The study of speech sounds - the ways in which they are produced and used as well as their acoustic properties. A mild obsession of Jono Dowse '06, who TA'd the phonetics course for two years (LING 210, formerly LING 111) and runs a sweet, sweet WSO webpage devoted to the International Phonetic Alphabet. Seriously, check that ish out.
Also, awesome class (at least, if taught by Nathan Sanders). Take it!