2004 OPEN FORUM Abstracts
A STETHOSCOPE COMPARISON OF BELL VS. DIAPHRAGM OUTPUT.
Jonathan B.
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.