If we threw a cork out on the waves of a pond, it would rise and fall as they passed under it. From the time it rode one crest until it dipped down and came up on top of the next, would represent one double vibration, and the number of such dips it had to make in a second would show the speed at which the wave was travelling and could be represented in d.v./sec. So, if it made 100 such dips in a second, we would say the waves were travelling at the rate of 100 d.v./sec.