Stephen P. Tarzia

Sonar Power Manager

For better power-savings from your computer display

Computer Operating Systems typically try to save power by turning off the display after some keyboard/mouse activity timeout: maybe 10 minutes. Wouldn’t it be better if the computer actually knew when you were at the computer and when you have left?


The Sonar Power Manager uses a new user presence detection technique to shut off the computer display when you are not present. It was developed by myself and other researchers at Northwestern University and University of Michigan. The goal is to shut off the screen immediately after you leave, even if you just moved the mouse 10 seconds ago.

It uses a very simple type of sonar system to accomplish this:

The Sonar Power Manger uses ultrasound (sound above 20kHz in frequency and which humans cannot hear) on your computer’s regular sound system. Surprisingly, we have found that many laptops can use ultrasound. However, many laptops cannot. For these unlucky users, the Sonar Power Manager will not properly detect user presence. Other unlucky users will find that their speakers produce annoying lower-frequency harmonics or noise while the sonar is operating.


The blue line is the history of sonar readings, taken about once every second. The black line is the average of the last ten consecutive sonar readings (in other words it is a smoothed version of the blue line). The red line is the presence threshold. When the black line dips below the red line, the display is shut off.

  1. Calibration: After ten readings are taken, the presence threshold is set to one half of the blue line average.

  2. Activity: When the user moves the mouse or types a key, the sonar stops doesn’t start again until the user is inactive.

  3. Absence: In this example, the user walked away here.

  4. Sleep: Several seconds after the user left, the screen is shut off. Sonar resumes after the user has both woken up the screen and become inactive again.

Technical details

The following technical publications describe our sonar presence detection system in detail:


The software works on MS Windows (2000, XP, Vista, 7) and also on Linux. However, to use it on Linux you must compile from source.

Hardware requirements include speakers and a microphone; these are built into most modern laptop computers.


Click here to proceed to the download page (link is at the bottom).

SonarPM press coverage