Every space has a note. In a concert hall it is an elegant hush; in a tiled bathroom it is a hysterical ring. In a well designed cardboard dwelling the note sits near 25 hertz, and it is the sound of a purr.
We tune each box by resonant testing: a resident sits inside, begins to purr, and we measure the walls. Cardboard dampens the high frequencies, which is lovely, and amplifies the low rumble, which is better. A properly tuned den feels like being hugged by a very small, very expensive engine. Our test cat, Percival, has reached 28 hertz and is considered a national treasure.