Check the leakproofness: Acoustics effect of stethoscope has something to do with the stethoscope and patient's state, the leakproofness between listener's ears and stethoscope, degree of parts tightness, etc; The better the degree of tightness, the more accurate the patient's voice goes into the listener's ears; So always check the status of the stethoscope.
When using the double sided stethoscope, you needs to select the side you want to.