What Do You Hear Under Water?
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!
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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!
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 someone jumping into the water. Compare your perception of the sound when you are submerged to 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 due to 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, 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.
If you made a mess, do not forget to clean up the spills.
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Sabine De Brabandere, PhD, Science Buddies
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