ETYM Latin surdus deaf (whence the meaning, deaf to reason, irrational), perhaps akin to Eng. swart. Related to Sordine.
(Linguistics) Of speech sounds; SYN. voiceless, unvoiced.

A consonant produced without sound from the vocal cords; SYN. voiceless consonant.
Irrational number; expression containing the root of an irrational number that can never be exactly expressed—for example, root(3) = 1.732050808.