sthetoscop digital using electrec condenser mic (ecm)

Thread Starter

Ramdani Febrian

Joined Dec 12, 2016
6
i make sthetoscope digital using ecm,
can't capture heart sound (i mean low) but can capture sound outside the sthetoscope (audible).

note : if i using stetoscope without ecm and connected to my ear,
i can hear the heart sound (audible)

what is wrong with my stetoscope ???

my rating sound( low - audible - loud)

my circuit


my project

right is stetoscop with mic
center is circuit
left is audio jack to soundcard laptop
 

RichardO

Joined May 4, 2013
2,270
If you are hearing sounds from outside your stethoscope then it seems that you have enough amplifier gain. That is good.

I think there are a couple of improvements you can do.

The first is mechanical. You need to get sound from one side of the microphone but not the other. In other words, the sound from the chest should be picked up but not the sounds from the room.

The second solution is electronic. You should add a filter to limit the sound to a only the low frequencies that the heart produces. This is probably only a few hundred to a thousand Hertz.

In addition, you might want to try increasing the value of the coupling capacitors since the heart has a slow beat rate.
 

RichardO

Joined May 4, 2013
2,270
Why not a contact microphone instead ? (piezoelectric sensor)
This sounds like a reasonable idea to me. Foam rubber or other damping material would be needed on the non-contact side of the sensor. A J-FET buffer would also be needed to make it equivalent to a electret microphone.
 

Thread Starter

Ramdani Febrian

Joined Dec 12, 2016
6
If you are hearing sounds from outside your stethoscope then it seems that you have enough amplifier gain. That is good.

I think there are a couple of improvements you can do.

The first is mechanical. You need to get sound from one side of the microphone but not the other. In other words, the sound from the chest should be picked up but not the sounds from the room.

The second solution is electronic. You should add a filter to limit the sound to a only the low frequencies that the heart produces. This is probably only a few hundred to a thousand Hertz.

In addition, you might want to try increasing the value of the coupling capacitors since the heart has a slow beat rate.
thanks for the reply RichardO,
for mechanical, my sthetoscope have two side,
diphragm and bell side, if i use one side, the other side will close,
i still confuse, how can if i use diaphragm side and then i place it at my chest, and then other sound is picked up ???

for electronic,
sthetoskop >>> mic >>> mic circuit >>> pre-amp circuit (about 1.5gain) >>> souncard laptop >>> laptop ( i use labview, digital amplifier and digital filter (100-1000hz)),
is it wrong if i use digital amp and filter?

i just use one 100nf at the end of mic circuit, and one at the end of pre-amp circuit.
is higher the value is the better? could you tell me the relevancy between capacitor and slow beat rate??
increase capacitor? adding paralel capacitor or just replace with higher value?



Why not a contact microphone instead ? (piezoelectric sensor)
i dont know if it can replace by pizoelectric,
is piezoelectic can capture sound too?
i think piezoellectic just convert pressure to current???
 

RichardO

Joined May 4, 2013
2,270
You may find this interesting:
https://www.researchgate.net/publication/228412447_Frequency_analysis_of_the_heartbeat_sounds
Sound 4 in the paper has frequency peak at less than 50 Hz. I couldn't find in the paper the source of sound 4.


thanks for the reply RichardO,
for mechanical, my sthetoscope have two side,
diphragm and bell side, if i use one side, the other side will close,
i still confuse, how can if i use diaphragm side and then i place it at my chest, and then other sound is picked up ???
Could you explain more abut what you mean when you say: "diphragm and bell side, if i use one side, the other side will close"?

for electronic,
sthetoskop >>> mic >>> mic circuit >>> pre-amp circuit (about 1.5gain) >>> souncard laptop >>> laptop ( i use labview, digital amplifier and digital filter (100-1000hz)),
is it wrong if i use digital amp and filter?
What you did appears basically right. Looking at the paper shows that you might need to set your filter for a lower frequency of more like 30 Hz. You also might gain some reduction in room sounds by setting the upper cutoff of the filter to 300 Hz or a little less.

i just use one 100nf at the end of mic circuit, and one at the end of pre-amp circuit.
is higher the value is the better? could you tell me the relevancy between capacitor and slow beat rate??
increase capacitor? adding paralel capacitor or just replace with higher value?
The coupling cap at the input of the pre-amp is too small a value to pass 30 Hz well. I would make this capacitor 10 uF. If you use an electrolytic cap you should connect the plus side to the base of the transistor.

You can also put a capacitor in parallel with the 100 K ohm resistor in the preamp to roll off frequencies above about 1 KHz. I would use a 0.01 uF cap.

i dont know if it can replace by pizoelectric,
is piezoelectic can capture sound too?
i think piezoellectic just convert pressure to current???
Yes. In fact an electret microphone is a piezoelectric element with a J-FET buffer built into it. The reason for the J-FET is that the piezo is a very high impedance voltage source (not current). The J-FET buffer reduces loading on the piezo so that low frequencies can be reproduced.
 

Thread Starter

Ramdani Febrian

Joined Dec 12, 2016
6
sorry for the late response,
and thank you so much for the help richardo,
i've found my problem,
there's hole in my rubber tube,
so outside sound is enter the stethoscope.
i've repair it,

and i've recorded my heart sound, but there's clickk sound?
this is come from circuit ???
 

Attachments

Top