Juan and giancarlo are opera singers. juan is a tenor and giancarlo is a baritone. tenors sing with higher-pitched voices than baritones. with respect to this fact, the sound waves corresponding to giancarlo's voice are lower in than those corresponding to juan's voice; that is, they are lower

The missing words are (respectively):

frequency and hertz

To complete the sentences:

the sound waves corresponding to Giancarlo's voice are lower in frequency than those corresponding to Juan's voice;

that is, they are lower in hertz

Quick information:

Frequency has two distinctions and level for male and female. For male the voice frequency is 85 to 180 and a unit of hertz which is Hz and for the female is 165 to 255 hertz (Hz)