Talking Tapes

How does the speed of vibration affect sound?

As we talked about in Activity 1, sound is created by vibration. Changing the speed and intensity of those vibrations changes the sound. In this activity, the sound is caused by running your thumbnail down the talking tape, over the ridges, and vibrating the tape in a certain way. The students will experiment with running their thumbnails down the strip faster and slower, harder and softer so they can see these differences.

This experiment doesn’t just deal with how specific sounds are created but with how we can better hear those sounds. If you run your finger down the talking tape while holding it, you may hear it “talk,” but softly. The tape is vibrating your fingers and the air around it and that vibration is traveling to your ear and your brain hears it as words. When you hold the talking tape, the sound travels out in every direction. Only a small amount reaches your ear. Because the vibration is not very strong, you don’t hear the sound as very loud.

When the talking tape is attached to the PhysicsQuest box, the sound is much louder. When the sound waves come off the talking tape, they don’t just make the air around the tape vibrate, they make the whole box vibrate. When the box vibrates, it can vibrate much more air than the tape on its own. With all that air vibrating, your ear can hear it as a louder sound. Because the box is empty and made of something that reflects sound waves easily, the waves bounce around without absorbing.

This is very similar to plucking a string on a guitar. If you just pluck the string unattached to the guitar body, you will only hear a soft sound but when that string makes the whole guitar vibrate, you can hear beautiful music.

The louder sound created when the tape is held up to the cup or the balloon is made a little bit differently. Sure, there’s the same effect as the guitar, but there is something else going on. When the tape vibrates the air inside the cup, the sound doesn’t just radiate out all over. Instead — just like light waves — it reflects back and forth on the sides of the cup. It keeps doing that till it comes out the top of the cup and to your ear. The sound is guided down the cup and when you hear it, the waves are more intense and your ear hears it as a louder sound. The cup acts as a waveguide. When the tape is held up to the balloon, the waves bounce around inside the balloon just like with the cup. If you look at how the balloon is shaped you can see it is very similar to a lens that you would use to focus light. The balloon is actually focusing the sound waves as they bounce around inside. When you hear the sound, it sounds very loud — just like focused light rays seem very bright. When the tape is held to the book and vibrated, the sound is much quieter.

With the cup, box, and balloon, sound waves were moving around inside and reflecting off the surfaces. Most was in some way making the air around it vibrate. The vibrations you put in were in some way amplified, either by reflecting around, being guided to your ear, or making more air vibrate. With the book, the sound isn’t able to reflect around and amplify — it is absorbed.

Soft materials like cloth or paper are good at absorbing sound because the sound waves make them vibrate but they aren’t stiff enough to make the air around them vibrate. The soft pages of the book absorb the sound so it is much more difficult to hear.

Real world connections -

• Based on what you learned, why do people use earplugs to block out loud sounds?
• When musicians play instruments, they play different notes by adding or lifting fingers to the keypads. Using what was discussed in this activity, why does this change the note?

Suggestions for drawing, illustrating, presenting content in creative ways

• Have students explore different models of visualizing sound waves

Engineering and design challenges connected to the content

• Have students try and soundproof an area of the classroom using materials that could absorb sound waves.