A phonologist is a person who studies phonology — the system of speech sounds in a language. They ask which sounds “count” as different, how sounds are allowed to follow each other, and what happens to a sound in different parts of a word or in fast speech. Though actually, people outside the field often call everyone who studies speech sounds a phonetician. The two jobs overlap, and some researchers wear both hats, but the phonologist is the one who cares about the pattern, not only the physical sound wave. You will meet this word in universities and specialist books, not in daily talk. If you mean “someone who teaches children to read by sounding out letters,” that person is using phonics; they are not, by that fact alone, a phonologist.

音韻學家（對岸也常說「音系學家」。）