subaudibleadj.

1837 Dublin Univ. Mag. Apr. 398/2 The faint sub-audible ejaculation of Father Luke, when he was recovered enough to speak.

1879 C. M. Yonge Magnum Bonum III. xxxvi. 812 When Lord Fordham..spoke of them as future possessors, in the tone of a father speaking of his heir apparent, there was a sub-audible ‘No, no’.

1928 E. Blunden Undertones of War vii. 109 A good joke: but with this subaudible meaning.

1961 W. J. McGuire in A. A. Lumsdaine Student Response in Programmed Instr. xxvii. 417 Such responses presumably take the form of sub-audible verbal surrogates.

2000 Tampa (Florida) Tribune (Nexis) 26 Mar. 1 He speaks in an almost subaudible voice, a wisp of a mustache twitching.

1922 Proc. IRE 10 253 It may be at sub-audible, or super-audible frequencies.

1957 Pop. Mech. Apr. 114/2 Sigalert for the first time takes part of this unused, subaudible portion [of the radio spectrum]—the area from 0 cycles to 40—and puts it to work.

1974 New Scientist 6 June 612/2 One investigation..showed subaudible 30 Hz bursts on the unused tape.

2009 A. Reynolds Impossible Dream v. 109 The inevitable dull thump of sub-audible bass frequencies.