Two stationary sources each emitting waves of wavelength $$\lambda $$, an observer moves from one source to another with velovcity u. Then number of beats heard by him

The equations of two waves acting in perpendicular directions are given as
x = $$a$$cos($$\omega $$t +$$\delta $$) and y = $$a$$cos($$\omega $$t + $$\alpha $$), where $$\delta $$ = $$\alpha $$ + $${\pi \over 2}$$, the resultant wave represents