t - Time elapsed
a - Constant in the function k(ω)
ω₁ - Angular frequency of the first wave
ω₂ - Angular frequency of the second wave
The wave number $k$ is determined by the formula $48.14$ from The Feynman Lectures on Physics, Volume 1. $$k = \frac{\omega}{c} - \frac{a}{\omega c}$$