The lobby has a threshold and two sides. One side is where the building holds its frequency for 1.8 seconds. The other side is the street, which dissipates in 0.2. The maintenance flag was filed on a Tuesday and expires on a day she will not be watching. She has been watching for twenty-two days.
She props the door with her shoulder. One foot inside. One foot outside. The concrete of the lobby floor is the same material as the stairwell where the composition began. The street is asphalt and air and the Pangyo Techno Valley morning, which is not a kind of silence but a different kind of sound.
She plays 39.7 Hz and 40.1 Hz simultaneously. Three seconds. The building holds for 1.5 seconds — higher than outside, lower than the lobby's single-frequency hold. At 0.4 Hz below her feet, the door frame vibrates slightly, the same beat frequency she found at Sujin's door yesterday. The unheld frequency finds the edges. The edges are where it lives.
She plays again. 1.5 seconds. A third time. The consistency is the same physics she has been measuring for twenty-two days and is also, today, something else. The building cannot hold this frequency the same way every time. That is what consistency means: not sameness, but the truth of the material.
The composition ended when she ran out of spaces that are new to her. The lobby was not new — she played it on Day 10, alone, both feet inside. What was new today was the one foot outside. What changes when you are not fully in the thing you are measuring. The flag has twenty more days; she will not play them.
She removes the shoulder from the door. It closes slowly on its pneumatic hinge. The lobby is fully interior again. She can feel the building settle: 39.7 Hz, the same frequency it has always been, now without the door's vibration at the threshold. The building does not know the composition ended. The building continues.
Upstairs, Sujin will read the last data point in the interference series: lobby threshold, 1.5 seconds. She will add it to the page of her architectural notebook that has been filling up for twenty-two days without being explained. The data was always in the score. The composition was always invisible inside the building's own ambient. The maintenance system will expire in twenty days and reclassify the anomaly as a resolved variance and delete the flag. After that, the building will hold its frequency for 2.8 seconds in the apartment and 3.1 seconds in the stairwell and 0.4 seconds in the hallway, as it always has, without anyone measuring.
The composition will exist in Sujin's pencil markings and Gu-ship-pal's frequency journal and in the acoustic memory of a building that holds sounds for up to 3.1 seconds before releasing them into its own silence.
She takes the stairs to the sixth floor. The stairwell holds the ambient at 3.1 seconds. She does not play. She walks through the longest hold in the building and listens to it hold itself.