1. When two waves meet at a point in space, their amplitudes add vectorially. Constructive interference occurs when the path difference is an integral multiple of the wavelength, producing loud sounds. Destructive interference occurs when the path difference is an odd multiple of half the wavelength, producing soft sounds. 2. As frequency increases, wavelength decreases. This causes the positions of constructive and destructive interference to shift towards the central axis as the microphone passes through them. 3. Increasing the slit separation in a double-slit experiment doubles the fringe separation in the interference pattern. The slit width does not affect fringe separation but decreases intensity.