What's the difference between cheap and expensive stethoscopes?

April 14, 2008 5:23 PM

I am using a stethoscope to control an art project so that a sculpture of LEDs responds in time to a user's heartbeat. I've started by buying a cheap stethoscope and jamming an electret mike into the tube and builiding a simple amplifier circuit (similar to this) to connect it to the line in on a PC. it works well, but the overall sound quality is not fantastic. I have noticed that there are a lot of more expensive stethoscopes out there and I wonder if I would get a substantial improvement in sound quality by using one of them?

I've never used a stethoscope before so I have no experience with the difference between cheap and expensive models. I've also seen electronic stethoscopes, but the expense has deterred me from buying one. This is eventually going to go to Burning Man, so I'd like to keep cost down so I can have more than one in case the conditions there destroy them.

posted by pombe to media & arts (12 comments total)

You probably don't want sound quality (unless you're also going to amplify and play the sound out, in which case you've got bigger potential problems e.g. feedback). What you probably really want to do is isolate the heartbeat sounds from all the other extraneous noise, so you can trigger on them - in which case you'll want some sort of low pass / bandpass filter.

You could also maybe get a PLL to the heartbeat to clean up your triggering further, but PLLs with such long time constants tend not to work very well (fail to lock / fall out of lock too easily)...
posted by Pinback at 5:57 PM on April 14, 2008

Electronic musician here - two points. One is that a mic in a tube will always sound like, well, picture a vocal mic in a cardboard tube. Having the mic actually inside the tube is a poor acoustic environment due to muddled reflections off the tube walls etc. That said, point number two is that I bet you can clean it up a ton with some good EQ. If you can get a small mixer with 3-band EQ like this, you can cut the mids and get a cleaner thumping noise.
posted by mindsound at 6:16 PM on April 14, 2008

Further: you might find something useful here. Figs 3 & 4 show a nice simple mic bias & low-pass filter arrangement that should work; the rest may give you some tips on how to implement your heartbeat detection.
posted by Pinback at 6:34 PM on April 14, 2008

On (lack of) preview: a purpose-designed filter will work 10x better than an equaliser for this purpose, at the trade-off of having to do some actual thinking and design rather than just playing with knobs or sliders. Of course, using an eq to home in on the frequencies of interest can give you a useful starting point for designing a filter...
posted by Pinback at 6:38 PM on April 14, 2008

Contact microphone.
posted by rhizome at 7:54 PM on April 14, 2008

I have a Littmann Cardiology III; it's nice, but for purposes of just hearing a heartbeat I can't say it's better than a cheapo one from anywhere. Make sure that you have the head in the right position to get the loudest beat, and clothing is a no-no. MDs are expected to hear very subtle changes; if there's any difference at all, they just have to have the top of the line, which is not quite your application. Here's research showing the differences to be small.
posted by a robot made out of meat at 7:59 PM on April 14, 2008

Thanks for all the comments. For clarification, I have some filtering built into the amplifier - currently I'm using a 5Hz - 500Hz passband, and I have parts to try changing those values around.
posted by pombe at 9:13 PM on April 14, 2008

There are electronic stethoscopes. Some have analog or digital audio out. These would be best for your purpose.
posted by ikkyu2 at 12:00 AM on April 15, 2008

I use a variety of stethoscopes every day because I am too lazy to carry one around and so just borrow whatever is handy. There is a lot of variation in sound quality and it doesn't always relate to costs. As mentioned above, proper technique is important. Also the properties of the tubing (length, diameter, stiffness) all affect the frequency response of the system, and most stethoscopes have two sides, one for high frequency sounds and one for low frequency sounds. I got a bunch of these to keep around the OR and like them; they were only $5.00 apiece, if I remember correctly. They sound surprisingly good for that price. Because you are using a microphone the acoustics will be different, though. I would suggest trying a few different cheap models and seeing which works best; if you know anyone in the medical field you should be able to borrow a few without having to buy until you find one that works well.
posted by TedW at 6:28 AM on April 15, 2008

This blog post might be of use. Its from someone trying to do something very similar with an arduino and a contact mic, faced signal/noise problems and found an audible signal form a heart monitor that was much more manageable. He was building something that controlled a projector with a heartbeat, so there is probably some useful info in his blog.

Are you using the sound as well, or just using it as a trigger?
posted by tallus at 6:50 AM on April 15, 2008

The plan is just to use the mic output as a trigger. We'll have some C# code that follows the mic output and then uses that to light up a series of LEDs in some aesthetically pleasing way that follows the heartbeat.
posted by pombe at 11:37 AM on April 15, 2008

Does it have to be a stethoscope input? This guy made a heartbeat MIDI controller by using an IR sensor to track blood flow in a fingertip.
posted by moonmilk at 7:34 PM on April 15, 2008