Connect with us

OTish Distortion in Conderser microphones

Discussion in 'Electronic Design' started by martin griffith, Mar 4, 2006.

Scroll to continue with content
  1. Guest

    I imagine that the effect is real, but there is way of using negative
    feedback to get around it.

    You've got to be able to drive the microphone sensing membrane as an
    electrostatic speaker, and then you can use an AC-excited capacitance
    bridge to measure the capacitance of the microphone, and compare the
    instantaneous capacitance with the long term average.

    Any deviation in measured capacitance then generates an electrostatic
    feedback signal to drive the capacitance back to its nominal value.

    J.J.Opstelten and N. Warmholtz describe a pressure-seinsor based on
    this pronciple in App. Sci. Res Hague, volume B4 from page 329 in 1955,
    and again with J.J. Zaalberg van Zelst in volume B6 from page 129 in
    1956.

    I'm quoting the references from my Ph.D, thesis - which dates back some
    25 years before we moved to the Netherlands.

    The problem with using this technique in a pressure gauge is that the
    electrostatic field required to counteract more than about 0,2% of
    atmospheric pressure is enough to induce conduction across the gap.
    This shouldn't be problem with a microphone.

    Obviously, the electrostatic restoring force/voltage is a linear
    function of the presure being counteracted. You've got to use a bridge
    excitation frequency a good bit higher than the higest frequency you'd
    want to follow - well over 100kHz, ata guess - and you'd want to design
    the feedback loop on the basis that you were sampling the capacitance
    of the microphone twice per bridge-excitation cycle rather than
    continuously.

    As a project it would offer a microphone that would be expensive enough
    to fascinate the audiophool exploitation industry - pity I can't patent
    it.
     
  2. Genome

    Genome Guest

    This am rubb coz musurment sytem do not account fur brake up ov bitz.

    DNA
     
  3. Glen Walpert

    Glen Walpert Guest

    The author should have read up on the subject before publishing his
    simplistic and unrealistic (10% capacitance variation!) analysis of
    1/x error in condenser microphones for two load cases (not two
    distortion mechanisms as he claims). A much better analysis can be
    found in Acoustical Engineering by Harry F. Olson, copyright 1957, for
    example. Other distortion mechanisms related to diaphragm and air gap
    dynamics limit diaphragm displacement with low distortion to the
    extent that 1/x error is very small. Never nonexistent, however, and
    I think his analysis of the effect of load capacitance on 1/x error is
    correct even if it is so simplistic as to be virtually useless. Other
    than that, I liked it :).
     
  4. Ken Smith

    Ken Smith Guest

    Problems:

    (1) assuming 100M of input resistance of the amplifier is unreasonable. A
    good design will be at least 1G. With bootstrapping, it can be many G.

    (2) 30Hz is not below the audio range and 60Hz certainly isn't. The
    distortion will largist at the 2nd harmonic.

    (3) That is a huge signal you are assuming. What are you trying to
    record?

    (4) 1pF is not an extreme case. A fairly simple input stage using 2SK170s
    will get under 1pF of effective capacitance, if the drop on the source
    resistor can be high enough. Values down to 0.25pF can be done with real
    components.

    (5) You need to review the math by which you come up with the distortion.
    The capacitance varies as (1/X) but the voltage goes as (1/C) so the
    voltage varies as X for the unloaded case.

    (6) You left out all the interesting effects.

    (7) Your yellow background is annoying.
     
  5. Phil Allison

    Phil Allison Guest

    "Glen Walpert"
    "martin griffith"

    Found this, maybe of interest to audio dudes here
    http://lavryengineering.com/forum_images/Condenser.pdf


    ** Given that a condenser mic capsule is typically polarised to circa 60
    volts DC, a +/- 5% variation in capsule C produces +/- 3 volts peak
    variation. This equates to a just over 2 volts rms of signal available at
    the gate of the FET used to buffer the capsule to the outside world.

    Also, typical studio condenser mics have a rated output voltage of circa 10
    mV at 94db SPL, = 1 volt at 134 dB SPL = 2 volts at 140dB SPL. Specified
    ( pre-amp) overload levels of 150 dB SPL or more are commonplace.

    The pre-amp circuitry in the body of such mics is of unity or more often
    *less* than unity voltage gain - but huge power gain of course. So, a +/-
    5% variation in capsule C is a very reasonable estimate of what happens in
    practice with high SPL sound sources.

    However, the polarising voltage is fed to the capsule via a resistor of 1
    Gohm (sometimes more ) - placing the 3dB down point at about 3 Hz for a
    50 pF capsule. Hence, the second harmonic generation alluded to in the URL
    can only occur at the very lowest audio frequencies and become at all
    significant at extreme SPLs that are most unlikely to exist at those
    frequencies from a musical source.

    Poking the mic down the port tubes in a 15inch sub woofer going full tilt
    not withstanding !



    ........ Phil
     
  6. Robert Baer

    Robert Baer Guest

  7. Guest

    Condensor microphones are small. Admittedly, the instability of the
    membrane is a big problem with electrostatic speakers, and part of my
    (unsuccessful) job interview at Quad was a discussion of Quad's magic
    "conductive"coating on the memebrane in its origianl electrostatic
    speakers that could be relied on the give the 100M per square (IIRR -
    this is twenty years ago) needed to prevent the membrane getting
    small-pox.

    I'd be less worried about a microphone membrane, The Dutch pressure
    gauges used thin metal membranes, which are a lot tougher than speaker
    membranes, if a lot heavier. The weight might conceivably limit the
    high frequency response - I haven't crunched the numbers - but a decent
    amount of negative feedback would counteract this too,

    Bear in mind the intended market - the system doesn't have to work
    well, but it must look impressive. A big glowing valve or two
    compensates for a lot of technical short-comings.and valves are
    reasonably neat devices for driving electrostatic speakers.
     
  8. Pieter

    Pieter Guest

    The linearity changes with capacitance change. The capacitance does
    not change when the voltage does not change. So to prevent voltage
    changes!

    To do this, the microphone must be short-circuited (can also be into a
    DC offset). The way to do this is feed the signal into the - input of
    an opamp that has capacitive feedback. This way this circuit does not
    function as voltage amplifier, but as CHARGE amplifier. The charge the
    sound delivers is amplified.

    I designed an accelerationmeter based on a crystal device (capacitive)
    that had a very large frequency range this way.

    Another advantage is that the signal is very low-impedant, this means
    low disturbances, low noise. And it is not sensitive to the length of
    the cable (the cable only sees a short-circuit), and it isn't
    sensitive to movement of the cable.

    Feel free to ask any questions.

    Pieter
    email: without the NOSPAM
     
  9. Robert Baer

    Robert Baer Guest

    Tanks for your eloquent expansion of my suggestion.
     
  10. Phil Allison

    Phil Allison Guest

    "Robert Baer"

    ** Yep - he made a real omelette out of one of your turds.






    ......... Phil
     
  11. Ken Smith

    Ken Smith Guest

    This statement is simply incorrect. It is also incorrect in a complex way
    too but we'll ignore this for now.

    Actually, it is more likely to act as a slightly charge sensitive RF
    oscillator with a lot of noise.

    No, it means very high noise. The input noise voltage of the op-amp
    divided by the impedance gives a noise current. Also the lower a resistor
    is, the more its noise current. When it comes to noise, you can't win,
    you can't break even and you can't get out of the game.
    Try tapping on a coax with a little bias voltage on it connected to such a
    circuit. You will find that the circuit is sensitive to moving the cable.
    I've had to shock mount cables.
    If a train is leaving the station at 11 miles per hour ..........
     
  12. Pieter

    Pieter Guest

    Not when you do a proper design. The opamp is feedback capacitive,
    making it very stable. A small series resistor at the input may be
    wise.

    Low sensitivity to outside disturbances.
    There will always be noise.
    The other circuits are sensitive. The one is decribe here isnt. When
    then is a short-circuit, there is no voltage. Where there is no
    voltage, that voltage can't change either. But it is best not to use
    phantom feeding, but a separate power supply cable. The phantom's
    voltage does interfere.
    Two hours and 15 minutes.

    Pieter
     
  13. Ken Smith

    Ken Smith Guest

    Putting a feedback capacitor on a high performance op-amp does not make it
    "very stable". You have to use a "unity gain stable" one in this sort of
    application. Without the small resistor you suggest, the op-amp is fairly
    likely to oscillate even if it is a "unity gain stable" one if the input
    cable is very long. The cable and capacitor look like a tuned circuit.


    No, this is not the case. The circuit you suggest is this:

    Noise
    !
    ---Cnoise
    ---
    ! A -------------
    Signal -[Zgen]---+-------! Z Amplifier !---- GND
    -------------


    You can't improve the signal to noise at "A" by making Zamplifier low.
    How ever much you lower the amplifiers input impedance, you must also
    raise its voltage gain by too to maintain the same signal output.

    Just for fun, lets assume that Zgen=Cnoise and the Signal(RMS) is equal
    to the noise(RMS).

    High Z amplifier case: SNR = 1:1

    Low Z amplifier case: SNR = 1:1

    When there's no impedance, the current is infnite and thus the noise is
    infinite. It was exactly the sort of circuit you are suggesting that
    required the shock mounting of cables.
    Try tapping of a length of COAX even with "no voltage" on it. You will
    still see spikes when you strike it. Just hook a BNC-BNC cable onto your
    scope's input and wack it against the work bench.
     
  14. Terry Given

    Terry Given Guest

    that it does. unless its terminated.

    I didnt do a very good job, but I tried to isolate both connector ends,
    and only whack the middle. simply waving an end about picked up a lot if
    noise. my crude attempt at isolation greatly reduce the resultant spike.
    its extremely sensitive at the connector.

    I wonder if its because the open end is moving thru an otherwise
    stationary E field. so I wrap it in tin foil, and it makes no difference....

    so whats going on here? is it simply VdC/dt, with dC/dt due to the
    pressure wave propagating thru the cable?

    Cheers
    Terry
     
  15. Ken Smith

    Ken Smith Guest

    If you hit it hard enough, you get triboelectric effects. This is the
    voltage generated when you mechanically break most materials. At lower
    hits, I believe, it is charges trapped in the plastic materials that is
    doing it. At the terminated end, the parts can move with respect to each
    other more easily.
     
  16. Terry Given

    Terry Given Guest

    makes sense. same reason semiconductor physics tests all assume an
    un-stressed material.

    Cheers
    Terry
     
  17. Joseph2k

    Joseph2k Guest

    Not just triboelectric effects, piezoelectric effects as well.
    Picocoulomb/g accelerometer sensors require very special cables which
    require very special connectors in turn. I have had to use these and
    special microwave cables (vibration resistant) when testing waveguide relay
    switches under vibration. I think i still have a made-up cable somewhere.
    The tooling for terminating such an accelerometer cable is probably in
    excess of US$10,000 by now, 20 years ago the cable was US$20/ft.
     
  18. Terry Given

    Terry Given Guest

    as a general rule of thumb, everything is far more complex than at first
    suspected....

    Thanks for some informative posts guys :)

    Cheers
    Terry
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-