If two pure tones of different frequencies occur simultaneously they will cause fluctuations in the smooth curve to create a Complex tone.
These noise suppressors are reminiscent of Maxwell Smarts "Cone of Silence," (if you are old enough to remember the TV show "Get Smart.")
On the other hand, I have an unconfirmed report that there is a spot in the U.S. Senate Chambers where sounds are reflected back in phase and amplify confidential whispers made across the room.
What happens if the to two simultaneous pure tones are different frequencies and are integral multiples of each other?
Obviously, they can't cancel each other out. But they do combine in an alternating enhancing and inhibitory fashion.
The end result is that the smooth curve of the sine wave (pure tone) becomes bumpy. If more pure tones are added (being all integral multiples) the curve becomes even more bumpy. This now describes a Complex Tone.
NOTES: Hear some pure tones and other noises.