Uh, Alta's whistle is above the pitch of human hearing.
5 seconds sound clip from the Forbidden Planet (1956) movie soundboard.
You can hear this line at 00:26:20 in the Blu-ray version of the movie.
Quote context
[...]
- Friends?
- Uh, yes. Perhaps you better call them, my dear.
- Felt something go right through my head.
- Uh, Alta's whistle is above the pitch of human hearing.
- Look.
[...]