Why do equivalent notes played on different instruments sound different?

Question

So if an A is 440hz, why does a piano playing an A sound different than a violin, a guitar, or someone’s voice making that same A 440 note? It’s obvious that the pitch is the same on each instrument but each instrument has a distinct sound. I’ve never heard an A on a piano and thought, is that a piano or a cello. Why can we distinguish between instruments?

In: 151

Answer 1

If you looked at the sound waves from both instruments, you would see two waves with the same frequency (number of repetitions over time) since frequency controls the pitch of a sound, and maybe even the same amplitude (height of the wave) which controls the volume. However, these are not the only two qualities a wave has: there is also the shape of the wave.

The shape of the wave is what determines everything besides the pitch and volume, and we refer to this as the timbre of the sound. There’s no easy way to tell exactly what kind of sound a certain wave shape will produce, though in general the more jagged the wave is the more harsh the timbre will be.

This is getting a bit technical but this is why: the “purest” shape of wave, in a sense, is a sine wave, the wave you get by measuring the height of something as it moves around in a circle. Any other shape of wave can be represented as a series of sine waves of different frequencies overlapping one another. Thus, if a sound wave is anything other than a pure sine wave, what you are really hearing is a root note with a bunch of higher frequency notes stacked on top of it. These higher frequency notes are called overtones, and our ears magically interpret them all as adding timbre to the root note rather than as their own individual sounds, but you can pick them out some times if you listen closely. Anyway, this is why jagged waves sound more grating and harsh, because the overtones present in those shapes of waves are really really high frequency.

All of this is still assuming that sound waves repeat forever and don’t change over time. Another big reason instruments sound different from each other is the envelope of the sound, or how the sound changes over the duration of a note being played.