subsonic area

例句：

1. 1.The aircraft flew at a subsonic speed.
   飞机以亚音速飞行。
2. 2. The sound of the waves is subsonic.
   海浪的声音是亚音速的。
3. 3. The rocket reached hypersonic speeds before it broke apart.
   火箭在解体前达到了超音速。