A confident voice can be achieved in two ways, either by vocal or verbal cues. And they add up — if you say "I'm certain" in a confident tone, people rate you as more confident than the tone alone would get you. Say "maybe" and it pulls you down further than the tone alone would.
Now, what's interesting is that women and men have different ways of displaying a confident voice. Men usually follow the script of a low pitch — their most confident statements are their lowest ones, and the pitch goes up as they get less sure. Women don't do that. They go higher in pitch for confident statements too. What they use instead is a lot of variety in the pitch, plus more volume. Confident female speech had the widest pitch swings and the biggest jump in loudness, while men's loudness barely moved. Men marked doubt differently — with longer and more frequent pauses.
In both cases, what words are used is a key element. Words like "I'm sure," "definitely," and "for sure" show confidence. It's probably important to match those words with the right tone, but that's my guess, not something the study tested. They only used cues pointing the same way. What happens when your words and your voice say opposite things wasn't measured here.
Interestingly, people often see a male voice as sounding more self-assured. Whereas female voice, not — at least on average. But that average hides something. When a woman did pull off a confident delivery, she was rated as more confident than a man saying the same thing. The catch is it works both ways: when women sounded unsure, they were rated as less confident than men sounding unsure. Female voices just got spread out over a wider range. More reward at the top, more punishment at the bottom.
So basically, when a woman speaks with confidence, it really shows. It lands harder than it does for a man. But there's less room for error. One caveat — the study only used three speakers of each sex, so don't treat the men vs. women stuff as settled.
Jiang, X., & Pell, M. D. (2017). The sound of confidence and doubt. Speech Communication, 88, 106–126. https://doi.org/10.1016/j.specom.2017.01.011
