by Tara Tiger Brown

March 12, 2017

Think about the last time you had your ears underwater. What did you hear? Most likely, not much. Our ears aren’t designed to hear underwater. Scientists use a device called a hydrophone, which is an underwater microphone, to pick up underwater sounds and reproduce them clearly for humans to hear. A hydrophone in the ocean can pick up sounds from whales and dolphins as much as it can pick up sounds from passing boats.

Waveform Diary

Cartoon by The Waveform Diary

A technique called sonar uses hydrophones to map the ocean floor and locate underwater objects. You may have heard that dolphins can navigate using echolocation. They emit sounds and listen for the reflection of the sound wave off nearby objects. People use the same technique by using hydrophones to capture reflected sound and determine the distance of underwater surfaces.

Just imagine all the sounds around you that you haven’t heard before! With a hydrophone, you can explore your environment in new and exciting ways.


Compare Sound Waves In Air To Water

Our ears are designed to pick up vibrations from the air around us. Since water is much denser than air, our ears don’t pick up those vibrations well. In fact, any sound you hear underwater is more likely due to your skull vibrating than your ears actually hearing anything.

However, sound actually travels faster in water than in air! About four times faster actually. The density of water makes it easier for vibrations to pass from molecule to molecule. And yes, the denser the material, the faster sound can travel through it. Have a friend stand on one end of a long metal rod, such as a bicycle rack, while you hold your ear close to the other end. You might notice that you hear a ringing through the metal before you hear the sound in the air.

Also, sound doesn’t pass easily between air and water. When you’re underwater, you might hear sounds that start underwater, but you probably can’t hear much from someone talking above the water. The person outside the water also can’t hear the sounds that you heard underwater.

Experiment With Underwater Sounds

I know we said you can listen to underwater sounds without getting wet, but this is an easy and fun experiment. Fill your bathtub with water. Have a friend uses objects of different materials to tap on the outside of the tub while you listen using your hydrophone. Now, listen with your ear underwater and compare the sounds you are able to hear. You’ve just made your first step to discovering a world of underwater sounds you haven’t heard before.

With a hydrophone, you’ll be able to hear what the fishes hear. You might be surprised just how much noise small animals make! Ever heard of snapping shrimp? These tiny creatures shoot out water jets that result in the formation of bubbles that pop to release a loud crack. The sound is actually one of the loudest produced by any animal. Get an idea of what bubbles popping underwater sound like by placing your hydrophone inside a cup of soda.

If it gets below freezing in the winter where you live, try listening when the ice is melting and cracking. Since ice floats, it produces sounds both above and below water as it moves around and changes from ice to water. If you don’t have a nearby water body that freezes over, just put out a bucket of water overnight and let it melt the next day.

  • recording from a river/stream
  • recording of small animals (e.g. snapping shrimp)
  • recording from open ocean
  • recording of large animals (e.g. whales)
  • recording from ocean with mechanical noise pollution

Try It As A Contact Microphone

Even though a hydrophone is optimized for listening to underwater sounds, it can be used outside of water as well. A Contact Mic or Pickup uses a piezo that senses audio vibrations through contact with solid objects such as glass, wood, and concrete. Listen on a nearby surface and see if you can figure out the source of the sounds you hear. Remember that the denser the material, the faster sound will travel, so the source of what you are hearing might actually be pretty far away!

How Scientists Use Hydrophones

It turns out that humans do a lot of things that produce underwater sounds, such as operate boats and other machinery. It’s important for scientists to listen to these sounds so that they know when human activity is affecting the lives of animals that live underwater and also to make important decisions. For example, if a hydrophone on a shipping lane picks up noises that sound like whales, a shipping director can direct ships to wait while the animals finish crossing.

Hydrophones are used for numerous environmental monitoring and measurement techniques.

  • Changes to ocean currents can be detected by comparing two recordings at the same location.
  • With global monitoring information, we can find out how the ocean climate has been changing.
  • Hydrophone recordings in rivers are combined with river height readings to learn more about the river bed.

Scientists such as marine biologists also use hydrophones to study animal behavior. For example, we know that whales produce sounds at very low frequencies while they swim and migrate. With more studying, scientists can figure out what different sounds mean. Marine biologists also track the movements of keystone species, such as sharks. Using RFID to locate sharks in the ocean, scientists then follow them by boat and use hydrophones to gather information about shark habits and patterns.

A Duet With A Whale

According to Dr. David Rothenberg, professor of Philosophy and Music at the New Jersey Institute of Technology, in the US, there is a definite possibility that whales do appreciate music. The professor, who is himself a clarinetist, embarked on a preliminary investigation in January-February 2007, when he played a live gig to a group of humpback whales off the coast of Maui, Hawai’i. The music was relayed to the humpbacks (which are renowned for their ‘singing’ abilities) via a waterproof aquatic speaker system, and the resulting ‘duets’ were recorded via hydrophones for subsequent spectrographic analysis. 

Whale Duet

Become A Citizen Scientist

With your new tool, you can also contribute to science. Take your hydrophone and check out the underwater sounds produced in your local areas. Share your recordings and your observations with a connected community of scientists who might be able to identify your sounds. It’s especially interesting when you come across a sound that you don’t expect! Altogether, data from you and people like you will build a more complete picture of our natural environment.

Underwater Microphone Kit

About the author 

Tara Tiger Brown

Tara Tiger Brown is a technologist and author exploring the intersection of the environment, education, and wellbeing. As founder and CEO of KitHub, she develops STEAM education programs focused on hands-on learning and environmental monitoring.

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