10.3. Sound Needs a Medium for Propagation

When you call up your friend who is standing at a distance, your friend is able to hear your voice. How does the sound propagate or travel to her?

Sound travels through a medium, which could be air, water, or solid objects. When you speak, your vocal cords vibrate, creating sound waves that move through the air. These waves then reach your friend's ears, allowing them to hear your voice.

Imagine throwing a stone in a pond. The ripples that form are similar to how sound waves travel through the air. Just as the ripples move across the water, sound waves move through the air (or other mediums) to reach another person.

Activity 10.7 Take a metal or glass tumbler. Make sure that it is dry. Place a cell phone in it. (Remember that the cell phone must not be kept in water.) Ask your friend to give a ring on this cell phone from another cell phone. Listen to the ring carefully. Now, surround the rim of the tumbler with your hands. Put your mouth on the opening between your hands. Indicate to your friend to give a ring again. Listen to the ring while sucking air from the tumbler. Does the sound become fainter as you suck air?

In this activity, when you place your hands around the tumbler and suck air out, you are creating a partial vacuum. Sound travels best through mediums like air, and when there is less air in the tumbler, the sound waves have a harder time propagating, making the sound fainter. When you remove your mouth, allowing air back in, the sound becomes louder again.

Think of listening to your voice in a tunnel versus an open field. In a narrowed space (like the tumbler), sound may not travel as effectively if there is less air, akin to how sounds echo and resonate differently in large, open spaces compared to confined areas.

Activity 10.8 Take a bucket or a bathtub. Fill it with clean water. Take a small bell in one hand. Shake this bell inside the water to produce sound. Make sure that the bell does not touch the body of the bucket or the tub. Can you hear the sound of the scratching? Ask your friends around you if they were able to hear the same sound?

When you shake the bell underwater, the sound it produces travels differently compared to when it is in air. Water is denser than air, allowing sound waves to move more effectively. This shows that sound can travel through liquids; however, it's important to note that the medium affects how the sound is perceived.

Think of how you can hear someone talking clearly while underwater but find it hard to hear outside. This is because sound travels faster and more efficiently through water than it does through air. It's similar to how you can hear someone shouting outside on a clear day much more easily than when you're in a noisy room.

Activity 10.9 Take a metre scale or a long metal rod and hold its one end to your ear. Ask your friend to gently scratch or tap at the other end of the scale. You can also perform the above activity by placing your ear at one end of a long wooden or metallic table and asking your friend to gently scratch the other end of the table.

In this activity, when you scratch or tap the rod, the sound vibrations travel through the solid material. Sound travels faster through solids than gases or liquids because the particles in solids are much closer together, allowing sound waves to transfer energy more effectively from one particle to the next.

Imagine the way you might hear a friend tapping on a table more clearly by putting your ear directly on the surface. The vibrations travel through the table (the solid), allowing you to perceive the sound once it reaches your ear, just like how a child hears music through the ground when a band plays on the street above.

We find that sound can travel through wood or metal. In fact, sound can travel through any solid. You can perform interesting activities to show that sound can also travel through strings. Have you ever made a toy telephone?

Overall, we understand that sound is produced by vibrating objects and travels through different mediums: gases, liquids, and solids. Each medium allows sound waves to propagate, but the efficiency may vary based on density and particle arrangement.

Just like how we can talk through a string telephone made from two cups and a string, sound can travel effectively through these materials. The string acts as a medium that transports the vibrations from one cup to another, demonstrating that sound needs a medium to travel.

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