History: What is filtering?

What is filtering?

• The process of increasing the amplitude of selected frequencies in the spectrum of a sound signal.
  The opposite is true.
• The process of reducing or eliminating the amplitude of selected frequencies in the spectrum of a sound signal.
  Yes, this is correct.
• The technique of varying parameters to sound generators and sound modifiers over time.
  This is modulation.
• Filtering reduces the proportion of the amplitude over a threshold amplitude.
  This is filtering.