A team of student hackers have demonstrated a method for using music to turn smart devices into tools for spying. The system is based on sonar, and embeds an inaudible signal into songs played on a smartphone or TV. The system can then use the device's microphone to listen to how the signal bounces, and track the movements of anyone near the audio source.
The University of Washington research team behind the technology, known as CovertBand, tested it using a 42-inch Sharp TV in five different Seattle homes.
They found that the method is able to track the physical movements of multiple people to within 18 centimeters of accuracy, and even differentiate between particular gestures and motions. The tech can also track people, though less accurately, through walls.
Get Data Sheet, Fortune's technology newsletter.
They also demonstrated that listeners couldn't distinguish between songs containing the hidden sonar signals, and those without it.
As far as getting the malicious code onto a device, that seems like the easy part. Smart devices have proliferated in recent years, and they've proven remarkably vulnerable to hacking. To a somewhat lesser degree, so have smartphones - and all CovertBand needs to work is a speaker and a microphone.