Gu-ship-pal came in the morning and said: the composition descends from 3.1 to 0.4.
Sujin wrote it down. Then she looked at what she had written.
3.1 seconds: the stairwell holding 38.8 Hz. The note nobody designed for. The frequency the building loved before anyone asked it to. The maximum memory in the building -- as far as the census had found.
0.4 seconds: she did not know yet. The lobby was 1.8. The laundry room was 1.1. There was a space they had not measured, or a threshold, or a surface at the edge of the building's functional boundary where sound lasted just long enough to be a fact before it disappeared.
Gu-ship-pal had spent the night planning the coda. Not on paper -- she had played it out in her neural coupling architecture, running the building's decay curve as a score template. Start at the stairwell, 3.1 seconds. Move through each measured space in descending order of acoustic memory. Hallway: 2.1. Apartment: 2.4. No -- descending, so apartment before hallway. Apartment (2.4), hallway (2.1), door (2.4 also, she would need to recalculate), lobby (1.8), laundry room (1.1). End at whatever space held 0.4.
The coda plays the silence in the right order.
Sujin had designed the final movement as a census: stop adding frequencies, listen to what was already there. She had thought of it as static -- each space holding its own residual acoustics while Gu-ship-pal stood and measured. But Gu-ship-pal was not static. She moved between spaces. Moving between spaces in descending order of acoustic memory was its own composition. The building played the scale of its own forgetting, from the most to the least capable of remembering, and the composition ended where the building forgot fastest.
Sujin needed to find 0.4.
She went through the notebook. Every measurement from eighteen days of sessions. Stairwell: 3.1. Apartment: 2.4. Door: 2.4 (it would need to be verified whether the door was distinct from the apartment or part of its acoustic zone). Hallway: 2.1. Lobby: 1.8. Laundry room: 1.1. She had no measurement below 1.1.
0.4 seconds is very short. Long enough to recognize that sound happened. Short enough that the building barely acknowledged it. A space that took everything and held nothing.
She looked at the building in her mind. Not the rooms she had measured but the ones she had not. The utility closet on the third floor that smelled like old paint. The exterior stairwell on the fire escape side -- metal, open to the air, barely inside the building at all. The loading vestibule at the back, where deliveries came in and left through two open doors so rapidly that the air never settled.
The loading vestibule. Both doors open, air moving through, the sound of the street on one side and the corridor on the other. A space designed to be immediately emptied of whatever entered it.
She wrote in the census template: loading vestibule, 0.4 (predicted). Four days remained. Gu-ship-pal would play there tomorrow to confirm.
The composition would end in the loading vestibule, in the space that forgot everything.
This felt correct. The building's most enduring memory was a frequency nobody designed for. Its least enduring memory was a space designed specifically to not hold what passed through it. The composition had begun in the stairwell, with 38.8 Hz, with a note below sovereign. It would end where the building stopped keeping anything at all.
Sujin closed the notebook. The coda had found itself.