People can now be detected by some Echo speakers.

Ultrasound-based motion detection is coming to fourth-generation Echos and Echo Dots.

Newer Echo and Echo Dot speakers will be able to use ultrasound to detect occupancy in your home and turn on and off other connected devices like lights and your Fire TV starting this week. The fourth-generation Echo and Echo Dot speakers can now emit a “inaudible ultrasound wave” to detect if there are people present in a room, as announced at Amazon’s fall hardware event in September.

In the Alexa app, you can enable or disable this feature, as well as set up occupancy routines to use this new capability to do things like turn on lights when you enter a room and turn them off when the room is empty. You could also have Alexa play music or a radio station when motion is detected near an Echo device for a certain amount of time, then turn off the music when you leave.

For each compatible Echo device, the function is listed under Motion Detection in the Alexa app settings, where you can turn it on or off.

This feature is similar to the motion-sensing capabilities of the newer Echo Show devices, but those rely on their cameras to determine whether or not there are people in the room. Because the Echo speakers lack cameras, they detect movement by emitting an inaudible ultrasound wave that bounces off nearby objects before returning to the device’s microphones.

In its Nest smart displays and Nest Mini speakers, Google uses ultrasound to detect how close a person is to the display or speaker and present different interfaces. Volume controls, for example, will be displayed on the Mini speakers. You can’t use ultrasound sensing to activate Google Home Routines right now.

The Philips Hue, Aqara, and Centralite motion sensors are among the stand-alone motion sensors that work with Alexa to trigger Routines. This new feature means you won’t have to rely on one of these extra devices to turn on your lights or music in theory. It also advances Amazon’s vision of the ambient smart home, in which your home responds to you without you having to do anything.

However, Amazon will have to put more effort into the Alexa app to get there. Setting up Routines to turn lights on at the appropriate brightness based on the time of day and then off when the room is empty still takes a lot of patience and technical know-how.