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The Ultimate Acoustic Stethoscope Review (page 6)

The Tunable Diaphragm Effect in vivo:
3M advertises its Littmann stethoscopes as having a tunable diaphragm. Light pressure on the diaphragm creates a Bell effect with better low frequency transmission, and firm pressure favors higher frequency sounds. We tested this effect on 2 volunteers, one male, one female. The volunteers held the stethoscope to their chest wall at the lower left sternal border with light pressure for 10 seconds, then firm pressure for the next ten seconds. We found no tunable diaphragm effect with our male volunteer, probably because the light pressure was still too firm. We did see a tunable diaphragm effect with all of the Littmann scopes in our female volunteer. But we only saw this at a pressure so light as to impair acoustic coupling, as shown below. The preamp gain settings were 10 Db greater than in the prior spectra.

Figure 3: Tunable Diaphragm Effect In Vivo Littmann Master Classic II

Tunable Diaphram Effect Littmann Master Classic II

Figure 4: Lack of Tunable Diaphragm Effect With ADC Adscope 603

Lack Of Tunable Diaphragm Effect ADC ADScope 603

The Tunable Diaphragm Effect In "Vitro", And Bell versus Diaphragm
We tested the effect on pressure on the diaphragm frequency response for all scopes using  a layer of acoustic foam over our transducer element, and frequency sweeps from 60-560 Hz. Light pressure was 50 grams, firm pressure was 240 grams. We also tested the response of the Bell, compared to the diaphragm. The results for several scopes are listed in the table. Click on the thumbnails to see the full size photos or spectra.

The results show a tunable diaphragm effect for the Littmann scopes, and for a lesser extent, all of the other scopes except the the Welch Allyn. The results also show that the Littmann Cardiology scopes are the most efficient at transmitting low frequency sounds. The Cardiology III is the most efficient in this system and its low frequency transmission exceeds the test jig alone below 80 Hz. The Welch Allyn has the flattest frequency response, in accordance with its claim that sounds from 200-500 Hz are accentuated relative to other stethoscopes.  Both the Omron and Welch Allyn had better high frequency transmission than the other stethoscopes which probably account for their subjective impressions of superior clarity.

Model Photo Bell vs Diaphragm Spectra
ADC Adscope 603 ADC Adscope 603 ADC Adscope 603 Bell vs Diaphram
Allheart Cardiology Allheart Cardiology Allheart Cardiology Bell vs Diaphram
DRG Puretone Traditional PT3 DRG Puretone Traditional PT3 DRG Puretone Traditional Bell vs Diaphram
Littmann Cardiology I Littmann Cardiolgy I Littman Cardiology I Bell vs Diaphram
Littmann Cardiology III Littmann Cardiology III Littman Cardiology III Bell vs Diaphram
Littmann Classic II SE Littmann Classic II SE Littman Classic II SE Bell vs Diaphram
Omron Sprague Rappaport Omon Sprague Rappaport OmronSprague Rappaport Bell Vs Diaphram
Welch Allyn Tycos Elite Welch Allen Harvey Tycos Elite Welch ALlyn Harvey Tycos Elite Bell vs Diaphram

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