You may've heard of these before . . . but some parks in Philadelphia have been installing sound machines that play a high-pitched sound all night.
Why? Younger people . . . like people under 25 . . . can hear the sound and they HATE it. Older people can't really hear it.
And the theory is that it keeps teenagers out of the parks at night.
But some people are criticizing the city for putting the noise machines in their parks, saying it discriminates against teenagers.
The city has already got them in 30 parks and they cost $5,000.
Now let's see if you can hear the sound that bothers teens....