Auscultating heart sounds with both the diaphragm and bell of the stethoscope is essential because each part is designed to detect different frequencies. The diaphragm is best for high-pitched sounds such as S1, S2, and murmurs of aortic or mitral regurgitation, while the bell is optimal for low-pitched sounds like S3, S4, and some mitral stenosis murmurs. Using both enhances diagnostic accuracy by allowing thorough assessment of all heart sounds and murmurs, which is critical for identifying valvular and other cardiac abnormalities during a cardiovascular examination.

Reference: Bates' Guide to Physical Examination and History Taking, Lynn Bickley, 13th Edition.