Noise reduction headphones actively produce their own sound waves in order to cancel

If you want to use it to cancel out the external wave, then it has to be exactly the same frequency and exactly 180 degrees out of phase.

mass is always a constant for both kinetic and potential energy..no matter what situation any object is the mass doesnt change..velocity is always the factor that kinetic energy changes with..the lower the velocity the lower the knetic energy is ke=1/2 mv^2 they are directly related..when velocity= to zero then ke=0 however if there was no mass then that means there is no object which means kinetic energy is not present either..but usally its all about the velocity