Proving the Irrationality of $\frac{\sin(a \theta)}{\sin(b \theta)}$

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I want to find a real $\theta$ such $\frac{\sin(a \theta)}{\sin(b \theta)}$ must be irrational for all positive integers $a$ and $b$. How could I go about finding such a $\theta$ and then prove that this $\theta$ satisfies the aforementioned condition for all positive integers?