Detecting very low level low freq?

Discussion in 'The Projects Forum' started by DC_Kid, Jan 15, 2016.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    so, in dead of night when outside has low ambient noise, from inside i can hear a low level (faint) hum, in the range of 250Hz. any ideas to build a simple directional detector that 1) can detect the frequency, and 2) allow me to get an idea as to the direction the sound is emanating from. i suspect the sound is coming from a nearby pool pump, but would be neat to build a detector that can help solve the mystery (and stop annoying me).

    since i am working another project that may use piezo element, how about mounting a bare piezo element to a 1/2" thick vertical AL plate or wood with heavy base, couple that to a high gain opamp? just not sure if 250Hz would couple well to a piezo element, might get ran over by noise in the gain. i guess one side of piezo should be on heavy mass while the other side couple to a "fork" that has resonance down low near 250Hz?

    your thoughts?
     
    Last edited: Jan 15, 2016
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Use two microphones back-to-back (one on each side of a 3/4" piece of plywood (to block/deaden sound). Then connect to differential amplifier. You will need to zero the signals with a pot on one of the microphones to zero the delta in a silent room. Then rotate and find the biggest response to the + input on your op amp or instrumentation amp.

    Have fun.
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Get a stethoscope and you will find your noise
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    Remember the old conical megaphones? Make one out of cardboard, press it against your ear and go hunting.
     
  5. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    the diameter around 1.5ft ?
     
  6. Lestraveled

    Well-Known Member

    May 19, 2014
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    Sounds Good
     
  7. #12

    Expert

    Nov 30, 2010
    16,247
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    Styrofoam works, too.

    ps, 250 Hz isn't consider, "Low" in some frames of reference, like power line frequency or musical instruments. 250 Hz is near the fifth string on a guitar.
     
    Last edited: Jan 15, 2016
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I have a piece of plywood in the garage - if I would have had Styrofoam, I would have suggested that.
     
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  9. #12

    Expert

    Nov 30, 2010
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    I have a piece of styrofoam in the laundry room...left over packing from a garbage disposer. That's why I thought of it. Besides that, styrofoam heads are used for testing headphones, so styrofoam probably works in the audio range.
     
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