Reaching resonance frequency: The highest amplitude

You can drive things at a frequency that maximixes the amplitude of vibration. For instance, consider the sound vibration in the speaker-and-tube-setup, which is driven by the speaker. As you increase the frequency of the sound wave from the speaker, you find that the amplitude of the sound vibration peaks whenever the speaker drives at one of the harmonics- that is, one of the frequencies of the ormal modes. So the tube has an infinite number of resonance frequencies.