Grades 3–5 · Arts & Music
Why can a glass sing?
A glass can produce a clear musical tone even though it has no string. Its walls can vibrate, moving back and forth very slightly. That motion pushes on the surrounding air, carrying sound away from the glass.
In the diagramA glass can turn friction into a toneDrive → glass vibration → surrounding airfriction driveair vibrationFriction can supply repeated pushes.maintained by input.
When a suitable damp contact moves along the rim, friction can alternate between sticking and slipping. These repeated pushes feed energy into the glass. The glass responds strongly near one of its resonant frequencies, meaning a vibration rate it readily supports. A steady input can keep the tone going.
Once the driving contact is removed, stored vibration can continue briefly and then fade as energy leaves the glass. A touching contact that resists motion can make it fade faster. Adding water also changes the vibrating system and commonly lowers the tone of a rubbed glass. The water is not simply a volume control: pitch concerns vibration rate, while loudness concerns other features of the sound.
About this explanation
Starting knowledge: Knowing that vibration is repeated motion and distinguishing higher pitch from greater loudness.
Sources
- Royal Institution — Singing wine glasses
- Science Buddies — Singing Wine Glasses
- Royal Institution — Singing wine glasses: Activity information sheet