To make a wine glass sing, you will need the following materials:
- wine glass - a crystal, thin-walled one works best
- separate glass of water
- wet fingertip
To make the wine glass sing, do the following:
- Hold the empty wine glass on a tabletop at the base of the stem with one hand.
- Wet the index or middle finger of your other hand with some water.
- Lightly rub your wet finger along the rim of the glass.
- As you rub the glass, you will hear the "singing" sound of the glass. You may have to re-wet your finger periodically and/or adjust the pressure of your finger on the rim of the glass to keep producing the sound.
- You can change the pitch of the sound by adding water to the glass.
Every material (such as glass, steel, concrete) has a natural frequency at which it vibrates, called a resonant frequency. If you put energy into the substance at its resonant frequency, you will force it to vibrate or resonate (resonance is a forced vibration). In the case of the wine glass, your finger slides and sticks along the surface of the glass as you rub the rim (a wet fingertip has no oil and makes a better contact with the glass). The rubbing imparts energy to the glass molecules and causes them to resonate. The motion of your hand sets up a wave of vibration traveling through the glass. The vibrating glass causes air molecules to vibrate at the same frequency. The vibrating air molecules are the sound wave that you hear (the frequency or pitch of the sound wave is the same as the resonant frequency of the glass).
So, how does the water change the pitch of the singing wine glass? As the resonant wave moves around the glass, it drags the water molecules with it, creating a wave of water that you can see near the edge of the glass. The dragging water molecules effectively increase the mass (both the water and the glass molecules) and reduce the energy of the wave traveling through the glass. When the energy is reduced, so is the frequency of the wave in the glass, which is reflected in the pitch of the sound wave that you hear.
If you impart enough energy to the glass at its resonant frequency, you can cause the glass to shatter. However, this takes more energy than you can provide by rubbing the rim. Some singers can sing a note equal to the resonant frequency of a wine glass and cause it to shatter (see Resonance for a video of this phenomenon).
Resonance has also been shown to cause bridges to collapse. Marching troops of soldiers will often break cadence when crossing a bridge to prevent a resonance collapse. The most famous example of resonance was the Tacoma Narrows Bridge in Washington State (also called Galloping Gertie). In 1940, just months after its completion, winds in the Tacoma Narrows matched the bridge's resonant frequency and caused the suspension bridge to sway uncontrollably. Within hours, the bridge collapsed (see The Tacoma Narrows Bridge Disaster for photos and a video of this event).