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The Ultimate Acoustic Stethoscope Review (page 4)                                                                                   

Objective Clinical Testing:

   Materials and Methods: Prior published testing used artificial systems to test stethoscopes for frequency response (transfer function). The human heart has very low frequency sounds and complex resonances which artificial transducers don't accurately reproduce. Thus we conducted loudness testing using human volunteers by measuring the relative loudness of their heartbeat with the various stethoscopes.  For other our tests we used  the transducer element from Sennheiser HD600 Professional Headphones, and commercial software to generate frequency sweeps and dirac pulses (clicks). We developed a test jig to mate both stethoscope earpieces to a sound pressure level meter. The sound pressure level meter has a microphone, preamp output, and a calibrated variable gain control. The preamp output was attached to the line input of a Creative Audigy II soundcard on a Windows XP computer, and were recorded as .wav files. Power spectra analysis and Fourier analysis of the .wav files were done with commercial software.

    Loudness testing was done using human volunteers. We used two volunteers, one male, and one female and recorded heart sounds for each using each scope, in duplicate. The control was the microphone of the sound pressure meter placed directly against the chest. We found that most of the stethoscopes amplified sounds below 100 Hz, and attenuated sounds above 200 Hz. Results were measured as the decibels of sound amplification at the peak of the human heart sound power spectra, typically at 30 Hz in our subjects. Amplification of 15 Db was assigned a value of 10 points on our chart. The DRG scope had the highest peak sound intensity.

   Clarity testing was done using the Sennheiser transducer to generate Dirac pulses (clicks). We found that all of the scopes attenuated the clicks.  The DRG had the worst performance in this test with 18 DB of attenuation. Results are again expressed on a ten point scale, with 10 representing the least attenuation (better).

  Sine wave frequency sweeps (transfer function) were also done.  By themselves frequency sweeps of stethoscopes have little meaning, as the main factor determining the transfer function is internal resonances from the stethoscope tubing. Each stethoscope has a resonant frequency which depends on the length and internal width of the tubing, similar to a pipe organ.  We used the sine wave frequency sweeps to study the difference between Bell and diaphragm in the dual head scopes. We also used the frequency sweeps to study the effects of diaphragm pressure on the transfer function. (the tunable diaphragm effect). Light pressure was  50 grams, firm pressure was 240 grams. Frequency sweeps were done from 60-560 HZ, to avoid damage to the transducer (At sine wave frequencies below 40 HZ the transducer was clipping in our test setup). The tunable diaphragm effect was also studied by recording heart sounds in our volunteers for 10 seconds under light pressure, and 10 seconds under firm pressure.

Table 4 Objective Testing

Model Intensity Clarity (Click Detection) Score
ADC Platinum Cardiology 6158.2 4.6 12.8
ADC Adscope Professional 603 6.2 5.6 11.8
AllHeart Cardiology 7.0 5.9 12.9
DRG Puretone Traditional PT3 10 1.3 11.3
Single Head Nursing Stethoscope7.3 7.0 14.3
Littmann Cardiology I 8.1 8.2 16.3
Littmann Cardiology III 9.0 6.9 15.9
Littmann Master Classic II 8.7 6.8 15.5
Littmann Classic II SE 7.3 7.1 14.4
Omron Sprague Rappaport 8.1 7.6 15.7
Prestige Sprague Rappaport Stealth 7.1 3.8 10.9
Welch Allyn Tycos Elite 6.7 7.9 14.6

Figure 1 Sound Intensity Littmann Cardiology III Compared To DRG
Sound pressure levels of the human heart were determined as a function of frequency for all scopes. Below is a comparison between the Littmann Cardiology III and the DRG scope.

Human Heart frequency respinse Littmann Cardiology III versus DRG Puretone Traditional


Figure 2 Clarity of Welch Allyn Compared To DRG
Waveforms of Welch Allyn and DRG stethoscopes overlay, using clicks generated one second apart. The DRG soft diaphragm attenuates the clicks, and artifactual resonances are generated.

Welch Allyn Versus DRG Stethoscope Clarity

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