"A is proportional to B"

A -------> B

A = k(B) -------the mathematical expression

where k is the constant of proportion

-------------------

For the posted problem,

Let A = amplitude

So, Ao = initial amplitude

Ao = Ao

A = (Ao) /1000

Io = k(Ao)^2

I = k(A)^2

I /Io = k(Ao /1000)^2 / k(Ao^2)

I /Io = 1 / (1000)^2

I /Io = 1 / 1,000,000 -----------------answer.