Why murmur present in aortic stenosis is crescendo decrescendo type

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Why the aortic stenosis murmur is crescendo-decrescendo (diamond-shaped)

The shape of the murmur mirrors the instantaneous rate of blood flow (and turbulence) across the stenotic aortic valve during systole, which in turn tracks the changing pressure gradient between the left ventricle (LV) and the aorta throughout systole. This is illustrated in Harrison's classic diagram of the LV-aortic pressure tracing superimposed on the phonocardiogram, where "a midsystolic murmur (MSM) with a crescendo-decrescendo configuration is recorded" corresponding to the "systolic pressure gradient... between left ventricle and aorta" - Harrison's Principles of Internal Medicine, 22nd Ed.

Breaking down the physiology across systole:

Early systole (crescendo phase) - As the LV begins to contract, intraventricular pressure rises but is still relatively low, and the gradient across the narrowed aortic valve is small. Flow velocity across the valve is modest, so the murmur starts soft.
Mid-systole (peak intensity) - LV pressure continues to climb toward its peak, driving flow across the fixed, narrowed orifice at increasing velocity. Turbulence (and therefore murmur loudness) increases in parallel, peaking when the LV-aortic pressure gradient and transvalvular flow rate are maximal - this is why the murmur is described as "late-peaking" in more severe stenosis (the tighter the valve, the later and louder the peak, since it takes longer for LV pressure to generate maximal flow through a smaller orifice).
Late systole (decrescendo phase) - As the LV finishes ejecting and begins to relax, LV pressure falls faster than aortic pressure, so the gradient - and thus flow velocity across the valve - progressively declines. The murmur fades out (decrescendo) and terminates before or around S2, since flow essentially ceases once the LV-aortic gradient reverses and the valve closes.

So physiologically, the murmur isn't crescendo-decrescendo because of the valve itself changing shape - it's because the driving pressure gradient and resultant transvalvular flow/turbulence rise and fall in a smooth "diamond" pattern over the course of ventricular systole, and turbulent flow is what generates audible sound. This also explains why:

It's a midsystolic ejection murmur (starts after S1, once enough pressure has built up to open the valve and generate flow, and ends before S2 as flow ceases).
The murmur is best heard at the right second intercostal space with radiation to the carotids (along the direction of turbulent jet flow) - Symptom to Diagnosis: An Evidence-Based Guide, 4th Ed; Textbook of Family Medicine, 9th Ed.
Later peaking = more severe stenosis: a more critically narrowed valve requires a longer, higher pressure buildup before flow (and thus murmur intensity) reaches its maximum, shifting the peak later into systole - Frameworks for Internal Medicine.

Summary: The crescendo-decrescendo shape is a direct acoustic representation of the rising-then-falling left ventricular-to-aortic systolic pressure gradient (and the flow/turbulence it generates) across the fixed stenotic orifice during a single cardiac cycle - not a fixed property of the valve, but a dynamic one tied to the physiology of ventricular ejection.

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