It works against all sound, both in and out. The only thing that makes a difference is how close you are to the source of sound. Someone talking right next to you, you may hear them simply because the sound vibrates air in your body and on your bones. You only hear because a membrane transfers vibration to a bone. So it's near impossible to stop all sound. You can hear without your ears essentially. Plug your ears, you still hear. Vibration transfers. So again, close sounds will be louder simply due to that. Also, some frequencies are going to penetrate and vibrate more than others. You will hear a piercing sound before you hear a low rumble for example. Background noise will be pretty much gone. Get 'em off Amazon. If you don't like how they perform, get a refund. Simple.
Very best,
