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Microphone Impedance

Can someone clarify what 'nominal impedance' is when given in mic specifications. I believe that with loudspeakers the nominal impedance is the lowest impedance above the resonant frequency of the speaker itself. Is this true of mics also or am I shooting in the wrong direction...



BigG Sun, 03/26/2006 - 21:06

I have come across evidence that 'nominal' impedance is usually measured at 1k. Although I would love to see an impedance plot for a microphone to see how much the impedance does vary with frequency. If someone knows of a link to one I would be very gratefull. Nominal as a word generally means minimal or small, so I don't know if 1k is a frequency where the impedance is lower than at other frequencies but that seems to be the standard test frequency.

Some manufacturers just quote 'Impedance' so who knows what frequency they are quoting their specification at (ie. Shure).

RemyRAD Sun, 03/26/2006 - 21:38

I do understand where you are going with this. You would truly like to see a time display plot of frequency response versus impedance. You are absolutely correct in your summation of impedance varying by frequency. Speakers do the same thing! Most of these specifications are all based on average data with reference to something. In most cases when dealing with audio from 20 -20,000 hertz, 1kHz pretty much lies within the middle response of human hearing and for ease and consistency, most specifications are centered there. Some manufacturers actually provide this data for their products and some don't. That's why we have testing laboratories. Have you considered contacting some of the professional audio trade publications like Pro Audio Review? They may be able to point you to the data you crave? I'm sure other anal audiophile oriented retailers and/or publications may also have hard data on how awful all of our recording equipment is!

Just don't tell them that I don't know the difference between Total Harmonic Distortion and Total Migraine Distortion! Ouch!


Kev Wed, 03/29/2006 - 13:43

BigG wrote: I understand that 1-4K is around the frequency of human speech, but mathematically the mid-point of our hearing would be around 8-10K somewhere, would it not?

don't think of it that way
humans and their senses can be very logarithmic and not linear

using your 1 to 4k as near the middle for speech

31-62 = -5
62-125 = -4
125-250 = -3
250-500 = -2
500-1K = -1
1-2k - octave middle
2-4k = octave +1
4-8k = +2
8-16k = +3
16-32k = +4

perhaps 500 to 1K is nearer the middle

BigG Wed, 03/29/2006 - 15:41

I stand corrected. If our hearing is logarithmic then I can understand that our ears tend to group a lot of the higher frequencies rather than hear them individually. I like that table Kev.

Makes sense...

kowloonbusno2 Mon, 04/03/2006 - 13:19

This question may not related to this topic but I also wanna ask something about setting mic pre-amp impendence. There is an option of tuning impendence (150-13k) on my mic pre-amp, all i know is the higher u turn the smaller noise you can get. But what else can it do and how it works with differ mics e.g. condenser or dynamic? thanx

Kev Mon, 04/03/2006 - 14:07


loading the mic output trafo can cause the sound to change
it could be very subtle

low end could get rounder but weaker perhaps ... even saturated as the current increases

the top end could be damped at the lower impedance and may do the reverse at higher impedance and cause an overshoot (ringing) at high frequencies

you will also hear level changes as the loading effects signal transfer ... same goes for the noise floor and signal to noise ratio ... as you have already noticed

very general statements there ... you need to listen

mpd Mon, 04/03/2006 - 17:00

Kev wrote:
Measuring a Mic's impeadance probably would be done by comparing it to a know impedance and comparing the voltage drop/ potential difference between the know and the unknow
delta stuff

In the RF world, it is done with a box called a network analyzer. I flipped through my Agilent and Rohde & Schwarz catalogs, and a bunch of the models do claim bandwidth down to 10 Hz or so. Some will also do spectrum analysis over the same band. You can download the PDFs of the catalogs, which have good descriptions of how they work.

Kev Wed, 03/22/2006 - 14:00

BigG wrote: ... with loudspeakers the nominal impedance is the lowest impedance above the resonant frequency of the speaker itself ...

cool question
and I don't know the answer
I have a bunch of old AES papers on mics and if I get time I'll try to find something

gut reation tells me it is more about max energy transfer
things are different for the mic in that it is a source impedance ... the speaker is a destination impedance

A dynamic mic may have similarities with a speaker but I feel that the others like Condenser/plate, Ribbons and PZM(back electret) may all look a lot closer to a flat line rather than the resonant peak of a speaker

I'm sure there are still bumps and I bet they coiside with both machanical and acoustic resonances in the mic

Measuring a Mic's impeadance probably would be done by comparing it to a know impedance and comparing the voltage drop/ potential difference between the know and the unknow
delta stuff

The Manufacturer would be looking for max energy transfer into the mic-pre to give max signal to noise
what is the imput imp of the mic-pre

chicken and egg stuff here

looking way back to old Radio Engineering Handbooks and you can find the original reasoning behind this sort of stuff

what was I saying
shut up Kev

Kev Tue, 04/18/2006 - 14:25

just a little addition on mic impedances in general
see the bottom of the pdf for the drawing with the pot for adjustable impedance
lots of OLD mics and many of them did had the Rheostat fitted for the imp adjustment
(rheostat = pot - potentiometer)

this same sort of thing goes for line level stuff as well
read the old Crown Amp manuals and the Neve manuals for correct interfacing of the equipment

... even the OLD Urei manuals also had a few pages devoted to correct interfacing

TheRealShotgun Wed, 03/22/2006 - 23:03

"Nominal" means "of, being, or relating to a designated or theoretical size that may vary from the actual". A/C Impedance (as opposed to D/C resistance) varies with frequency. So the nominal impedance of a device is the theoretical impedance. You could even sorta say "average" or "working" impedance. Remember that a dynamic microphone is just a speaker working in reverse.


RemyRAD Thu, 03/23/2006 - 00:35

A nominal microphone impedance is 150 ohms. Sometimes adjustable depending on the manufacturer from 50 to 600 ohms average.

Average girl
Ms. Remy Ann David

BigG Thu, 03/23/2006 - 04:02

Geez I wish I had some of those old AES papers floating around. It's ironic that you don't get all the information on spec sheets. For example if a 'nominal' impedance is given what frequency are they basing that on. Or as Kev has said, what load impedance might they be using to derive such a figure.

I'll keep digging... thanks for your help.

Profile picture for user Boswell

Boswell Wed, 04/19/2006 - 15:01

"Nominal impedance" of a microphone is the manufacturer's design figure for maximum power transfer. It should always be qualified by a frequency at which it is measured.

Maximum power transfer will happen when the microphone is driving a resistive load of the same nominal value, even though the voltage will be half the open circuit value. This condition also theoretically gives maximum signal-to-noise ratio, but very few microphone pre-amps are designed to exploit this configuration.

However, manufacturing tolerances mean that individual microphones made to the same design could well vary by (say) +/-20% from their nominal impedance figure at any given frequency.

Kev Wed, 04/19/2006 - 18:52

Kev wrote: ... gut reaction tells me it is more about max energy transfer ...

max energy or max power transfer
this can lead to the theory that it gives maximum signal-to-noise ratio

this is the way things were designed in the past ands can still hold true today

but newer components and methods can give just as good signal to noise and can make for a more robust or forgiving circuit when the impedances are higher
perhaps even less operational variations between mics and models of mics

it is also parallels in the drift towards bridging inputs at line level

In this world of music producing
it is cool to have more choices and these choices are not always about what is technically correct

nice to know a few rules and especially the old rules ... so you can better break them and have some fun along the way.

BigG Tue, 03/28/2006 - 21:22

Hi Remy, thanks for your response. I will be contacting manufacturers in search of such documents at some stage. Where did you get the information that 1K is about the middle range of human hearing. I understand that 1-4K is around the frequency of human speech, but mathematically the mid-point of our hearing would be around 8-10K somewhere, would it not?

Just curious...

Kev Thu, 03/23/2006 - 15:50

to give a complete picture an impedance plot for a given load would be needed

the internal elecronics can provide the impedance conversion to interface with a standard mic-pre

these standards probably developed over time and have roots in the deep past

the nominal imp is probably on the lower side of the average for the usable frequency response ... say 150 to 15k

I can find a plot of a typical mic we all know
so post I can post here
mostly we only see Freq plots

what the hell was Kev trying to say ... ???
I think it was supposed to be
I can't find a plot of a typical mic we all know
so I can't post it here

any of the PC softwares like AudioTester and some ingenuity should be able to get a result

RemyRAD Tue, 03/28/2006 - 21:41

BigG, I think you'll find that much of this information was originally based upon early telephone technology. United States telephone systems were originally rated from 300 hertz to 3000 hertz and 1000 hertz was pretty much determined to be the middle of that frequency spectrum.

And no, what would make you think that a cymbal hit between eight and 10kHz is the midpoint for human hearing? Along with the fact that 1kHz was at many times the turnover frequency and midpoint for many preemphasis and deemphasis recording and playback frequency response curves. Many high and low frequency equalizer's also have no effect frequently near 1kHz.

Unfortunately after reading your response and secondary question I find that my head really Hertz!
Ms. Remy Ann David