Asound wave travels through air. in which regions are air molecules the least dense?

The atmospheric pressure is an indicator how dense is the air. at higher altitude where the atmospheric pressure is low is where the air is least dense. one proof is that mountain climbers needs oxygen tank at higher altitude because air is less so as well as the oxygen present

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.