Tapping on a cup with your finger generates a sound wave through a process called percussion or impact excitation. When you tap on the cup, several physical phenomena contribute to the creation of the sound wave:
Impact: When your finger strikes the cup, it causes a brief and sudden deformation of the cup's surface. This impact creates vibrations in the cup.
Vibrations: The impact generates vibrations that propagate through the cup's material. These vibrations travel as mechanical waves, causing the cup's surface and surrounding air molecules to oscillate.
Acoustic Waves: The vibrating cup's surface displaces air molecules, creating compressions and rarefactions. These compressions and rarefactions travel outward from the cup as sound waves, which can be perceived as sound.
Resonance: The cup's shape and material properties influence the characteristics of the sound wave. The cup acts as a resonator, emphasizing certain frequencies and harmonics that correspond to its resonant modes. This resonance enhances specific frequencies and gives the sound its unique timbre.
The resulting sound wave consists of a mixture of frequencies and harmonics, influenced by the cup's shape, size, material, and the force applied when tapping. Different cups will produce distinct sounds due to variations in their resonant properties.
It's worth noting that the sound produced by tapping on a cup is typically a percussive or transient sound with a relatively short duration. The shape of the cup and the force of the tap contribute to the decay rate of the sound wave, determining how quickly it fades away.