A wheel within a wheel

This is a convent wheel, currently in the Santa Chiara museum in Naples. I’d never seen one before. It was used in the convents of closed orders, whenever communication with the rest of the world was required. The nun would stand on one side of the wheel and speak into the drum. The sound would travel into the other half of the drum and out of the opening on the other side. I was there with my sister, we tried it and it works extremely well, though it wasn’t clear to me why it should be a wheel rather than, say, a wall, unless the idea was that it could be spun round to accommodate particularly tall or short petitioners. Although presumably the nuns were accustomed to kneeling… The wall of the cloister closest to the museum contains scenes from the martyrdom of St Catherine. I wonder if the wheel ever reminded the cloistered nuns of the rather gruesome images outside.