2004 OPEN FORUM Abstracts
A STETHOSCOPE COMPARISON OF BELL VS. DIAPHRAGM OUTPUT.
Waugh, PhD; Dale W. Callahan, PhD; George A. Mathew, MS; Wesley
M. Granger, PhD; University of Alabama at Birmingham, Birmingham, AL.
BACKGROUND: Early stethoscopes had only bell-shaped chest pieces. Modern stethoscopes often have both bell and diaphragm chest pieces. Manufacturers’ rationale for this was that diaphragms act as a high-pass filter to eliminate the lower frequency heart sounds to allow better appreciation of higher frequency lung sounds. We tested high-quality stethoscopes to see if this were true.
METHODS: Seven different models of adult “cardiology” stethoscopes were measured using a pure tone (sine wave) sweep input from 50-3000 Hertz (Hz). Four new copies of each model were tested and the data averaged for comparison by pooled t-test. The 19,000 data points sampled during each test were divided into 491 subgroups and the average of each subgroup was used for analysis and graphing.
RESULTS: The bell group overall average was different from the diaphragm group (p<0.001). When the two groups were plotted there was a point of divergence at about 1250 Hz threshold for the two groups (see figure). This is based on group averages, it is possible some individual models may not show a difference between bell and diaphragm.
DISCUSSION: The results show a difference in signal output between the bell and diaphragm chest pieces and there does appear to be a selective filter effect associated with the diaphragm between 200-1250 Hz. The diaphragm plot appears to closely follow the bell plot at measured frequencies greater than 1250 Hz. The commonly held notion that, in general, a diaphragm filters out some of the lower heart sounds appears to be valid.
CONCLUSION:Based on these results there may be reason to have both a bell and chest piece available but the threshold of the filter-effect may be too high, unintentionally attenuating important lung sounds as well as heart sounds.