Headphones started out as simple devices: just two mini speakers connected by a cable. But over time, they’ve added microphones, control buttons, and Bluetooth chips. And those chips have become increasingly advanced, allowing manufacturers to add features like adaptive active noise cancellation, heart-rate tracking, body-temperature measurement, location tracking, onboard voice controls, AI-powered transcription and translation, audiogram-based sound adjustment, and even brainwave-activity monitoring.Many headphones and earbuds we’ve recently tested — including the AirPods Pro 3, JBL Tour One M3, Google Pixel Buds Pro 2, Beats Powerbeats Pro 2, and EarFun Air Pro 4 — utilize these types of advanced features. And these functions collect data and send it to your phone, often to be processed through the headphone manufacturer’s app.And that’s where things get complicated. Where does that data go, and who controls it?