Parasites, they do not synchronize well, they do not charge well, and overall they are not worth it. Money wasted for anyone who buys them.

STAY AWAY!!
If you want to listen without others hearing, simply DO NOT buy them.

If you increase the volume above 30%, then all the sound comes out into the room. Imagine it like the old radio of your grandfather. It can be heard outside.

Also, the volume above 70% distorts.

And these two things, only during the first hour of use.. (after that, I gave them as a gift to a friend of mine.. :) )

When the source is at a very low level, the headphones become disconnected, auto shut off or something like that. They don't completely shut down, they just emit interference. This is not a manufacturing problem but supposed to save battery!
A month after I bought them, they stopped charging. The service kept them for 3-4 months and returned them saying there was no problem. They did work fine when we tested them in the store except for the power plug that had lost some insulation and didn't always make contact. I didn't even bother sending them back.
Stay away...