Ear picked up the stimulus from vocal vibrations. When you hit a vibration fork, the vibrations move back in forth and creates sounds that the ears pick up. The air molecules that are right next to the fork collide when being hit, it creates pressure and at each vibration it tries to spread everything about. So you see pressure air molecules being put under pressure and being released from that pressure then it comes back. If you are under water you will hear the same sound. There is limitation to thiswhen you are under water the distance and speed will be reduced. The sin graph represents the pressure going up and down. Notice the repetition of the graph, we would like to know how many times it repeats itself in a given period of time: usually measured in a second. Frequency is this characteristic (repetition or cycle per sec: cycle per second which can be called hertz.