Key concepts
Sound
Physics
Music
Resonance

Introduction
Thanksgiving can be a wonderful time of year, with friends and family getting together. The table is full of delicious food—a big feast—and there might even be some fancy dishes and glassware on the table to make the celebration even more special. But did you know that you can make wine glasses sing? Perhaps you have done this many times before, but have you ever wondered why the wine glass makes this sound? Do this activity, and you will not only make wine glasses sing, but you will also learn how to generate different notes; you might even get inspired to play a little song.

Background
Glasses have been used for making music since the Middle Ages. The first musical instrument made of upright wine glasses was called the glass harp and was invented around 1750. About 10 years later, Benjamin Franklin—one of the founding fathers of the United States—invented a mechanical version of the glass harp, called the glass harmonica. Both of these instruments are based on the principle of generating musical tones by means of friction. What does that mean?

When you rub your moistened finger along the rim of the glass, your finger will stick to the glass as it encounters resistance, or friction, when it moves over the glass surface. The water on your finger, however, will allow your finger to slip, as it forms a cushion that reduces friction. When the pressure and amount of moisture are just right, this so-called stick-slip motion (the slight friction between your finger and the rim of the glass) will cause vibrations in the sides of the glass. The sides of the glass transmit the vibration to the surrounding air, creating a sound wave with a specific frequency. The frequency specifies the rate at which a vibration occurs and is usually measured per second or Hertz (Hz). There is a particular frequency, called the resonant frequency, at which the sides of the glass will vibrate most easily. The resonant frequency of wine glasses is typically within the range of human hearing (20-20,000 Hz), and this is why you hear the resulting resonant vibration as a tone. Now let's make some music and you can play your very own wine glass instrument.

Materials

  • Wine glasses and permission to use them carefully (optional: different size and shapes)
  • Water
  • Graduated liquid measuring cup
  • Metal spoon
  • Ping pong ball (optional)
  • Chromatic tuner (optional, available as free phone app)
  • Piano or keyboard for comparing notes (optional)

Preparation

  • Find a working area that can tolerate some water spills.
  • Carefully gather your wine glass(es) and your measuring cup, as well as some water.
  • Wash your hands with dish soap and rinse your hands thoroughly.

Procedure

  • Take an empty wine glass and put it in front of you on the table.
  • Hold down the base of the glass with your non-dominant hand.
  • Rub your dry finger around the rim of the glass while pressing down gently. Observe how it feels. Does your finger stick to the glass or is it gliding smoothly along the rim? Do you hear a sound?
  • Wet the index finger of your dominant hand with water.
  • Rub your finger around the rim of the glass again while pressing down gently. Does it feel different than with a dry finger? How much resistance do you feel this time? Does the glass start to sing? Tip: If you hear no sound, try changing the speed and applying more or less pressure to the rim.
  • While you hear the glass sing, stop rubbing your finger along the rim and take it away from the glass. Does the glass continue to sing or does it stop?
  • Now make the glass sing again and stop rubbing your finger along the rim but this time leave your finger on the rim on the glass. What happens to the sound this time?
  • Take a metal spoon and very gently tap the glass on the side. Does this generate a different sound or note compared to when you used your finger to make the glass sing?
  • If you are using a chromatic tuner, use it to determine the note the glass makes when rubbing it with your finger or hitting it with the metal spoon. You can also compare the note frequency with notes sounded on a piano or keyboard. What note did the glass generate? Can you sing the same note?
  • Now fill one-third of the glass with water and repeat the procedure. What note does the glass make now? Is it a higher or lower pitch?
  • Watch the water surface in the glass carefully while you are generating the sound. Does the water surface stay calm or do you see any disturbances?
  • Take the metal spoon again and very gently tap the glass on the side. Do you get the same note again or is it different?
  • Again, you can use the reading from the chromatic tuner to determine the note the glass makes when rubbing it with your finger or hitting it with the metal spoon. You can also compare the note frequency with notes sounded on a piano or keyboard. What note did the glass generate this time? Is it different from the previous one? How?
  • Add more water to the glass so that two-thirds of the glass is filled with water. Rub the rim of the glass again with your damp finger. How does the pitch of the sound change with the different water level? Did the sound become higher or lower?
  • Take the metal spoon for the last time and very gently tap the glass on the side. What note do you generate?
  • Compare the three notes that you generated with each water level (empty, one-third full and two-thirds full). Can you find a relationship between the water level in the glass and the resulting pitch of the note?
  • If you are using a chromatic tuner, use it again to determine the note the glass makes when rubbing it with your finger or hitting it with the metal spoon. You can also compare the note frequency with notes sounded on a piano or keyboard.
  • Extra: Try the activity with wine glasses of different sizes and/or shapes. How does the size or shape of the glass change how it sounds? Can you draw any conclusions about the relationship between the size/shape of the glass and its resonance frequency?
  • Extra: Put one glass close to another wine glass and let it sing. Now hold it so the sound stops. Does the second glass sing instead?
  • Extra: Take an empty wine glass and put a lightweight object in it, such as a ping pong ball. Rub your damp finger around the rim of the glass, and once the glass starts making a sound, tilt the glass on its side. Be careful not to drop the ping pong ball on the floor. Continue rubbing the glass to make it sing and observe the ball inside. What happens to the ball? Does it sit still on the walls of the glass or does it move?

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Observations and results
Could you make the wine glass sing with your dry finger? Probably not, as there was too much friction between your finger and the glass rim. To create a stick-slip motion, the finger has to be a little wet. But once the conditions were right, the glass should have generated a clear and beautiful sound. By rubbing along the glass rim with your finger, you made the walls of the glass vibrate; you might even have felt the vibrations in your finger. When you take your finger away from the glass, the glass walls continue to vibrate, thus, still producing a musical note. However, if you stop rubbing the rim and leave your finger touching the glass, your finger will stop the vibrations and the sound stops as well. In a wine glass filled with water, you actually should have been able to see the generated vibrations in the form of a little wave pattern that develops, especially around the glass walls. If you did the extra activity with the ping pong ball, you also probably made the vibrations visible. The ball should dance inside the glass as it gets pushed around by the vibrations of the walls each time it comes in contact with the glass edges.

In each case when you tapped the glass with the metal spoon, you should have heard the same note as when you used your finger. Using the metal spoon is just another method to get the glass walls to vibrate, and the generated sound waves of the glass thus stay the same. Sometimes, even vibrations of one glass can make another glass vibrate and sing if you put them very close together. However, when adding different amounts of water to the glass, you should have noticed that the pitch of the sound changed depending on the water level; the more water you add to the glass, the lower the pitch of the sound you hear. This is due to the fact that the water volume inside the glass makes it much heavier and therefore, it is more difficult for the glass walls to vibrate. That means that the sound wave generated by the vibrations is much slower or has a lower frequency. As the frequency of a note is correlated to its pitch, the pitch produced by the glass goes down as you add more water. If you can use more than one glass, you can play around with different water levels or glass shapes and sizes; each one will have a different resonance frequency and will produce a different note. Try to find different glasses and change water levels to generate each note of the musical scale. Can you play a simple song with your glass harp?

More to explore
Why do glass cups make a ringing noise, from SeattlePi
Singing Wine Glasses, from Science Buddies
Harry Potter theme on glass harp, at YouTube
Sonorous Science: Making Music with Bottles, from Scientific American
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies