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I tried to make a quick casual recording of rain and thunder outside the window today by setting my Zoom H4n on my desk (internal mics, 96 kHz, 24-bit stereo, lithium batteries). When I loaded the WAV file to edit it, I noticed a rhythmic noise below 50 Hz and -50 dB. I removed everything above 50 Hz, amplified by 20 db or so, and shifted the pitch up by several octaves so I could hear it, and the noise sounded just like a heartbeat. The spacing of the beats also corresponded to my heart rate at the time, which was about 67 bpm.

This surprised me. The recorder was resting on the desk. I was leaning with my elbows on the desk. So I reasoned that the recorder had actually picked up my heartbeat. This sensitivity surprised me, but it seemed at least plausible.

So I tried putting the recorder on a table across the room and started recording. The ambient noise level was around 38 dB(a). I walked in and out of the next room, doors open. Then I took a look at the recording. The "heartbeat" was there again.

Now that I found hard to believe. It's not a fancy digital recorder. The room was quiet, but not that quiet. I walked in and out of the room, but the amplitude of the "heartbeat" did not change. And yet it sounded like a heartbeat, and it matched my pulse.

So, what could this be? Is it really possible that a little recorder like this could record heart sounds even across a room? Or is this noise from some other source that just happens to sound like a heartbeat and just happens to match my pulse?

I don't have enough experience with sound engineering to draw conclusions. Can someone provide me with some insight on what this might actually be? The suspense is terrible!

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  • $\begingroup$ It's certainly not quantum entanglement, but then what is it..? May be you should make more experiments and provide exact details of the setup so that someone else outside may also conduct it to see for himself... $\endgroup$
    – Fat32
    Jun 5 at 3:43
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Is it really possible that a little recorder like this could record heart sounds even across a room?

No. There is no physical mechanism that could transfer enough of your heart beat for the recorder to pick it up.

Or is this noise from some other source that just happens to sound like a heartbeat and just happens to match my pulse?

Could be all sort of things. Residential spaces LOTS of low frequency noise. Your number of 38dB(A) is misleading since the A weighting totally disregards low frequencies. Unweighted it's probably more like 70dB or 80dB. That's mostly infrasound, so you can't hear it unless you pitch shift it up.

At those frequencies a lot of the sound is not only transmitted but also through "hard stuff", walls, floors, furniture, etc.

Major sources for infra sounds are HVAC systems (air condition, heater), appliance (washer, dryer, fridge, freezer, dish washer etc.), environmental, traffic, normal air movements (wind, door opening/closing), other people or pets in the building, etc.

It could also be something that's happening in the zoom recorder itself, maybe the analog front end picks up noise or there is EMI (electro magnetic interference).

If you want to explore this a bit further you can try a few things

  1. Do it again and see if your can reproduce it
  2. If yes, work up a good sweat and record again. See if the recording tracks your elevated heart beat.
  3. Lift the recorder up and hold it in your hand. This will eliminate some structure born sound.
  4. Repeat the experiment with the recorder in a different place in the house.

Have fun! Nothing wrong with being a mad scientist

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