___ 1 (speed of sound) (4)

I believe the answer is:

mach

(Other definitions for mach that I've seen before include "Number referring to speed of sound" , "Ratio of a body's speed to the speed of sound" , "-- Number relates to speeds" , "Austrian physicist, gave name to speed no." , "Ratio of the speed of a moving body to the speed of sound" .)

I've seen this clue in the Universal and the USA Today.

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