Voice allows Hizashi to increase the volume of his voice, giving him the ability to create loud, high-pitched sounds.
The sound that comes from Hizashi's mouth can reach extremely high levels, enough to incapacitate people by disrupting their eardrums (which may cause bleeding). The loud noise will also drown out other sound-based Quirks, giving him an advantage against those Quirk types in battle.
Because sound does not travel well to the underground, it's ineffective against anything moving through it.
In the anime, the words shouted by Hizashi will travel alongside Voice's sound wave, which is depicted in a yellow color. In the manga, the speech bubbles that are used through Voice have noticeable borders.
Hizashi wears a special Directional Speaker System support item around his neck area that allows him to direct his sound waves. This prevents his Quirk from affecting allies that are close to him.
- Human voice - Wikipedia article about the specific sound source Hizashi's Quirk is based after.
- Loudness - Wikipedia article about the specific property of sound that Hizashi has actual control of.
- Sonic weapon - Wikipedia article about sound being used as weaponry.