The ~ of an event is the number of times it happens during a particular period.
The ~ of Kara’s phone calls increased rapidly...
The tanks broke down with increasing ~.
In physics, the ~ of a sound wave or a radio wave is the number of times it vibrates within a specified period of time.
You can’t hear waves of such a high ~.
...a ~ of 24 kilohertz.
...low ~ waves.