This study evaluated stethoscope acoustics by using a sound frequency generator and an active artificial ear. Six popular, currently available stethoscopes were compared in their various modes involving bells, diaphragms, etc.: Littmann Classic II, Littmann Cardiology II, Littmann Master Cardiology, Hewlett-Packard Rappaport-Sprague, Tycos Harvey Triple Head, and Allen Medical Series 5A RPS Binaural. The transfer function was measured from 37.5-1000 Hz, the range where nearly all heart and lung sounds are found. Sound in the low-frequency range (37.5-112.5 Hz) was in most cases amplified by the bells and attenuated by the diaphragms; however, there were no significant differences. Both bells and diaphragms attenuated sound transmission in the high range, and this increased with frequency. The Tycos Harvey Triple Head ribbed diaphragm attenuated sound transmission to a significantly greater extent than the other diaphragms (P less than 0.01). The results show that the bell and diaphragm for a given stethoscope usually have different transmission characteristics, particularly at low frequencies. The Littmann Classic II is an exception. The Hewlett-Packard and Tycos Harvey stethoscopes showed the greatest differences in low frequency response between the bell and the diaphragm. While the differences found in sound transmission between stethoscopes were in most cases small, the Littmann Cardiology II, bell and diaphragm, appears to possess the best overall performance by this study design.