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Jim Dalrymple's Tour of Apple's HomePod Audio Lab: Materials, Custom Rooms, and More - The Mac Observer
Jim Dalrymple was given a tour of Apple's audio lab as part of Apple's HomePod release blitz. He described the custom anechoic chamber built to test the HomePod, development time (6 years), and the fact that it was started as a speaker project, not a Siri product, and materials. It's a good read.