Their vocal cords are different, and produce a unique “formant” and “timbre” – essentially a different set of overtones that change the tone of the sound.
It’s the same reason a violin playing the same pitch, in the same octave, as a flute, won’t be mistaken for a flute. They have different timbres.
There’s a further difference when you consider someone talking with their head voice or their chest voice, which changes how breathy their voice sounds and therefore changes the overtone series too.
There is a part of every sound (don’t know the English word for it, sorry) that will make you recognize what it is. A guitar, drums, voices… anything.
If you remove that part or make it lower, you can still hear and recognize that sound, but it’s weird…
I can’t give you a 100% answer, but I can only imagine there are hundred of little things that make a difference to our ear…
A sound is actually a pressure wave traveling through a medium (usually the air). Unless that wave has a perfectly sinusoidal pattern (a sine wave), it will contain some amount of variation in its shape. Those variations are interpreted by our brain as timbre, or tone color.
Topics for further research:
Harmonic Series, Fundamental Frequency
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