interference in 2d and 3d
two speakers playing the same note. in some places the crests arrive together and the sound doubles; in others a crest meets a trough and they annihilate. the quiet lines don't drift — they're nailed to the room, and you can walk in and out of them.
the same field, as a surface
height is loudness. drag to turn it — the valleys running away from the sources are the silence.
wavelength
49 cm
separation ÷ λ
2.45
dead lines
4
the count follows straight from the geometry: a quiet line exists wherever the two distances differ by half a wavelength, and the difference can never exceed the gap between the speakers. so the number of dead lines is set by separation ÷ λ and nothing else. drop the separation below half a wavelength and they vanish entirely — the speakers are too close to disagree anywhere.
this is why bass is forgiving and treble is fussy. a 60 Hz note is nearly 6 metres long, so a normal room can't fit a dead zone in. a 4 kHz note is 8 cm, and moving your head shifts you through several.
switch to “what you'd hear” and the motion stops. your ear can't follow a 700 Hz wave — it only registers the average, and the average is a fixed pattern of loud and quiet.
one source is never one source
set the sources to 1 and the field is boring — smooth rings, no dead spots anywhere. but a room does this to you regardless: every wall is a second speaker, reflecting a delayed copy back at you. the comb of loud and quiet you get from two sources here is the same thing that makes a single speaker sound different in every corner of a room.