We can now come back to our definition of what resonance is:

Resonance: If a periodic force is applied to a system, the amplitude of the resulting motion is greater than normal when the frequency of the applied force is equal to or nearly equal to the natural frequency of the system.

Our simple pendulum system only had one natural frequency, whereas now we have standing-wave systems, such as guitar strings, which can have a whole set of natural frequencies. We can drive these systems with a force equal or nearly equal to the natural frequencies to increase the amplitude of motion or sound volume.

We can demonstrate this with two tuning forks of equal frequency.

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