Key Concepts
Physics
Sound
Waves
Biology

Introduction
Have you ever listened to noises underwater? Sound travels differently in the water than it does in the air. To learn more, try making your own underwater noises—and listening carefully. 

Background
Sound is a wave created by vibrations. These vibrations create areas of more and less densely packed particles. So sound needs a medium to travel, such as air, water—or even solids. 

Sound waves travel faster in denser substances because neighboring particles will more easily bump into one another. Take water, for example. There are about 800 times more particles in a bottle of water than there are in the same bottle filled with air. Thus sound waves travel much faster in water than they do in air. In freshwater at room temperature, for example, sound travels about 4.3 times faster than it does in air at the same temperature.

Sound traveling through air soon becomes less loud as you get farther from the source. This is because the waves’ energy quickly gets lost along the way. Sound keeps its energy longer when traveling through water because the particles can carry the sound waves better. In the ocean, for example, the sound of a humpback whale can travel thousands of miles!

Underwater sound waves reaching us at a faster pace and keeping their intensity longer seem like they should make us perceive those sounds as louder when we are also underwater. The human ear, however, evolved to hear sound in the air and is not as useful when submerged in water. Our head itself is full of tissues that contain water and can transmit sound waves when we are underwater. When this happens, the vibrations bypass the eardrum, the part of the ear that evolved to pick up sound waves in the air. 

Sound also interacts with boundaries between two different mediums, such as the surface of water. This boundary between water and air, for example, reflects almost all sounds back into the water. How will all these dynamics influence how we perceive underwater sounds? Try the activity to find out! 

Materials

  • Bathtub or swimming pool (a very large bucket can work, too)
  • Water
  • Two stainless steel utensils (for example, spoons or tongs)
  • Two plastic utensils
  • Small ball 
  • Towel
  • Adult helper
  • An area that can get wet (if not performing the activity at a pool)
  • Floor cloth to cleanup spills (if not performing the activity at a pool)
  • Other materials to make underwater sounds (optional)
  • Access to a swimming pool (optional)
  • Internet access (optional)


Preparation

  • Fill the bathtub with lukewarm water—or head to the pool—and bring your helper and other materials.


Procedure

  • Ask your helper to click one stainless steel utensil against another. Listen. How would you describe the sound? 
  • In a moment, your helper will click one utensil against the other underwater. Do you think you will hear the same sound? 
  • Ask your helper to click one utensil against the other underwater. Listen. Does the sound appear to be louder or softer? Is what you hear different in other ways, too?
  • Submerge one ear in the water. Ask your helper to click one utensil against the other underwater. Listen. How would you describe this sound? 
  • Ask your helper to click one utensil against the other underwater soon after you submerge your head. Take a deep breath, close your eyes and submerge your head completely or as much as you feel comfortable doing. Listen while you hold your breath underwater (come up for air when you need to!). Does the sound appear to be louder or softer? Does it appear to be different in other ways? 
  • Repeat this sequence but have your helper use two plastic utensils banging against each other instead.
  • Repeat the sequence again, but this time listen to a small ball being dropped into the water. Does the sound of a ball falling into the water change when you listen above or below water? Does your perception of this sound change? Why would this happen? 
  • Switch roles. Have your helper listen while you make the sounds. 
  • Discuss the findings you gathered. Do patterns appear? Can you conclude something about how humans perceive sounds when submerged in water? 
  • Extra: Test with more types of sounds: soft as well as loud sounds, high- as well as low-pitched sounds. Can you find more patterns?
  • Extra: To investigate what picks up the sound wave when you are submerged, use your fingers to close your ears or use earbuds when submerging your head. How does the sound change when you close off your ear canal underwater? Does the same happen when you close off your ear canal when you are above water? If not, why would this be different? 
  • Extra: Go to the swimming pool and listen to the sound of someone jumping into the water. Compare your perception of the sound when you are submerged with when your head is above the water. How does your perception change? Close your eyes. Can you tell where the person jumped into the water when submerged? Can you tell when you have your head above the water?
  • Extra: Research ocean sounds and how sounds caused by human activity impact aquatic animals. 

Observations and Results
Was the sound softer when it was created underwater and you listened above the water? Did it sound muffled when you had only your ear submerged? Was it fuller when you had your head submerged? 

Sound travels faster in water compared with air because water particles are packed in more densely. Thus, the energy the sound waves carry is transported faster. This should make the sound appear louder. You probably perceived it as softer when you were not submerged, however, because the water surface is almost like a mirror for the sound you created. The sound most likely almost completely reflected back into the water as soon as it reached the surface. 

When you submerged only your ear, the sound probably still appeared muffled. This happens because the human ear is not good at picking up sound in water—after all, it evolved to pick up sound in air. 

When you submerged your head, the sound probably sounded fuller. That is because our head contains a lot of water, which allows the tissue to pick up underwater sound—without relying on the eardrum. It also explains why closing your ear canal makes almost no difference in the sound you pick up while you are underwater. 

If you tried to detect where the sound came from when submerged, you probably had a hard time. Our brain uses the difference in loudness and timing of the sound detected by each ear as a clue to infer where the sound came from. Because sound travels faster underwater and because you pick up sound with your entire head when you are submerged, your brain loses the cues that normally help you determine where the sound is coming from. 

More to Explore
Discovery of Sound in the Sea, from the University of Rhode Island and the Inner Space Center
Can You Hear Sounds in Outer Space?, from Science Buddies
Talk through a String Telephone, from Scientific American
Sound Localization, from Science Buddies 
Ears: Do Their Design, Size and Shape Matter?, from Scientific American
STEM Activities for Kids, from Science Buddies 

This activity brought to you in partnership with Science Buddies

Science Buddies