According to Guinness: 'In tests conducted on 19 November 2021, the ambient sound level inside the room was measured at -24.9 decibels.'
It's built very specifically to reduce any noise reverberations inside. Guinness notes that it's a masonry and concrete chamber lined with 10.5-cm-thick steel plates, with a smaller steel chamber inside, supported by vibration absorbing springs.
All this, and there's also a layer of heavy insulation - glass-fibre wedges that extend 85cm into the room.
This means the room isn't actually very big, and if you go in you need to stand on a suspended mesh over these wedges… More here.
Something about this room really scares me…
I was in one at Bose headquarters in Massachusetts. It’s not scary, but you really can hear your own blood pumping through your veins. It’s actually calming.