“Quindar-tones” are the twinned beeps best known from the Apollo and Space Shuttle missions. They’re named for Quindar Electronics, Inc., which produced the related equipment.
50: The difference in Hz between the two tones
2,525 Hz: The higher of the two pitches, in hertz
250ms: The length, in milliseconds, of each of the two tones, individually
I thought to do this entry because someone at some point searched for the word “Quindar” on disquiet.com. There was no entry, so I figured I’d make a brief one, and the Sound Ledger seemed like a good format in which to do so.