What’s an Allophone?
A phoneme is a distinct unit of sound. There are about 40 unique phonemes in American English, 24 consonant sounds and 16 vowel sounds.
But of course, this division of sounds is not absolute. There is a lot of variation in each of these sounds. Take for example, the letter T /t/. Say each of these words and listen closely to the sound of the letter T in each.
Do you hear the different T sounds in each? Probably not. Not because they don’t sound different, but because you associate all these sounds with the written letter T so strongly that you have stopped hearing the difference. To a native ear they all sound like a T, but in fact they are all created a little differently and sound a little differently.
Put your hand in front of your mouth and say “top” and “cat”. Notice how you feel air against your hand when you say “top” (aspirated T) and no air against your hand with “cat” (unreleased T). Other examples of aspirated T’s are tip, hotel and guitar and examples of unreleased T’s include: want, print, wet and light.
When you say the word “button” you stop the air with your throat before the T sound is made (glottalized T). You’ll also find this with words like fountain, curtain. “Let me know if you can’t,” has a glottalized T at the end of “let” and “can’t”.
With “water” the T becomes more of a D sound (flapped T). Other words where you’ll hear the flap T are: beetle, wanted, get in, fatal, hospital, turtle and letter.
With the word “winter” you combine the N and the T together and all that’s left of the T is some air coming out of your nose (nasalized T). Listen for six nasalized T’s in this sentence: “The important dentist wanted ninety winter paintings.”
Then there is just the plain old T sound like in the word stop. This allophone doesn’t have a name, but we can think of it as a flat T.
Although we aren’t consciously aware of making these various T sounds, being able to distinguish them allows us to hear the differences between words like: nitrate (aspirated) and night rate (unreleased).
If a flame is held before the lips while these words are spoken, it flickers more during aspirated nitrate than during unaspirated night rate. The difference can also be felt by holding the hand in front of the lips. For a Mandarin speaker, to whom /t/ and /tʰ/ are separate phonemes, the English distinction is much more obvious than it is to the English speaker who has learned since childhood to ignore it.