Stethoscope Acoustics: Explaining Low/High Frequency Filtering

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Discussion Overview

The discussion revolves around the acoustics of stethoscopes, specifically focusing on how different components, such as diaphragms and bells, filter and amplify low and high frequency sounds. Participants explore the underlying physics principles that explain these phenomena, aiming to clarify concepts for a broader audience.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant notes that stethoscope diaphragms filter out low frequency sounds, amplifying higher frequencies, while bells amplify low frequency sounds due to their design and interaction with skin vibrations.
  • Another participant suggests that the size of the diaphragm or bell relative to the eardrum contributes to the amplification of different frequencies, with the bell's larger mass favoring low frequencies and the diaphragm's lower mass favoring high frequencies.
  • A participant expresses confusion regarding the relationship between diaphragm size and frequency transmission, comparing it to drum sets where smaller drums produce higher pitches, and seeks clarification on why stethoscopes operate differently.
  • A later reply acknowledges a misunderstanding about diaphragm sizes, confirming that larger diaphragms are associated with higher frequencies, while smaller diaphragms are used to filter out low frequencies more effectively.

Areas of Agreement / Disagreement

Participants exhibit some agreement on the general principles of how stethoscope components function, but there remains confusion and differing interpretations regarding the relationship between diaphragm size and frequency amplification, indicating unresolved aspects of the discussion.

Contextual Notes

Participants reference conventional wisdom and personal understanding, which may not fully align with established acoustic principles. There is an indication of missing clarity on how size affects frequency transmission in the context of stethoscopes versus musical instruments.

pkubin
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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
 
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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
 
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?
 
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.
 

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