Sage Gateshead contains three performance spaces; a 1,700-seater, a 450-seater, and a smaller rehearsal and performance hall, the Northern Rock Foundation Hall. The rest of the building was designed around these three spaces to allow for maximum attention to detail in their acoustic properties. Structurally it is three separate buildings, insulated from each other to prevent noise and vibration traveling between them. The gaps between them may be seen as one walks around inside. A special spongy concrete mix was used in the construction, with a higher-than-usual air capacity to improve the acoustic. These three buildings are enclosed (but not touched) by the now-famous glass and steel shell.
Sage One was intended as an acoustically perfect space, modeled on the Musikverein in Vienna. Its ceiling panels may be raised and lowered and curtains drawn across the ribbed wooden side walls, changing the sound profile of the room to suit any type of music. Sage Two is a smaller venue, possibly the world’s only ten-sided performance space. The building is open to the public throughout the day.
Addendum 5 March 2019: Here’s a view from inside.