Stethoscope Acoustics: Explaining Low/High Frequency Filtering

[pkubin]

Hi, everybody. I run a blog about my training as a physician assistant, and I'm planning a video that discusses just how stethoscopes work. I understand the general principle, but I'm having trouble with the WHY for two of the facts I intend to share:

1) Stethoscope diaphragms filter out low frequency sound, and therefore tend to amplify the higher frequency ones

And,

2) Stethoscope Bells -- the side that has a smaller diaphragm, or none at all (just a concavity) -- tend to amplify low frequency sounds.

Is there a fairly simple physics principle that will explain to the non-physicist WHY those two things are true?

Thanks,

Paul

 

[Andy Resnick]

The main difference, from what I can tell, is that the diaphragm picks up sounds (that is, the diaphragm is excited by vibrating air) while the bell is directly excited by (acoustic) movement of the skin.

The amplification in both cases occurs because of the large size of the diaphragm/bell as compared to your eardrum. The bell amplifies low frequencies due to the large mass, while the diaphragm amplifies high frequencies because of the low mass.

http://www.blurtit.com/q505558.html

 

[pkubin]

Okay, that makes sense to me, but I'm still not clear on one issue.

Some stethoscopes, instead of having a diaphragm and a bell, have a diaphragm and a smaller diaphragm that is used in place of a bell. The conventional wisdom is that the smaller diaphragm will transmit the lower tones and the larger will transmit the higher ones.

This confuses me a little, because in a drum set, the smaller the drum diameter, the higher the pitch (if you're not a musician, think of the tympany drums - huge diameter for the low booming sounds) -- and that's just the opposite of the way it seems to work with stethoscopes. Why is that?

 

[pkubin]

Nevermind. It seems I have the large vs. small diaphragm thing backwards. The large is for high frequencies, and the small is for even higher ones in the presence of low ones (that are then more effectively filtered out).

thanks for your answer, Andy.