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Microphone advise for monitoring heart sounds in animals under anesthesia

Discussion in 'General Electronics Discussion' started by oliverfunnell, Jun 12, 2018.

  1. oliverfunnell

    oliverfunnell

    4
    0
    Jun 12, 2018
    Hi Folks

    I wonder if I can get some advice on microphone selection. I am currently making a combination Esophageal ECG and stethoscope for animals (I work in the animal healthcare field)

    Essentially a traditional Esophageal stethoscope is a plastic tube with multiple holes in one end (protected from blocking up by a thin plastic cover) that is passed into an animals esophagus (the food tube) while under anaesthesia (the esophagus passes directly over the heart and you can listen to the heart through the wall of the esophagus) . A regular stethoscope is attached to this simple sealed plastic tube and you can hear the animals heart beat through the stethoscope ear pieces.
    Oesophageal stethoscope.jpg

    We have an amplifier which you can connect to this (so someone does not have to remain listening for the duration of the procedure) but the sound despite being amplified is still quiet, the microphone for the amplifier is place at the exit of the tube and some distance from the heart. I think a lot of the sound is lost in the length of the tube
    Eos Steth amp.jpg

    I am looking to make some custom esophageal stethoscopes that have a microphones placed further down the tube, hopefully to pick up a louder sound for amplification. I am hoping to get some advice on the type of microphone that would be best to pick up these low frequency sounds (somewhere between 50-200Hz ) that are travelling in a plastic tube . It would be a sealed unit , so the microphone would not need to be waterproof , the tubes would be somewhere between 4mm internal diameter and up to 6mm or even 8mm internal diameter (larger units for larger animals) . I hope to combine this with external electrodes that will enable as ECG to be read from the same unit. ecg-probe.jpg

    I wonder if I get some advice as to the best microphone for such a unit at these frequencies and even the availability. As a youngster I dabbled in electronics but my knowledge of microphones is a little lacking.

    Thanking anyone for their advice in advance

    Best Regards

    Oliver
     
    Last edited: Jun 12, 2018
  2. Externet

    Externet

    665
    135
    Aug 24, 2009
    Welcome.
    An electret microphone tight fitted at the end of the stethoscope tube will be extremely sensitive to sound and anything touching/rubbing the tube. An audio preamplifier for it is commonly available.

    Is your current microphone tight fitted inside the tube end ? The length of the tube should not produce major loss, I think.
    Is the microphone healthy ? An electret diaphragm could easily rupture with the tube pressure levels when mishandled, I think.
    Perhaps a dynamic earphone will be sturdier instead of an electret type microphone.

    I believe Audioguru designed one; he should show up any moment.
     
  3. AnalogKid

    AnalogKid

    2,229
    628
    Jun 10, 2015
    50 Hz seems way too high. My dog's heart rate is around 100-110 once she calms down at the Vet's. That's under 2 Hz.

    Separate from that, what is the inside diameter of the tube? You probably can improve coupling of the acoustic energy in the tube into a microphone if a small microphone capsule is stuck into the end of the tube like a cork.

    ak
     
  4. FuZZ1L0G1C

    FuZZ1L0G1C

    279
    93
    Mar 25, 2014
    He probably means although the fundamental (beat) is at heartbeat rate, the acoustic resonance is around 50-200Hz (Lo-fr Bass drum).
     
  5. oliverfunnell

    oliverfunnell

    4
    0
    Jun 12, 2018
    Thanks for the replies , yes the freq refers to the sound waves from the heart beat as they pass down the plastic tube being 50-200hz , ie low freq , different from the actual rate of the heart beating which depending on the size (and emotional state) of the dog can be 40-180 beats per minute

    Cheers

    Oliver
     
  6. Audioguru

    Audioguru

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    520
    Sep 24, 2016
    Use an electret mic. Most have a very flat frequency response to less than 20Hz.
     
  7. oliverfunnell

    oliverfunnell

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    0
    Jun 12, 2018
    Thanks very much , I have checked these mic's out on ebay and they seem very reasonably priced and very small which would be ideal for this application.

    Much Appreciated

    O.
     
  8. Audioguru

    Audioguru

    2,382
    520
    Sep 24, 2016
    Don't use cheap Chinese ebay products for medical applications. ebay sells manufacturer's rejects and fakes. Also they might be contaminated with poisons and/or diseases.
     
  9. oliverfunnell

    oliverfunnell

    4
    0
    Jun 12, 2018
    Thanks for the heads up.

    O.
     
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