Why are voices so unique and distinguishable? Even a single word can let you recognize someone.
In: Biology
**TL;DR**: we all have different bodies and backgrounds and that affects the timbre of our voice and it has been evolutionarily beneficial for us to be able to distinguish between voices and voice characteristics.
**Full Response**
All of the responses so far discuss how the ears receive sounds and how the brain decodes the sound, but no one has given a deeper explanation as to why voices are so unique to begin with. This is an extensive topic so I won’t be able to cover everything, but I’ll provide enough of an explanation to get you going.
All sounds that we hear are really just acoustic waves. When you breathe out and make any “sound” from your mouth, you are pushing a certain amount of the air in front of you with a certain amount of energy which causes the air to move at a certain speed. Your body causes this energy to change rapidly over time, going up and down as you speak, and that causes the acoustic wave moving through the air to change rapidly as well. If I put a little spring in front of your mouth and an electromagnet behind the spring to record signals into a computer about how the spring moves when it is struck by the acoustic wave, I can plot this movement on a graph that shows the position of the spring at any given time. This generally looks like some sort of wave that goes up and down.
A “pure” wave can look like a smooth sine wave, flowing evenly up and down over time at a particular frequency (i.e., the number of ups and downs over a period of time). But in reality, most acoustic waves aren’t so smooth. Some get tall very fast — this is often called a fast “attack”. Some stay loud for a long time and slowly get soft — this is called a slow “decay”. Some might have a fast or slow decay, but continue making a sound for a long period of time — this is called “sustain”. And when the sound suddenly falls off, it can happen abruptly or slowly — this is called “release”.
In addition to these aspects, there are other subtle nuances that can change the acoustic wave called “overtones”. These are very important to your question. If I sing a note an A note, which is 440 Hertz (a measure of frequency), the fundamental frequency of my sound is 440Hz. But, if I make the sound more nasal sounding, certain overtones will be produced. These overtones occur at a higher frequency than the fundamental frequency. On a graph of this signal, the overtones can look like little bumps along the fundamental frequency, but what is really happening is that the overtones are being added to the fundamental frequency tone to produce a complex signal.
With that in mind, it’s a bit easier to understand this excerpt from the Wikipedia article on overtone:
> Most oscillators, from a plucked guitar string to a flute that is blown, will naturally vibrate at a series of distinct frequencies known as normal modes. The lowest normal mode frequency is known as the fundamental frequency, while the higher frequencies are called overtones. Often, when an oscillator is excited — for example, by plucking a guitar string — it will oscillate at several of its modal frequencies at the same time. So when a note is played, this gives the sensation of hearing other frequencies (overtones) above the lowest frequency (the fundamental).
> Timbre is the quality that gives the listener the ability to distinguish between the sound of different instruments. The timbre of an instrument is determined by which overtones it emphasizes. That is to say, the relative volumes of these overtones to each other determines the specific “flavor”, “color” or “tone” of sound of that family of instruments. The intensity of each of these overtones is rarely constant for the duration of a note. Over time, different overtones may decay at different rates, causing the relative intensity of each overtone to rise or fall independent of the overall volume of the sound. A carefully trained ear can hear these changes even in a single note. This is why the timbre of a note may be perceived differently when played staccato or legato.
In addition to the above, complex waves have something called an “envelope”. If I were to draw a smoothed out line from peak to peak over a complex sound wave, the envelope is essentially that smoothed out line. Our brain often picks up that envelope as well. The crazy thing is, our brains can actually hear and decode the envelope of an ultrasonic sound. In other words, we can hear the envelope of a signal that has a fundamental frequency that is too high for the human ear to hear. One fascinating corollary and consequence of this is that ultrasonic sounds have a smaller cone of dispersal — they can be aimed more precisely at, say, a single person in a crowd. But if I modulate the ultrasonic sound to have an envelope in a frequency that your ear can hear, then if I aim that ultrasonic signal at you while you’re standing in a crowd, only you will hear that sound. Creepy stuff, right? I actually wrote a patent on this technology and casinos are working to implement it into their games (which can be used both positively and with devious effect).
But to finally answer your question, voices are so unique and distinguishable because each person’s body is very unique. Some people might have a larger soft palette than others, some may have a uniquely shaped nasal cavity, some may have a stronger diaphragm, some may have lungs with a larger air capacity. And those are just physical aspects. Social aspects can result in tons of unique pronunciations and and habits in how a person uses their unique physical system. Similarly, various other things can come into play. For example, cortisol levels, particular diseases such as the common cold and covid-19, and tons of other biological aspects that can result from external sources can affect the voice. If you’re interested in that last bit, look at the research of Rita Singh.
In conclusion, there are tons of different aspects that can change from person to person, and these aspects all affect the timbre, overtones, and many other factors that can be picked up in an acoustic wave. That is at least a good starting point for why voices are so unique and distinguishable.
As for why our brains are able to perceive and decode those different sounds so precisely, that’s an equally long discussion that deals with evolutionary psychology, auditory processing pathways in the brain, social psychology, neuroscience, and much, much more. The high-level answer is because being able to distinguish between voices and voice characteristics has been beneficial to the survival of our species. But the detailed answer as to why and how we do that is far more than any one response can cover.
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