Meaning of whisper

[Estersitta]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #58 "Stages", question 7

As he didn't want anyone else to hear what he was saying he spoke to her in a stage ..........

(a) whisper
(b) hush
(c) voice
(d) tone

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #58 "Stages", answer 7

As he didn't want anyone else to hear what he was saying he spoke to her in a stage whisper.

Correct answer: (a) whisper
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check this answer
meaning of whisper, please

Estersitta

[Ceo Neeraj]

Hi,

Is this "Stage whisper " an idiom??