What’s a timbre of a human voice? Let’s start with the voice! The human voice is created when vocal folds vibrate. It’s this vibration of air molecules when travelling through different cavities like pharynx, sinuses, nose, and mouth, that’s converted to speech. And here is the interesting part that makes them unique to a person.
Like any physical objects, these cavities through which the sound travels have their own properties as well. They have their own distinct natural frequencies because of the geometries of the muscles that are unique to a person and the composition of the air.
And the sound from the vocal folds is made of not just a uniform sine wave with a fundamental frequency, but a composite of other distinct frequencies as well. So when certain frequencies of that sound wave hit the natural frequencies of the cavities, resonance happens, and those parts alone get amplified. The end result of all this is the distinct voice of a person. In other words, the lowest resonant frequency that’s modulated by the rest of the frequencies is what gives that unique tone to your voice. And that’s what we call the ‘timbre’ of the voice.
The voice you hear when you inhale helium, that’s because of the timbre change too. Not the frequency shift!