The interference of the two waves at P is fully constructive if their path
length difference DL = |L_{2}-L_{1}|
is an integer multiple of the wavelength l.
DL = n l,
n = 0, 1, 2, ... (fully constructive
interference)
The interference of the two waves at P is fully destructive if their path length
difference DL = |L_{2}-L_{1}| is a
half-integer multiple of the wavelength l.
DL = (n+0.5) l,
n = 0, 1, 2, ... (fully destructive
interference) Review wave interference in
chapter 1

Click on S_{1}, S_{2},_{ }or P_{ }and drag them
to change the two waves path length difference DL = |L_{2}-L_{1}|.

Consider a point P where we want to detect the sound from the sources. Let L_{1}
be the distance from P to S_{1} and L_{2} be the distance from P
to S_{2}. We assume that L_{1} and L_{2} are much
greater than the distance between the sources so that we can approximate the
waves as traveling in the same direction at P.

interference

Wave 2

L_{2}

S_{2}

S_{1}

L_{1}

Wave 1

Consider two sound point sources S_{1} and S_{2}. S_{1}
emits wave 1 and S_{2} emits wave 2. The two waves have identical
wavelength l. The phase of wave 1 at S_{1} is
the same as the phase of wave 2 at S_{2}. Because of this, the two
sources are said to be in phase.