As part of our “fun sound projects for kids” series, this week we listen in on sound transmission and propagation.

A big part of what we do here is review Environmental Impact Statements and provide “technical comments” on their proposed actions – making sure that the impacts expressed align with the most up-to-date peer-reviewed science, and that the models used to determine those impacts are mathematically accurate.

In our case the models are the “propagation models” used to determine how far, and how loud a sound is from the source – whether it is a seismic airgun array, or a high frequency communication sonar.

Sound propagation, with its cute twin sister, sound transmission helps us determine if a noise source exposes marine life to noise levels that exceed regulatory thresholds. As our concern is the ocean, most of our work deals with propagation characteristics of seawater and its surroundings.

But sound propagates in different materials in different ways, so this week’s social distancing “fun things to set the kids on while you try to work from home” involve exploring sound transmission in various materials.

This is all stuff you can do with items found around the house – although I was not able to secure paper cups given that I eschew disposability. But I believe that you can find them at coffee take-outs. And aren’t coffee joints “essential services” anyway?

Here is to hoping you are staying well, and even keeled.

Little Red Riding Hood getting long distance instructions from the Wolf.

When we think of sound, we usually think of sound in air, because we usually hear sound in air through our ears – which are specifically adapted to hear airborne sound. But if you were a termite, or a mole, you might be hearing sounds in wood, or soil, not through ears, but through your feet! And if you were a fish, or a whale, where do you think you might hear sounds?

What is perceived as sound is actually acoustical energy that can transmit, or propagate through any material. Ocean Conservation Research is focused on sounds (and noise) in the ocean. Because sunlight does not penetrate into the ocean depths, ocean animals rely on sound more than they rely on light to sense their surroundings, so sound propagation (and transmission) in seawater is central to our work.

These experiments take advantage of the sound propagation characteristics of various materials under various conditions.

Cup Telephone

This is a good experiment for “social distancing!” When you talk to a friend, your voices travel from your mouths through the air (a gas) to each other’s ears. What happens when you connect your mouths and ears with a piece of string (a solid)? What happens when he string is not taut, and why?

Materials for Sound Experiment

• Two paper or plastic cups (small yogurt containers are durable)
• Scissors
• String
• Tooth pick
• A friend

Procedure for Sound Experiment

1. Using the point of the scissors, poke a hole in the middle of the bottom of each cup. (Depending on your age, it may be better to have an adult do this step.)
2. Stand a few feet away from a friend and talk to each other in normal (not yelling) voices.
3. Keep moving apart until it is hard to hear each other.
4. Hold one of the cups up to your mouth and speak into it while your friend listens into the other. Now can you hear each other?
5. Next cut a length of string long enough to stretch the distance between you and your friend when it was hard to hear each other.
6. Poke the ends of the string through the holes in the bottoms of the cups (poke it upwards through the bottom), tie the string in a knot around a small section of the toothpick.
7. Hold onto one cup, have your friend hold onto the other, and walk away until the string is taut (pulled straight, not sagging).
8. Talk into one cup while your friend holds the other cup over her ear. (Remember to keep the string taut.) Switch. Now can you hear each other through the cups? (Remember to move the cups from your mouths to your ears and back depending on if you’re listening or speaking.) Why do you think attaching the cups with the string allows you to hear each other?
9. More fun with this project! Try it with longer and longer pieces of string. How far apart can you get and still have the “phone” work? What if someone grabs onto the string while you’re talking?

Explore the science of sound with a just a wire hanger and some string! Can you hear the difference between materials when you knock the coat hanger against wood, brick, metal, or seat cushions? Why would that be?

What’s Going On

Sound from your voice propagates in air in all directions, so the acoustical energy in your voice dissipates in all directions and thus attenuates pretty rapidly. But if you capture the sound of your voice in a cup, it contains it, and the bottom of the cup acts like a drum head or diaphragm and vibrates. This vibration is transferred to the string and knotted toothpick section and transmitted down the length of the string. This in turn vibrates the bottom of the remote cup where the listener can hear the sound vibrations.

Coat-Hanger and String-A-Phone

Materials for Sound Experiment

• Wire hanger
• String

Procedure for Sound Experiment

1. Tie the hook of a wire hanger to the center of a large piece of string. (About 3 feet long)
2. Stretch the string over the tips of your index fingers.
3. Now place the tips of your fingers into your ears, with the string inside your ear duct.
4. Lean over and swing the hanger so that it taps against a table or door. What do you hear?

What’s Going On

Sound waves are created by the vibration of an object (the wire hanger and string).  When vibrations hit your ear drum, your brain interprets the vibrations as sound. The sound waves can travel through air, liquids, and solids. When we listen to the hanger hit an object with the string poked into our ears, the sound waves are traveling through the solid string and hanger and transmitted to your eardrums. Since sound waves travel more quickly through solids, we hear the sound more clearly. When we bang the hanger without putting the string to our ears, the sound waves are traveling through air to get to our ears making the sound quieter.

What happens when you tap the hanger on wood, or brick, or a couch? Why?

Physics at Your Desk: Drumming Finger

Materials for Sound Experiment

•             Quiet room
•             Your finger
•             Table or desk

Procedure for Sound Experiment

1. In a quiet room, tap your finger gently on the surface of a desk or table. Pay attention to how loud the noise is.
2. Now put your ear down on the desk and continue gently tapping your finger. Your finger should stay about the same distance away from your ear as it was when you tapped the first time. How is the noise different?
3. Do the same experiment on a wall, the side or edge of a bathtub, the floor, the sofa. What differences do you hear? Why?

What’s Going On

Sound is produced when something vibrates. Guitars and pianos produce sound when their strings vibrate. Drums make sound when their drumheads vibrate. And the sound of your favorite singer is produced when his or her vocal cords vibrate.

Vibrating strings, drumheads, and vocal cords bump into nearby air molecules and cause them to vibrate too. Then these molecules bump into other molecules close by and cause them to vibrate. In this way the vibrations spread out through space like ripples in a pond, by way of sound waves.

Sound doesn’t just travel though air; it can travel through solids, liquids, and other gases too. When you tap the desk you cause the molecules in the desk to vibrate and the vibrations travel through the desk like they do through the air.

How well – and efficiently sound transmits through various materials is correlated to the properties of the material. The speed of sound in a material is dependent on density. The efficiency of sound transmission in a material is dependent on compressibility, or “elasticity” (how readily a material returns to its original shape).

Sound propagates in air at approximately 1,100ft//second. It propagates in water at approximately 5,000ft/second, wood at 8,000 –12,000ft/second, and steel at ~19,000ft/second.

You could easily make a sound in water that can be heard 100 miles away. It would be very hard to make a sound in air that could be heard 100 miles away.

Credit: Physics Central

• Learn about Ocean Noise
• What is Ocean Noise Pollution?
• How Do Fish Hear?
• What are Decibels?
• What is Bioacoustics?
• Explore Sounds of the Bay
• Listening to the Ocean: Reading Audiographs
• Deploying Hydrophones
• Activities: Play with Sound
• Timeline: The Roots of Understanding Sound

Want to know more about sound?

“Hear Where We Are” by Michael Stocker