Summary
Introduction
Have you ever wondered how fish hear the sounds we make when we splash into the water or pass by in a motorboat? Now that the summer vacation is here, you might be spending more time in the water: an ideal occasion to investigate how we hear in the water! Maybe you will feel inspired to research how that transfers to aquatic animals, too!
Background
Sound is a wave created by a vibration, or something that moves very quickly back and forth. Vibrations create a sequence of regions of denser and less densely-packed particles in the substance surrounding it. These regions are called compressions and rarefactions respectively. This explains why sound needs a medium; without particles, there cannot be regions where particles are more or less densely packed! It also explains why sound can travel through air, liquids, and solids. Although particles in solids cannot move much, they can move enough to create compressions and rarefactions, so they can transmit sound.
Compressions move through the medium because particles bump into other particles, compressing the next set of particles together. The speed with which these compressions travel through the medium is called the speed of sound. Sound travels faster in denser substances because neighboring particles will more easily bump into each other. Solids have more particles per unit volume than air, so sound travels faster through them.
The same is true for liquids. There are about 800 times more particles in a bottle of water than there are in the same bottle filled with air. Because of this, the speed of sound in water is much higher than the speed in air. For fresh water and air at atmospheric pressure and 20°C, it is about 4.3 times faster!
Sound interacts with boundaries. The surface of the water is one of these boundaries. It reflects almost all sounds that are not too high-pitched back into the water.
In air, we perceive sounds that carry more energy as louder. Sounds traveling through air become less loud quickly as you distance yourself from the source because energy gets lost on the way. Sound travels faster in water, so the energy reaches us at a faster pace. In addition, sound keeps its energy longer when traveling through water—a non-compressible substance—compared to when it moves through air. These are some of the reasons why sound reaches farther underwater. The sound of a humpback whale can even travel across whole oceans!
In theory, energy reaching us at a faster pace and sound keeping its intensity longer should make us perceive sounds as louder when we are submerged. However, the human ear evolved to hear sound outside of water and is not as useful when submerged. Our head, being full of tissues that contain water, also pick up sound when we are under water. Your inner tissues guide it to the inner ear, bypassing the eardrum. How will that influence how we perceive underwater sounds? Do the activity to find out!
Materials
- Bathtub or swimming pool. A very large bucket can work, too.
- Water
- Two stainless steel utensils
- Two plastic utensils
- Small ball that can get wet
- Towel
- Helper
- Floor cloth to cleanup spills
- Other materials to make underwater sounds (Optional)
Preparation
- Fill the bathtub with lukewarm water, or head to the pool.
Instructions
- 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? What if you submerge your ear, or even your head or your whole body in the water?
- 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 submerged your head. Take a deep breath, close your eyes and submerge your head completely or as much as you feel comfortable doing. Listen. Does the sound appear to be louder, or softer? Does it appear to be different in other ways?
- Repeat the sequence but choose the sound of two plastic utensils banging against each other.
- 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. Your partner now listens, and you make the sounds.
- Discuss the findings you gathered. Do patterns appear? Can you conclude something about how humans perceive sounds when submerged?
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, but fuller when you had your head submerged? That is expected.
Sound travels faster in water compared to air because water is denser, so the energy it carries is transported faster. This should make the sound appear louder. You probably perceived it as softer when you were not submerged because the water surface is almost like a mirror for the sounds you created. The sound probably 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 under-water sound and conduct it to the inner ear. It also explains why closing your ear canal makes almost no difference in the sound you pick up while you are under water.
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. Since sound travels faster underwater, and you pick up sound with your whole head when you are submerged, your brain loses the cues that normally help you determine where the sound is coming from.
Ask an Expert
Cleanup
Additional Resources
- Discovery of Sound in the Sea, by University of Rhode Island and Inner Space Center
- Make Sprinkles Vibrate with Sound, from Science Buddies
- Make a Rubber Band Guitar, from Science Buddies
- Build a Disk Siren, from Science Buddies
- Make Your Own Harmonica! from Science Buddies
- Build a Paper Speaker, from Science Buddies
- Can You Hear Sounds in Outer Space? from Science Buddies
- Sound Localization, from Science Buddies
- Science Activities for All Ages!, from Science Buddies
