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.
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
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?
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.