What happens when two sound waves overlap?
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After you watchWhat happens when two sound waves overlap?
The short answer
Sound waves add as pressure changes. Equal waves at opposite phase can cancel at a listening position. Unequal waves leave a remainder, even with perfectly opposite timing.
The whole story
How it works
The practice uses equal waves. The hidden test uses amplitudes 2 and 1. Its combined amplitude is sqrt(5 + 4 cos phase), falling from 3 at matched phase to 1 at opposite phase. This is pressure amplitude, not perceived loudness.
What people get wrong
Opposite timing alone does not guarantee silence: the pressure amplitudes must match too. Doubling pressure amplitude is not the same as doubling perceived loudness.
The catch
Canceling sound really works, but it's picky. The opposite wave has to match almost perfectly — move it a little and the silence turns back into sound. A steady hum is easy to mirror and cancel, which is why noise-canceling headphones quiet a low engine rumble well, but voices and music change every instant, so there is no single opposite wave that can erase them all.
Questions kids ask
What happens when two identical sounds overlap?
Sound waves add as pressure changes. Equal waves at opposite phase can cancel at a listening position. Unequal waves leave a remainder, even with perfectly opposite timing.
What is different in the hidden experiment?
Explore equal-wave addition, then predict an unshown unequal case: pressure-wave heights 2 and 1 are slid to opposite phase. Watch the exact sum, meter, and optional sound leave height 1 instead of silence.
What limit should I keep in mind?
Opposite timing alone does not guarantee silence: the pressure amplitudes must match too. Doubling pressure amplitude is not the same as doubling perceived loudness.
Talk about it
- Before running the hidden experiment, explain your prediction.
- Compare the final picture with your prediction. Which visible change supports your explanation?
- What did the experiment keep fixed?
For grown-ups
The practice uses equal waves. The hidden test uses amplitudes 2 and 1. Its combined amplitude is sqrt(5 + 4 cos phase), falling from 3 at matched phase to 1 at opposite phase. This is pressure amplitude, not perceived loudness. Opposite timing alone does not guarantee silence: the pressure amplitudes must match too. Doubling pressure amplitude is not the same as doubling perceived loudness.
Keep going
What else makes you wonder?
- If two sounds can cancel into silence, can two beams of light ever cancel into darkness?
- Where in a room would the silence land — does it move if you tip your head a little?
- Could you cancel a really high squeak as easily as a low rumble, or does pitch change how hard it is?
