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Hi, we're currently working on the school project and need to pick up our stomach sound for a live performance.

First when we connected contact microphone with a room equalizer, it picked up some small trace of gurgling sound. But when we tried to amplify the signal by connecting the microphone to the DIY amplifier then to the room equalizer, it couldn't pick up anything.

i have some questions regarding to my little knowledge about sound and its theory (or any further suggestion that will make it easier or cheaper... because we have to draw everything from our own pocket ).

+ Contact or electret microphone is better for picking up stomach sound?
+ Around which frequency will be for the stomach/visceral sound?
+ Can we use computer instead of room equalizer or audio mixer?

Thank you very much!

When I recorded my heartbeat I used a electret microphone glued to a ~9cm diameter lid from a coffee jar,
(the plastic coffee jar lid had a ~3mm hole drilled in the centre to allow the sound into the mic).

With the benefit of hindsight a smaller lid from a plastic milk bottle would have been sufficient and blue-tak would be easier (and safer) than using thermal glue to seal the electret mic capsule over the hole.

The plastic lid acts as a cup, directing sounds from the body into the mic and helping exclude sound from the room, similar to a stethoscope.

The electret mic signal was first fed through a (home made) pre-amp with a gain of about 20dB.
(This type of toy may provide a cheap high gain pre-amp).

Faxe_Kondi
... need to pick up our stomach sound for a live performance.

hi Timbre!

thank you very much for the tips.
i think i 'll need to make the preamp (i did the plastic lid trick and it worked well with heartbeat but not the gurgling sound of stomach).

here's the disgram of the pre-amp i'm going to build:
http://stephane.aubert.free.fr/SAMSUNG.YP-T7FX/PREAMPLI.NE5532.jpg

thank you again!

Hi,

In a similar vein if you could borrow a stethescope ( Biology dept ?) and insert an electret into a tube connected to the stethescope (I used a funnel ) you might get some good results.

http://www.freesound.org/samplesViewSingle.php?id=62912

Hi.

I looked at the NE5532 pre-amp circuit, and it should work fine, but a couple of tips (that you might not need 'cos you know already).

1. It would be a good idea to build it in a metal box, or line the box with foil to screen hum and noise. A steel box/lining has the advantage that it reduces magnetic interference as well. Connect the screen to 0V. (Performance improvement will be most noticeable at the highest gain settings where the impedances are also highest.)

2. I only say because it's not shown on the diagram, but the output jack screen/sheath connection should also be connected to 0V.

3. When the unit is switched on it will produce a heavy "thump" as the DC voltages charge the capacitors to normal working voltage, so turn down the recorder/amplifier level (or don't plug into output jack) until pre-amp has been on for a couple of seconds.

4. For perhaps a few cents/pence more you can use an OP275 in place of the NE5532. It is a direct plug-in replacement and has a better spec all round. Also importantly for battery powered equipment it uses a lot less power.

If you already know all this stuff forgive my intrusion, but perhaps these notes might be useful to anyone else deciding to take up a similar project.

(b.t.w. I'm Stan. Long time ago I was co-founder of BSS Audio. I was the designer/Tech Director until we sold the company to Harman International. A product I designed in 1984 - http://www.bssaudio.com/productpg.php?product_id=13 - is still in BSS's product catalogue and still looks identical to my original design. I was pleasantly surprised to find it still being manufactured 25 years later.)

strangely_gnarled

3. When the unit is switched on it will produce a heavy "thump" as the DC voltages charge the capacitors to normal working voltage, so turn down the recorder/amplifier level (or don't plug into output jack) until pre-amp has been on for a couple of seconds.

BTW
To prevent feedback squeal I was thinking possibly a push-to-make switch could be incorporated into the cup pressed to the person, so the mic is only electrically connected when the cup is pressed against the skin. (I found when the cup was lifted from the skin a feedback squeal was more likely to occur as room sounds could then more easily reach the mic).

Hi Timbre, Faxe Kondi, all.

I assumed the circuit will be battery driven because of the 9v power rail, and yes, absolutely, a battery will eliminate the problem of earth loops and keep hum and noise at bay. Theoretically slowing down the "rise time" of the power will move the thump to the subsonic (say less than 0.1 Hz) and reduce it to a trivial amplitude, but unfortunately this would involve a capacitor in the order of, say, 1000uF and also the addition of a series resistor of about 100R between the battery and the rest of the circuit. Without the resistor the low impedance of the battery will charge up 100uF in no time at all. Putting in the extra bits is no problem, but I was keeping note of Faxe Kondi's comment on the limited budget, and taking care during the turn on sequence is for free. Besides, other than being annoying, the thump isn't likely to damage anything.

My thoughts on the microphone go like this:- Use a pair of in-ear headphones like the cheap ones that come with MP3 players. Put them inside the fingers of a latex glove (or condoms), and press them against the skin in a thick puddle of fluid, like the gel used for abdominal ultra-sound scanning. Any fluid will do, like hand cream, liquid soap, petroleum jelly or KY. This should improve the acoustic coupling between the transducer and the fluids of the body by at least 20dB and also attenuate the scritchy handling noises that plague this sort of situation where the mic can't be physically isolated in mid air. If the pre-amp has a stereo jack input then borrowing and trying a pair of ear pieces makes this part of the experiment essentially free. Going further (but possibly sacrificing) the ear phones, the latex/rubber sheath could be dispensed with and the gel allowed to penetrate to the transducer diaphragm. this would improve acoustic coupling even further. I think that with this sort of arrangement it should even be possible to have a live demonstration using loud speakers without the feedback problem.

As far as the frequency/phase etc. characteristics of whichever microphone is used go, they become pretty irrelevant when coupling to anything other than an airborne source, so earphones used in reverse as a microphone are likely to be an effective solution. Also well-matched stereo comes as a convenient bonus!

Regards.

thank you for all of the suggestion! i know a little about electronics so i really appreciate in all of your comments (and no, i hvnt known them already).
apparently, i was the one in the group who has the most knowledge in electronics, so i 'm the one who responsible for it. and when you're on school, there're no excuse not to try :wink: . but it's good, so i'll learn some more.

btw, i have one more question regarding to the schematic http://stephane.aubert.free.fr/SAMSUNG.YP-T7FX/PREAMPLI.NE5532.jpg

is the first capacitor after the on/off switch 10mF or 10uF?
(mF is quite huge isn't it?)

thanks!
+n

is the first capacitor after the on/off switch 10mF or 10uF?
(mF is quite huge isn't it?)

Hi Faxe_Kondi.

Good observation - I missed it. Quite right, 10mF is a typo I'm sure. It should probably read 10nF. For a commercial design I would use a 100nF instead, and I would position it in the circuit so that it was close to pin 4* and pin 8 of the IC . It's purpose is to act as a very low impedance to shunt high frequencies on the power lines but in a simply circuit with only one stage it's value and position aren't particularly critical.

Out of interest, whereabouts are you located? I'm guessing US. (I'm in the UK.)

Good luck, and I hope you keep us up to date with how the project's going.

*Edit. I corrected typo -"pin 5" changed to "pin 4".

The electronics company Maplin (in the UK) have a mono preamp kit for £4 ($6) ...
http://www.maplin.co.uk/module.aspx?TabID=1&criteria=usb&ModuleNo=220052&C=SO&U=Strat15

Here is the tech spec on the kit ... http://www.maplin.co.uk/Media/PDFs/N46FL.pdf
(note the 100uF capacitor across the supply, were have I heard that before ? )

@ Timbre: thanks for the suggestion. unfortunately, i've already ordered the components to build the preamp. i also have a bad experience with Maplin service. last year, i ordered some gear-motors from them and paid by Paypal account (it's the only payment method ordering from Sweden). however, it seems the Maplin website just fetch the 'delivery address' from Paypal without asking. so the package went all the way to US (my friend's office which i previously used Paypal to buy her a present). the Maplin company took no responsibility... when i made a phone call to their customer service, all they did is rejecting everything.

@ strangely_gnarled: yes, i'm in Sweden. i think about moving to UK every time i have to buy components... everything you possibly need is cheaper there (even IKEA!!! :lol: ).

So how did it all come out in the end. Did the pre amp work? Did you get the sound you wanted? What was the final setup?

Joe