(1)音楽の伴奏なしで
(2)without musical accompaniment; "they performed a cappella"
さらに詳しく
(1)同時に、いっせいに
(2)すべて同時に
(3)at the same instant
(4)all at the same time; "Let's say `Yes!' all at once"
(3)at the same instant; "they spoke simultaneously"