History: Which of the following is NOT why we use filters?

Which of the following is NOT why we use filters?

• To shape sounds.
  We often use filtering to shape sounds.
• To balance sounds together.
  We often use filtering to balance sounds together.
• To add spectral content.
  Filters do not, by themselves, add spectral content.
• To bring out new colors.
  We often use filtering to bring out new colors