Why Instruments Sound Different: A Look at Harmonics
By Henry Bahrou – Guitarist, Music Theorist, Music Academy Director
When you listen to a song, it’s common to hear multiple instruments playing together, either during certain sections or throughout the entire piece. Sometimes, two different instruments — like a piano and a guitar — might even play the exact same note, in the same octave, at the same time. Despite this, it’s easy to tell them apart.
While the material and shape of an instrument certainly influence its sound, the main reason instruments have unique tones comes down to something called harmonic partials.
When you pluck a string on a guitar, the sound you hear is mostly the fundamental — the main vibration that defines the note. However, if you listen carefully, you’ll notice additional, higher-pitched sounds layered above the fundamental. These are known as the overtone series. Every time a note is played, what you’re actually hearing is a complex mixture of pitches, not just a single frequency.
Different instruments highlight different parts of the overtone series, which gives each its distinctive voice. For instance, a clarinet strongly emphasizes only the odd-numbered partials, while an oboe is rich in both even and odd overtones. A piano’s highest partials are generally softer and slightly more distorted compared to a harpsichord, largely because piano strings are thicker in proportion to their length. This is why a full-size grand piano sounds clearer and more vibrant — its longer, thinner strings produce more accurate overtones.
How the overtones evolve over time also shapes an instrument’s color. A plucked string, like on a guitar, starts with many high overtones that gradually fade as the note dies away. In contrast, a trumpet note might start simple and become richer as more partials develop.
Understanding how harmonic partials work not only explains why each instrument sounds unique, but also reveals the incredible complexity behind even the simplest musical sounds.
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