Re:xpen Mic for recording internal body sounds / help please.
 

It's common place in pro recording studios to use a 'star' or other grounding scheme, allowing only one path to ground.
A balanced power transformer/configuration is also an option, though quite expensive, it cures many types of power/noise issues. Even single-coil guitar pick-ups become relativity silent.

Re: Mic for recording internal body sounds / help please.
 

The audio is definitely audio and needs a continuous return path.

The DC (whether phantom or "plug in power") also needs a continuous return path.

No RF is ever fed on the mic line.

So you definitely need a negative return connection between the mic and the preamp input. If you run a separate, insulated, negative return conductor, then the shield should be connected at one end only.

Noise induced into the shield, when the shield is part of the mic signal circuit, would never be a problem with a balanced mic, as there is no audio on the shield in that case. It could potentially be a problem with an unbalanced mic. In practice, it's normally not an issue, unless you're running terribly low levels and terribly high gain, or unless you're running terribly long mic wire, or unless you're in a terribly noisy environment.

Re: Mic for recording internal body sounds / help please.
 

Re: Mic for recording internal body sounds / help please.
 

Bingo, Thanks Richard


1 to 600hz, I wonder if that range will catch everything, hearing is from 20 to 20k

Re: Mic for recording internal body sounds / help please.
 

Re: Mic for recording internal body sounds / help please.
 

I wouldn't use it to record bats, or even cymbals, or even voice. Based on heart recordings I've heard, I would guess 600 Hz is adequate... but then again, I don't know how some of the irregularities (murmurs, etc.) might sound.

For whom are you building this device? Does the client have any idea what frequency range is needed?

Re: Mic for recording internal body sounds / help please.
 

Remember it is not just for the heart, although the heart can make rather unusual noises. The lungs have many high pitch sounds as well as the stomach etc.

I am in uncharted waters and consider the range to include all of human hearing as the stethescope would provide to a listener. I think it would be unwise to create any limitations. The client is a heart patient whose mechanical heart beats do not match the holters electrical recording and wants to provide his physicain with the 5 to 8 second recording of their artythmia which induces syncopy. My how nice it would be to have some kind of timing between the holter and the audio. I figured if I made it for the heart it might as well include the lungs and others as well.

Re: Mic for recording internal body sounds / help please.
 

The tissue of the human body blocks high frequencies and conducts low frequencies, so any sound that travels through tissue is going to have that "underwater" sound (humans are mostly water). There's a reason they use relatively low frequency waves for sonar. So I wouldn't worry at all about trying to match the high end frequency response of the human ear IN AIR.

Re: Mic for recording internal body sounds / help please.
 

A friend of mine had a heart valve replaced with an artificial one. He swore he could hear it clicking at times. Whether "clicking" extends above 600 Hz is more than I can tell you.

In that case, buy a stethoscope, cut the tubing, and insert an electret capsule. That should sound pretty similar to the way a stethoscope sounds.

If the goal is to provide a recording for the client's physician, you could always consult with the physician about what frequency range he needs. If he doesn't know, perhaps he can refer you to appropriate medical literature.

Or else invest $5.00 in an electret capsule, or $59 in the pre-made sensor, make a recording, and let the doctor evaluate it.

Seems to me that if you want to make something with wider and wider application (lung noise, digestive noises, noise of arthritic joints, etc.), you're making your job harder and harder. It might make the project more interesting to think about, but it might put you further and further from finishing this contraption for the original intent.

I think you've gotten plenty of info on the audio side of things, and any further investigation will move into the medical realm.

Re: Mic for recording internal body sounds / help please.
 

I'm still interested: what results?

Re: Mic for recording internal body sounds / help please.
 

My just done web search results suggest that in most instances equipment designed for medical or mechanical diagnoses may or is more likely to give inferior sound. One exception is the fetal stethescope or pinard horn. The example given at My Stomach (updated) How Dare You?: appears to come somewhat close to using that principle for recording body sounds, though I could be gullible.

Most else degenerated into inanity; or likewise YouTube recordings done by placing mobile phone or compct camera on belly.....

and of on its own a quote from Michael Ondaatje: "Gentlemen who have placed a microphone beside a naked woman's stomach after lunch and later, after slowing down the sound considerably, have sold these noises on the open market as whale songs"