One note contains a whole family of sounds
When a piano and guitar play the same named note, they share a fundamental frequency: the vibration rate your ear hears as the note’s main pitch. But the strings also vibrate in faster patterns at the same time. These are overtones, and their relative strengths help shape a sound’s tone color, or timbre.
A string can vibrate as one broad segment, producing the fundamental, while also vibrating in two, three, or more segments. Those shorter patterns create overtones at higher frequencies. The instrument’s body and the way its string is set in motion affect which overtones ring strongly and which fade quickly.
A strike and a pluck start different vibrations
A piano hammer hits its string briefly, setting off a sharp burst of motion. The hammer’s contact and the string’s design help determine the initial mix of overtones. The piano’s soundboard then carries the vibration into the air, making the note resonate beyond the string itself.
A guitarist pulls a string aside and releases it. That pluck creates its own pattern of motion; where the string is plucked matters, because a pluck near the middle tends to excite a different balance of vibrations than one near the bridge. The guitar’s body also colors and amplifies the result.
The note changes as it fades
Timbre is not fixed at the instant a note begins. A piano note often starts with a distinct attack and then decays as the string’s vibration loses energy. A guitar note also fades, but its plucked beginning and the way its body resonates give that decay a different shape. Some overtones die away sooner than others, so the tone can shift while it rings.
That changing blend is why matching the pitch alone does not make the instruments sound alike. Your ear recognizes not only the note’s fundamental, but also its overtone mix and how that mix evolves from onset to silence.
