Ultrasonic repellers work by emitting sound waves at frequencies that interfere with an insect's nervous system, making the environment hostile enough that they leave, or stop reproducing.
The science is solid. The problem is in the execution.
Every ultrasonic device you find on Amazon, in pet stores, in supermarkets, almost all of them emit between 9 and 15kHz. That's the hearing range of mice and rats.
The manufacturers aren't lying. Those devices genuinely work on rodents.
But fleas hear between 25 and 66kHz.
For a flea, a device emitting at 12kHz is completely silent. It doesn't exist.
You could have one plugged into every room in your house and they would have absolutely no idea it was there.
It's the equivalent of trying to tune into an FM radio station with an AM antenna. The signal is real. It's just going to the wrong receiver entirely.
This is why the reviews are split.
People with rodent problems swear by them.
People with flea problems throw them away after three weeks.
Both groups are right, they're just using the same product for two completely different problems.