Suppose there is a speaker at $x=0$ that produces sine waves with a frequency of $\omega(t)$. The sound waves right at the speaker can be described by $f(t,0)=\sin(\omega(t)t)$, and suppose that the speed of sound is 1 m/s. Now what do I hear at f.e. $x=5$m. Well I should hear what someone heard right at the spreaker 5 seconds ago. So the „wave function“ at x=5m should be $f(t,5)=f(t-5,0)=\sin(\omega(t-5)(t-5))$. Or more general $f(x,t)=\sin(\omega(t-x)(t-x))$
But now if we consider the phase of the wave for every time t at x: $\phi(t,x)$ such that $f(t,x)=\sin(\phi(t,x))$, then $\omega(t)$ tells us by how much this phase grows per unit time so $d\phi=\omega(t)dt \Rightarrow \phi(t)=\int_0^t\omega(t‘)dt‘$, and since $\phi(t,x)=\phi(t-x,0)$ (the speed of sound is one) this leads me to believe:
$f(t,x)=\sin(\int_0^{t-x}\omega(t‘)dt‘)$
But these two formulas are only the same if $\omega$ is constant. I know I‘m probably ignoring something very trivial but I can‘t see it right now, so I‘d be very happy if someone could help me:)