Four independent waves are represented by equations :
(1) $X_{1}=a_{1} \sin \omega t$
(2) $X_{2}=a_{1} \sin 2 \omega t$
(3) $X_{3}=a_{1} \sin \omega_{1} t$
(4) $X_{4}=a_{1} \sin (\omega t+\delta)$
Interference is possible between waves represented by equations :