How far can sound travel through space, since it’s so empty? Is there an echo in space? – Jasmine, age 14, Everson, Washington In space, no one can hear you scream. You may have heard this saying.

Ask enough sleep nerds about the best sound machines and two very different front-runners will ultimately emerge: the Dohm and the LectroFan. The Dohm, sold by a company called Yogasleep, is a ...