CNN is reporting on a Thai computer programmer who is selling a
computer program that is supposed to repel mosquitos and rats.
The program supposedly generates sound waves that the little disease bearing blighters can’t stomach. It’s absurd of course.

Your typical PC cannot reproduce ultrasonic sounds. The sampling rates of sound cards usually maxes out at 48khz, making 24khz the highest frequency that is possible to be emitted. Typical sound cards have analog filters in their output stages to limit actual emitted
frequencies to probably around 19khz, which is right at the limit of
human hearing. Ultransonic bug repellers are supposed to emit frequencies in the 20khz to 50khz range, which is impossible for your sound card to emit. Don’t believe me? Well, send me $50 then and I’ll send you my bug repelling program. It’s guaranteed to be just as effective as the one listed by CNN, and it even works when your PC is off!

The real reason to scoff at stuff like this is that despite the popularity of such devices they don’t work. In virtually every scientific test conducted, ultrasonic repelling devices do absolutely nothing to repell insects. Their claims about repelling rodents are also extremely suspect. You can read the
Straight Dope on the subject, or a researcher at Rutgers.