|Statue at Wat Phra Kaew, Bangkok (photo by Vladimir Samoilov)|
In Thai, for example, a syllable can have one of five possible tones. Here's a set of five words that differ only in their tone. You can hear these words being spoken here (just click on the words). Can you hear the difference between the tones?
- naa21 [a person's name] -- low falling tone
- naa51 'face' -- high falling tone
- naa45 'aunt' --high rising tone
- naa213 'thick' -- low falling-rising tone
- naa32 'field' -- mid falling tone
Many people who don't speak a tone language (including many students I've taught) have a hard time hearing the difference between tones, even when the tones are explicitly pointed out to them. This is normal, of course, because we're not used to listening for pitch in that way. But for speakers of a tone language, it's as easy as hearing the difference between p and t is for us.
And it's not just a matter of listening for specific pitches; we can't just say "in Thai, a low falling tone starts at 110 Hertz and drops to 90 Hertz". Why not? Well, the most obvious reason is that all people have different voices with slightly different pitch ranges. A man might produce a "high" tone that's actually a lower pitch than a woman's "low" tone. So when you meet a person for the first time, you have to be able to adjust to that person's natural pitch range. And one person's pitch range can change depending on many factors (their emotional state, if they're imitating someone else, etc.).
But worse than that, even if we're just talking about a single speaker, saying a single sentence, the exact pitch of a given tone will still vary a lot. This is because all languages use pitch for something called intonation. Intonation is the use of pitch to demarcate and emphasize various features of a sentence. For example, in English, when we ask a question, we generally put a rising intonation at the end of the sentence. To demonstrate, I recorded myself asking a question and saved the pitch track (which shows my pitch over time):
Another use of intonation is to emphasize words that the speaker wants the listener to pay attention to.
Tone languages make use of intonation too. So speakers of Thai would have to be able to take intonation into account when listening for tone.
There's yet another difficulty in the way of perceiving tone. Phoneticians have noticed that over the course of a sentence (or a longish phrase within a sentence), a speaker's pitch tends to gradually drift downward. So a word with a pitch peak (like the word "brownies" above) may still have a lower pitch than earlier words in the sentence. To illustrate this, I recorded myself reading the first line of Pride and Prejudice:
You can see that, although there are lots of peaks and valleys in my pitch (corresponding to intonational patterns), it generally decreases over time. Speakers of tone languages have to compensate for this too!
So tone seems pretty wild to us English speakers, but really it's just another example of the amazing power of the human capacity for language (not to mention the very fine control we have over our vocal cords!).
P.S. #1: Another thing I didn't mention, because I don't know a lot about it, is the fact that tones can actually mess with each other when they're adjacent!
P.S. #2: If you're wondering why there are some gaps in the visualizations of my pitch, it's because not all speech sounds have a pitch. During the sounds s, f, t, and p, among others, your vocal cords aren't vibrating; the only sound is noise caused by air bouncing around in a narrow channel (in the case of s and f) or the explosive release of air in t and p.