Key concepts

Physics

Sound

Vibrations

Material science

 

Introduction

Have you ever thought about the sheer number of words that exist in the English language to describe sounds? A noise can be a thud, a clang, a bang, a pop, a crash, a splash, a clatter, a buzz, a tinkle and many more! You can probably think of an example for each of these—but if you heard the sound, could you say what produced it?

 

Background

Start to hum and then touch the front of your throat. Can you feel it vibrate? That vibration in your throat also occurs in the air, which your ears pick up and together with your brain translates it into the humming sound you hear. This sequence occurs with any sound you hear: It starts with a vibration, which is carried by one or more media (air, water, the wall, etcetera) to your ears. Your ears register the vibration and transmit it as nerve impulses to your brain, which converts it into the sound you hear.

 

We hear a wealth of sounds. This is because vibrations come in a wide variety, and they all influence tone, which consists of sound quality, pitch and volume. Fast or slow vibrations are perceived as high or low pitch sounds, respectively. Each musical note has a specific pitch, but tones with the same pitch can sound very different. For example, the middle C on a piano does not sound the same as the middle C on a guitar, a violin or a flute because these objects each vibrate in a complex way. There are many faster vibrations—called overtones—on top of the main vibration, each with their own volume.

 

When a hard material crashes onto a hard floor, the sudden impact makes it vibrate and creates a sound with a particular tone. If the same object is dropped again, the crash sounds similar because the object vibrates in a similar way. This might make you wonder if you can tell what object dropped, what it is made of, how heavy it is or its shape just by listening to the sound it makes when you drop it. Try this activity to find out if you can!

 

Materials

  • At least two objects, each made from a different material, that you can drop without worrying about breaking them or damaging the surface or floor. Chose from the following materials: hard plastic (for example, a spoon, cup, food storage container); metal (silverware, serving plates, BBQ tools); and wood (serving spoon, blocks, cutting board)
  • Bag to hold your items
  • Hard floor (tile or wood) or another hard surface such as a countertop or table (Make sure you have permission to drop things on these surfaces—some objects could cause scratches.)
  • Carpeted floor (A thick carpet works best.)
  • Partner

Preparation

  • Show the objects you will use in your experiment to your partner. Gather these objects in a bag and walk to the area with the hard floor or surface.
  • Ask your partner to face away from you so he or she cannot see what you do.
  • Explain to your partner that you will pick an object out of your bag and drop it on the floor. He or she needs to listen to the crash, and from the sound identify what the dropped object is. Do you think your partner will know what you dropped? Why do you think this?

Procedure

  • Pick a metal object from the bag and drop it so it makes a sound when crashing onto the floor. Repeat this a few times so your partner can hear the sound well. Ask your partner what you just dropped. Can he or she tell? If not, did your partner pick one made of the same material or one that is similar in shape or weight?
  • Show your partner what you dropped.
  • Put the object back in the bag, pick a plastic object from the bag and repeat the sequence.
  • Put the plastic object back in the bag, pick a wooden object from it then repeat the sequence.
  • Put the wooden object back in the bag, randomly pick another object from it, repeat the sequence. Do this a few times. Do you see a pattern in what your partner can identify? Is it easy to know what you dropped? Does he or she frequently pick an object made of the same material, the same shape or the same weight? Why do you think this is the case?
  • Switch roles. Now your partner will drop objects and you will guess what it is. Do you think it will be easier for you to identify the objects because you just heard the sounds these objects make when crashing onto the ground?
  • Perform the tests.
  • Do you see a pattern in what you can identify? Is it easy to tell what was dropped? Do you frequently pick an object made of the same material, the same shape or the same weight? Why do you think you can or cannot do this?
  • Is your pattern similar to your partner’s?
  • Do you think the results of these tests would have been different if you had dropped objects on a carpeted floor instead? Why do you think this is the case?
  • Move to an area covered with carpet and try the same tests. Can you or your partner distinguish what fell just by hearing the crash? If not, can you distinguish the material, the shape or the weight of the falling object? Why is this so?
  • Extra: What noise does a soft object such as a scarf make when landing on the ground? Why is this so?
  • Extra: Investigate if you can distinguish between heavy and light objects made of the same material crashing onto a hard floor—or between hollow and filled objects made of the same material.
  • Extra: Can you find words to describe the different sounds you produced in this activity?

Observations and results

You and your partner could probably identify what the falling object was made of (wood, metal or plastic) but it was probably harder to guess what the object was exactly.

 

When hard objects crash onto a hard surface, the vibrations from the sudden impact are like a person on a swing: After an initial push, they will swing back and forth, a little less high each time, until they eventually stop. Similarly the dropped object gets a push at impact and starts vibrating but the movements are too small for you to see. They become smaller and smaller until they eventually stop. These vibrations create rhythmic disturbances in the air. Your ears pick up these disturbances, and that is how you hear the sound of the crash.

 

Because materials vibrate in many ways and our ears are designed to register tiny differences, we hear a variety of sounds. A metal object crashing onto a hard floor will vibrate in a specific way creating a specific sound. Other metal objects will vibrate in a similar—but not identical—way creating a similar sound. You learned to distinguish the sounds of metal, plastic and wooden objects crashing onto a hard floor, which is why you could probably tell what the falling objects were made of.

 

Heavier objects vibrate differently compared with lighter ones. (This also is true for hollow versus full or short versus long ones.) The sounds these types of crashes make are different, but they are often harder to tell apart. You might have had trouble differentiating the sounds made by two different objects of the same material if they were close in size or shape. When we drop soft materials onto a hard floor or drop a hard material onto a soft, carpeted floor the resulting vibrations are not as big. That is why these crashes are not as noisy.

 

More to explore

Ring on the Resonance!, from Scientific American

Talk through a String Telephone, from Scientific American

Making Sound Waves, from Scientific American

Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies