History: What are phasers and flangers?

What are phasers and flangers?

• Phasers and flangers modulate the amplitude of a sound source.
  That is tremolo.
• Phasers and flangers modify the spectrum of the sound source over time.
  Great work.
• Phasers and flangers modulate the frequency of a sound source.
  That is vibrato.
• Phasers and flangers modulate the measurements of a sound source.
  Read more carefully.