When sound reflects off a flat, rigid wall, the reflected wavefront looks exactly as if it came from a mirror-image copy of the source, positioned on the other side of the wall. Repeating this mirroring across every wall — and then mirroring those images again — builds a lattice of "image sources." A straight line from any image source to the listener represents one possible reflection path, with its length giving the extra distance (and therefore extra delay) that reflection travels compared to the direct sound.
The image-source method is exact only for simple rectangular "shoebox" rooms with flat, rigid walls — real concert halls use more elaborate ray-tracing or wave-based solvers, but the mirrored-source intuition still explains why hard, parallel walls (like a bathroom) produce noticeably more flutter echo than an irregular, furnished room.
A 3D shoebox room mirrors a sound source across every wall to generate "image sources," visualizing how sound reflections build up reverberation and how absorbent walls drain their energy.
Each wall reflection is geometrically equivalent to sound arriving from a mirror-image copy of the source. Chaining mirror reflections across all six surfaces builds the image-source lattice used to predict every reflection's arrival time and energy loss.
Resize the room, adjust wall absorption, raise the reflection order to reveal more bounce paths, and slide the listener across the room. Watch RT60 and reflection timing update live, and see higher-order rays fade faster as absorption increases.
The image-source method is exact for rectangular rooms with flat rigid walls — it's the same geometric trick used to design recording studios and predict flutter echo between hard, parallel surfaces.