We've seen earphones that double as health sensors, but they frequently require integrated (and sometimes bulky) equipment to get the job done. Bifrostec and the Kaiteki Institute have just solved that problem with processing technology that turns any pair of in-ears into a pulse wave sensor. The approach converts earphones into microphones that listen for eardrum movement, which is usually in sync with the pulse rate of a person's arteries. As the technique compensates for background noise, it lets users monitor their pulses while they listen to music -- and without having to wear head-mounted gadgets. While there's no word on when (or if) we'll see the pulse wave discovery reach shipping products, it's efficient enough that it could slip into conventional mobile devices almost unnoticed. Don't be surprised if a future smartphone can tell you when it's time to relax.