Sensing 5 dB and lower sounds

Hello,

I would like to built a heart beat sensor (note: no, it is not an ECG) that sense the sound made from the heart (phonocardiography) and converts it in a digital signal.

I am writing here for suggestion about how to achieve it.

I did a little bit of researches and found that a heart beat should be around 10-15dB (source: The Decibel ). Actually an heart beat is made by 4 different sounds so i guess that the article refers to the most audible (S1) so for my application I think that would be good to aim at around 5dB.

I wasn't able to find a dedicated sound sensor (maybe i didn't search enough?), i found a few comparator or amplifier like the lm393 used in these application. Also on google i found the existence of Surface acoustic wave sensors (https://en.wikipedia.org/wiki/Surface_acoustic_wave_sensor) that maybe could suit my needs.

What i would like to do has been already done in digital stethoscopes (https://www.worthview.com/stethoscope-amplify-sound/).

Do you think that what i would like to do is feasible? how would you do it?

Thanks in advance

Top Replies

shabaz over 5 years ago

Hi Vincenzo,

A piezo disc can be used. There's probably better ways, but as a schoolkid experimenting at the time, I certainly was able to hear heartbeat with just a piezo disc and something like an LM386, and headphones. The disc needs a plastic rim, and enclosure (could be 3D-printed).
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aster94 over 5 years ago in reply to shabaz

I thought that a piezo was something not very good for this, but i shall reconsider this, I just found an interesting table that shows that some of the most important stethoscopes (3M) is actually using it.
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michaelkellett over 5 years ago in reply to aster94

These people make good microphones but they are expensive:

https://www.gras.dk/microphone-guide/dynamic-range

You can see that you need a big diameter to get the performance you need.

I don't think that anyone make piezo microphones any more - the last one I designed was about 1977 !
(Not a real new design, re-working a microphone based on a Rochelle salt element to use a ceramic element - sensitivity was way lower with the ceramic.)

I've got no data on it now but here's an ad I found on the web with a picture of the original - the company I worked for made the microphone under license to Cosmocord in the mid to late 1970s.

If you can get this type of microphone it might be OK - the source capacity is many times greater than an electret or capacitive element so the LF noise can be lower.
A moving coil microphone could give low noise if you can find a suitable element.
Best I could see on Aliexpress were these:
https://www.aliexpress.com/item/32915426371.html?spm=a2g0o.productlist.0.0.5ea18047Oj3XvL&algo_pvid=4b0e9b7e-e98a-4a31-8…
(Shure clones - cardioid response so not ideal but 600ohm impedance and high sensitivty can give very low noise.)

MK
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aster94 over 5 years ago in reply to michaelkellett

unfortunately my knowledge in this field is near zero so if you suggest that microphone I am going to buy it, even if I would rather to use something smaller
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dougw over 5 years ago

Are you looking for a contact microphone (which responds to tissue vibrations) or a mic that responds to air pressure variations?
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aster94 over 5 years ago in reply to dougw

contact would be better
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