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Microphone Impedance

Discussion in 'Microphones' started by BigG, Mar 22, 2006.

  1. BigG

    BigG Guest

    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...
     
  2. jahtao

    jahtao Guest

    100-200ohms? (600ohm pre 1970?)
     
  3. Kev

    Kev Well-Known Member

    cool question
    :roll:
    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
    :roll:
    shut up Kev
     
  4. "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.

    ~S
     
  5. RemyRAD

    RemyRAD Well-Known Member

    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
     
  6. BigG

    BigG Guest

    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.
     
  7. Kev

    Kev Well-Known Member

    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

    :shock:
    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
     
  8. BigG

    BigG Guest

    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).
     
  9. RemyRAD

    RemyRAD Well-Known Member

    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!

    !hcuO
     
  10. BigG

    BigG Guest

    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...
     
  11. RemyRAD

    RemyRAD Well-Known Member

    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
     
  12. Kev

    Kev Well-Known Member

    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
     
  13. BigG

    BigG Guest

    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...
     
  14. 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
     
  15. Kev

    Kev Well-Known Member

    listen

    loading the mic output trafo can cause the sound to change
    BUT
    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
     
  16. mpd

    mpd Guest

    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.
     
  17. Kev

    Kev Well-Known Member

    just a little addition on mic impedances in general

    http://www.shure.com/pdf/discontinued/566.pdf
    see the bottom of the pdf for the drawing with the pot for adjustable impedance

    http://www.shure.com/datasheets/guides-discontinued.html
    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
     
  18. Boswell

    Boswell Moderator Distinguished Member

    "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.
     
  19. Kev

    Kev Well-Known Member

    yes
    max energy or max power transfer
    and
    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.
     

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