I've mentioned the Bluetooth technology...

Phoenix Freeze

...(of Phoenix BIOS) that locks a computer based on the proximity of a paired Bluetooth enabled phone. The limitation this product presents is that its enabling technology. I've never seen a Windows desktop PC with Bluetooth (although all current iMacs have it). And, even many netbooks and notebooks are Bluetooth-less.

Stephen Tarzia presents an alternative (and apparently Open Source) solution based on sound, specifically very high frequency sound...

Stephen P. Tarzia: Sonar Power Manager

Sonar Power Manager requires a notebook computer capable of emitting sound above 20kHz. Most humans can't hear this high frequency sound. The software uses this high frequency sound and the notebook's microphone like a sonar. The sound is emitted through the notebook's speakers, is bounced off of the human body (you), and detected by the notebook's microphone. Its function is to shut off the notebook's screen as soon as you move away from the notebook (echo reflection off of you stops).

There are a couple of problems with this though:

- As the author notes, note all notebooks are capable of creating 20+ kHz frequency sounds
- He also notes that some people hear annoying lower frequency harmonics of the 20 kHz wave

To this I will add this speculation:

- I wonder if animals that can hear very high frequency sounds (like dogs) will be annoyed by this?
- I wonder if teenagers and young children who may also be capable of hearing this frequency or its harmonic will be annoyed by the sound?

Sonar Power Manager is available as a free Windows installation file as well as in source code form for compilation under Linux or Windows.

Via SlashDot