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Microphones vs. ear sensitivity

Discussion in 'Electronic Design' started by Kryten, May 31, 2005.

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  1. Kryten

    Kryten Guest

    Hi all.

    Just been playing with some cheap (ECM-66B) microphone inserts and found
    they have a pretty poor performance compared with the human ear. I can hear
    some very quiet sounds in my room: the ticking of my clock and distant
    barking dog. This sensor can pick up sounds at this level but when the
    signal is amplified enough to hear them, there is a lot of noise with it.

    The S/N ratio is quoted at 40 dB, and sensitivity at 60 dB/microbar.

    It seems okay where it is next to or nearby a person's mouth, like a hand
    held microphone or a telephone. But if you stuck a pair on a dummy head and
    listened through an amplifier, you would not get a good sense of being in
    the room.

    So, do microphones get much better than this?

    Or is the ear such a marvellous organ that it is yet to be beaten?
  2. Pooh Bear

    Pooh Bear Guest

    In short, it's rubbish.

    Professional condensor mics are often specified in terms of their dBA
    equivalent noise. I haven't heard of one lower than 12 dBA.

    It is.

  3. James Meyer

    James Meyer Guest

    Of course, the ear is supported by quite a bit of additional signal
    processing. If you looked at the nerve signals between the ear and the brain, I
    suspect that any useable information is thououghly buried in the "noise".

  4. Pooh Bear

    Pooh Bear Guest

    The way the ear hears is not exactly understood. The fact that it can apparently
    'outperform' test instrumentation in critical istening tests in certain tests but
    not others perhaps adds to that puzzle.

    Note that the ear hears partly by means of many cilia that are quite high Q 'tuned
    circuits'. I can't imagine a simple model of how humans or other animals hear. For
    example the ear can extract a signal that's 'buried in the noise' if it's of narrow
    enough bandwidth.

  5. Yes. Microphones designed for close in work (telephones, handheld voice
    mics, etc.) are not very good for picking up weaker or more distant
    sounds. They are made intentionally insensitive so as not to be
    overdriven by close in sources.
    The ear is pretty good, but being attached to the human body, it comes
    with its own noise sources. In a very quiet environment, my breathing
    and heartbeat drown out sounds that could be picked up by a sensitive
  6. Ban

    Ban Guest

    I do not know if it is true but this mike from Bruel and Kjaer has extremly
    low noise:
    The model 4179 is claimed to have -5.5dB(A) thermal noise only, that would
    be more sensitive than the human ear.
    Neumann has one with +7dB(A)
    and another one: BCM104
    They also have a mike with built-in DSP with the same sensitivity.
  7. We can't beat the brain that processes the ear information, but as far
    as the ear itself goes, just what can it detect that test equipment

    Care to support this with some references?
    You must be using duff test equipment. I dont see that there is anything
    the ear/brain can hear that decent equipment can't measure. We just
    don't know what the measurements mean.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  8. Kryten

    Kryten Guest

    I didn't expect much for my £2. :)

    The dynamic range is certainly poor, but adequate for some applications.

    It is sensitive enough it can pick up feint sounds, only buried in noise.

    I was surprised by the amount of difference a simple plastic funnel made.
    Pointed out the window at a bunch of trees, the birdsong from them was much
    Might be fun to play with a parabolic microphone :)

    How much does a reasonable quality transducer cost?
    I've seen mikes for several 10s of £, but much of that cost must go to make
    the robust housing for the transducer.
    I suspect it has noise cancelling mechanisms.
    After all, it is buried in a mass of blood vessels and right next to

    As an aside, what is the noise heard when a sea shell is held to the ear?
    Usually explained as air currents, I find that hard to believe.
    I guess it is sounds coming from the head/ear itself, resonating and
    reflected back into the ear.
  9. René

    René Guest

    FWIW, I once built a remote microphone for my handycam, incorporating
    a junkbox electret, a BC547 (as "frontend"), a 4558 (one half used) -
    to create a AGC mike amp with line level out (9V battery fed). The
    thing could "hear" things that I could ony detect with my "bare" (then
    still quite good) ears after noticing it on the headset. It would even
    feedback over thesound leaked out of the earphones.

    In other words, I guess that with sufficient power line filtering and
    an adequate gain distribution - it may not be that bad.
  10. Kryten

    Kryten Guest

    Yep, I did that (BC550 replacing BC547) and used a CA3140 op-amp.

    I have a VOGAD (voice-operated gain adjustment) chip somewhere I will try
    Yep, noticed that too.

    I think the electronics are fine, so the electret is the noisy bit.

    Recently heard of the "microflown", which is a silicon micromachine that
    senses the flow of air instead of pressure. Since it is very small
    (<match-head) and low-mass I suspect it may be a lot more sensitive. However
    I can't see anywhere selling them off the shelf. The manufacturers are
    selling for novel applications.
  11. Zak

    Zak Guest

    Try it with a satellite dish... the nice thing is that your microphone
    will point up and not cacth much ground noise while the reflector
    determines to which piece of ground you listen.

  12. Mark

    Mark Guest

    most electret elements have an FET preamp built in so this bit of
    electronics may be adding some noise.

  13. Kryten

    Kryten Guest

    Yes, I'm just surprised that one FET can add so much noise.

    What technology do decent quality microphones use?
  14. Pooh Bear

    Pooh Bear Guest

    Cheap fets I expect !

    Professional conndenser mics use larger capsules and polarising voltages
    that produce more output for the same SPL. Hence the s/n is better. And
    they use quiet fets ( or toobs even in very old ones ! ).

  15. Pooh Bear

    Pooh Bear Guest

    It's a larger than average capsule ( see diameter ) which helps but even so that's an
    astonishingly low noise figure. Looking at the data it has a polarising voltage of 200V.
    That helps too.

    Not too surpised at Neumann. Good products. I was thinking of more affordable mics !

    I once owned an Neumann U87 - a classic mic. Last time I checked the RSP was £1600.

  16. Kryten

    Kryten Guest

    Okay, that makes sense.

    If the bias voltage is 20 times my 9V battery,
    signal should be 20 times greater.

    And that will be roughly multiplied by the area increase.

    Hmm... I'm tempted to see what kind of output I'd get from two sheets of
    tinfoil (say 30 x 30 cm)held under tension very close to each other? Not the
    most practical stage mike, but it does provide a large sensing area. I
    imagine it would be pretty directional too.
  17. Pooh Bear

    Pooh Bear Guest

    Tried something like that using metallised melinex once as an experiment to make
    an electrostatic speaker.

    I can see why it's best left to the experts !

  18. Kryten

    Kryten Guest

    Ah, but that is trying to put sound out, rather than sense sound in.

    AFAICT the change in C needs to be as big a fraction of the total C for best
    Thus the sheets need to be close as possible.
    I think that is the hardest bit.
  19. Kryten

    Kryten Guest

    Okay, it is pretty small so not got a great sensing area.
    And for £3 I did not expect much.

    I went out and bought a dynamic mic insert for £5, and this is labelled as
    having 76 dB sensitivity. It is a lot bigger and heavier. (type DM13)

    I made up this preamplifier
    which is all-discrete so should have low noise.

    However I didn't find it was much improvement.
    There is noticeable hiss and buzz.

    Hiss seems to be coming from the first transistor (BC109, common base) and
    the buzz from the mic insert, because the buzz disappears when the signal
    input is shorted to ground. The hiss and buzz disappears when there is no
    signal to the other transistors.

    Circuit constructed on a bit of veroboard - not ideal but fairly neat and

    So, is this mic insert also crap, and if so how much would I expect to pay
    for a good one?

    Also, is this BC109 transistor just too noisy?

    Or am I going to have to make a much more complex pre-amp like this one:
  20. René

    René Guest

    Cleaning the power line (1K - 220 uF, that sort of thing), may help
    tremendously. Batteries are noisy! Power supplies more so.

    Gain distribution, not too much gain in any stage. e.g. ~20x in the
    transistor, same in the opamp. Never let the frontend transistor
    amplify flat out (grounded emitter)

    As I had the pleasure of commenting before - a junkbox electret /
    transistor / low end opamp ckt. easily matched my hearing treshold.
    (when I built it -long ago- my hearing was not all that bad actually)
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