The physics
Two sounds of the same frequency meet. A squeeze can meet a squeeze, or a squeeze can meet a stretch. This adding of waves is interference.
The phase gap δ comes from the path gap Δx. Add any gap the sources start with.
δ = δ₀ + 360° × Δx / λ λ = v / f
In step (δ = 0°, 360° …), the waves add, and the spot is loud. Half a wave apart (δ = 180°, 540° …), they cancel, and the spot is quiet.
Swapping a speaker's two wires adds 180°. Then a path gap of one whole λ gives a quiet spot.
Stand d from the midpoint of two facing speakers. The path gap is 2d. A spot d from a hard wall has a gap of 2d too. So does Quincke's arm, slid out by d. The first quiet spot has 2d = λ/2.
quiet at d = λ/4, 3λ/4, 5λ/4 …
At a hard wall the air cannot move. So the pressure swings most there, and it is loud.