Four different independent waves are represented by

a) $y_1=a_1\sin {\omega }_{1}t$

b) $y_2=a_2\sin {\omega }_{2}t$

c) $y_3=a_3\sin {\omega }_{3}t$

d) $y_4=a_4\sin \left({\omega }_{4}t+\frac{\pi }{3}\right)$

The sustained interference is possible due to