphones with symmetrical conncetion since ever.

Now, cheap microphones are mostly electret type and the impedance is defined by the built-in electronic.

Noise matching of microphone amplifiers suggests an impedance in the kohm range. If microphones have a considerably lower impedance, they are usually connected through transformers for best performance.

 

[soham_2342]

Re: Microphone Impedance

Hi FvM / Yadavvisi

Thanks for the answers.

What I am using is ECM type mic. its a small one (Dia 4.3mm x 1.3 mm) sitting on the pcb and connected to analog codec and then digital. its impedance is 2.6kohms.
What I understood is the mics with lower impedance can be used for long distances (cables) and the once with higher impedance is well suited for less distance. for e.g. the speaker ohmage is calculated by the coil - no. of turns and dia and length of wire etc... --- and it comes out to be around 4 ohms or 8ohms (most commonly). similarly - how mics work is the sensing the change in pressure in air and then relatively change in the voltage. In this, why microphones' impedance is so high - upto 10k (for eg the one i am using has 2.6k). This is what I wanted to understand. I hope I conveyed my question correctly.

Thanks and Regards

 

[FvM]

Re: Microphone Impedance

ECM (electret condenser microphone has neither a magnet nor coil windings and it's impedance it defined by an electronic circuit (basically a FET buffer amplifier). See Electret microphone - Wikipedia, the free encyclopedia for details. These days, many professional microphones are also ECM type, e.g. those sugar cube sized lavalier microphones. They have impedance matching circuits or transformers to adapt standard symmetrical microphone inputs. Besides ECM, classical condenser microphones (CM), either high voltage or RF type, are still used in recording and broadcast studios.

 

[soham_2342]

Re: Microphone Impedance

Alright - Thanks for the explanation.