To obtain a static interference pattern at a point (that is, one that is constant with time) the two sources must have:
(a) the same wavelength
(b) a constant phase difference between them
Sources with synchronised phase changes between them are called coherent sources and those with random phase changes are called incoherent sources.
Two speakers connected to the same signal generator are coherent sources because the sound waves that they emit are continuous – there are no breaks in the waves.
Two separate light sources do not give a static interference pattern because although they may be monochromatic the light from them is emitted in a random series of pulses of around 10-8 s duration.
The phase difference that may exist between one pair of pulses emitted from the source may well be quite different from that between the next pair of pulses.
