The length of two open organ pipes are l and (l+Δl) respectively. Neglecting end correction, the frequency of beats between them will be approximately
(Here v is the speed of sound)
v2l
v4l
vΔl2l2
vΔll
λ1=2l, λ2=2l+2Δl ⇒n1=v2l and n2=v2l+2Δl
⇒ No. of beats =n1−n2=v21l−1l+Δl=vΔl2l2