The frequency of a sound wave is measured by counting the number of times the wave goes up and down each second. We can illustrate this by counting the peaks that would pass a stationary point in a given unit of time. Let's imagine that our unit of time is 1/100 of a second (10 milliseconds).

The short fork produced a wave with more peaks per unit of time, so we say that wave has a higher frequency, which our ears will hear as a higher pitch.

Graphic of a tuning fork with tall tongs next to a sound wave peaking 4 times during a 10 millisecond cycle. Tuning fork with short tongs next to a sound wave peaking 8 times during a 10 milisecond cycle.

Could you calculate the frequency for the top wave in peaks per second? (Remember that 10 msec equals 1/100 sec.)

Since we have 4 peaks per 1/100 second, there would be 400 peaks every second. The technical term for cycles per second is “Hertz”. So we say that the frequency of the top wave is 400 Hertz (400 Hz).