空喊

idle clamor / to prattle

empty / air / sky / in vain

to empty / vacant / unoccupied / space / leisure / free time

to yell / to shout / to call out for (a person)

to converse with sb by shouting from some distance away / (fig.) to say sth intended for the ears of sb who is not in one's presence (often, by making a public statement or by posting online)