Connect with us

Microphone for low frequency recording use

Discussion in 'Sensors and Actuators' started by Hopup, May 18, 2018.

Scroll to continue with content
  1. Hopup

    Hopup

    253
    35
    Jul 5, 2015
    I would like to measure very low intensity and also low frequency soundwaves emanating from surfaces. I would need microphone design which is capable of recording these soundwaves. Frequency range would be around 1-20Hz and the sound would be recorded near the surface of the object.

    If you have some idea how to do it, it would be useful.
     
  2. Chemelec

    Chemelec

    291
    47
    Jul 12, 2016
    Even if you have such a Microphone, Most devices won't Record them.
    Also unlikely you could hear those Low frequencies.
    And Most speaker won't reproduce them with any amount of loudness.
     
  3. Hopup

    Hopup

    253
    35
    Jul 5, 2015
    No, I can't hear them, would take much output power to hear them. I'm only interested about the data what I would get by measuring them. I don't want to reproduce them.

    I think it is certainly possible to do with simple parts and design, but I don't know exactly how. Recording them is not problem since I can easily amplify the signal and use it after I just get it.
     
  4. Chemelec

    Chemelec

    291
    47
    Jul 12, 2016
    Hopup likes this.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,361
    2,756
    Jan 21, 2010
    A speaker in a sealed enclosure could be used. However a small vent is required unless you also want to measure barometric pressure!

    In any conventional microphone, if both sides of the diaphragm are open, low frequency signals are lost due to the fact that the place of the signal arriving behind the diaphragm is almost the same as that arriving at the front.

    A similar feature also adds the directionality of a microphone where the size of the diaphragm becomes a significant fraction of the wavelength of the sound (in this case where both high and low pressure regions exist on the same side of the diaphragm). In your case, with low frequencies, you'll lose directionality - but it lets you get away with a larger diaphragm.

    The other thing you need to worry about is that any microphone which produces an output proportional to the rate at which it moves (and a speaker is one of these) will produce a smaller output at lower frequencies for the same excursion distance.
     
    Hopup likes this.
  6. Chemelec

    Chemelec

    291
    47
    Jul 12, 2016
    A Long Throw Voice Coil type Speaker is probably also Better
     
  7. Hopup

    Hopup

    253
    35
    Jul 5, 2015
    Thanks for your ideas. Idea of using speaker is fine, but I don't think it will work with such low power sound waves.

    The measuring device must be quite small, about 3-5cm diameter for accurate measurement. Length is not problem, but it does limit the surface area used for registering the vibrations of air.

    I agree Steve that It needs some kind of enclosure in any case for proper operation.
     
  8. Hopup

    Hopup

    253
    35
    Jul 5, 2015
    Is it possible to build some structure front of the element to amplify 1-20 Hz frequencies?
     
  9. Chemelec

    Chemelec

    291
    47
    Jul 12, 2016
    Possibly Cut some Tubes (Pipes) to Various Lengths, to Resonate at Frequencies between 1 and 20 Hz, with your microphone on one end of the tubes.
     
  10. Hopup

    Hopup

    253
    35
    Jul 5, 2015
    Ok, I figured the design out, I can use Helmholtz resonator. Would this kind of microphone work if I attach it in very light plane of plastic speaker cone?

    http://www.farnell.com/datasheets/2305668.pdf

    It has only 8Hz, but does that mean it won't work at all with lower frequencies?
     
  11. duke37

    duke37

    5,340
    753
    Jan 9, 2011
    Earthquake detectors have to respond to very low amplitude and frequencies. These are done by measuring the distance on a freely suspended weight from the earth.
    Use LVDT or capacitance?
     
    davenn likes this.
  12. Hopup

    Hopup

    253
    35
    Jul 5, 2015
    But that kind of design would not work when the device is not parallel to earth?
     
  13. chad786

    chad786

    1
    0
    Oct 24, 2012
  14. Audioguru

    Audioguru

    2,789
    633
    Sep 24, 2016
    A speaker has a resonant frequency where it is very sensitive which makes it sound "boomy" as a microphone.
    An electret mic does not show a resonant frequency and probably produces frequencies very low (lower than audio).
     

    Attached Files:

  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,361
    2,756
    Jan 21, 2010
    Before you say that, you should probably read this.

    When the microphone is exposed to air to receive a sound wave, a small amount of acoustic energy reaches the diaphragm whose threshold sound pressure level is about 40 dB. Most electret condenser microphones cannot faithfully capture the signal when the frequency of the acoustic wave is lower than 20 Hz. This is because when the corresponding sound pressure level is lower than 40 dB, the acoustic wave cannot provide enough force to actuate the microphone’s diaphragm.

    They also compare electret microphones against small speakers (with favourable results).

    @Hopup, Maybe an electret mic with a horn attached to it could be an option. Do you know how loud the signals are that you're trying to pick up?

    The attached article also suggests the (in retrospect) obvious answer of using an accelerometer which is attached to some surface.
     
    Hopup likes this.
  16. Audioguru

    Audioguru

    2,789
    633
    Sep 24, 2016
    A horn to concentrate very low frequencies must be enormous.
     
  17. davenn

    davenn Moderator

    13,555
    1,853
    Sep 5, 2009

    Yeah, I was going to eventually bring this up
    A geophone resonant around 10 Hz will easily handle 1- 20 Hz

    The geophones I use in my seismic recording system are centred on 4.5 Hz but easily record 0.1 - 25 Hz

    from my www page …..
    http://www.sydneystormcity.com/g_phones.htm
    This is a view of the Geo-Space, 4.5Hz, 4000 Ohm coil, geophone seismometers that I use for my E/W, N/S and VERT. Short Period system

    [​IMG]

    around 1.5 inch diameter and 2 inches high


    Dave
     
    darren adcock and bushtech like this.
  18. Hopup

    Hopup

    253
    35
    Jul 5, 2015
    I will do prototype using Helmholtz resonator chamber which is located in front of the some sort of cone, preferably made with super light material if that would help picking up the sound waves. This way I can amplify the sound waves I want in very small enclosure, I think.

    Dave's geophone is very interesting, but I can't use it in my device because of its size. I need to operate the device in many surfaces which are not parallel to earth.

    Steve's idea of using accelerometer is probably good one, I might use accelerometer or some sort of piezo mic which is attached to the cone.
     
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

-