(טו) כִּי֩ יְהוָ֨ה אֱלֹהֶ֜יךָ מִתְהַלֵּ֣ךְ ׀ בְּקֶ֣רֶב מַחֲנֶ֗ךָ לְהַצִּֽילְךָ֙ וְלָתֵ֤ת אֹיְבֶ֙יךָ֙ לְפָנֶ֔יךָ וְהָיָ֥ה מַחֲנֶ֖יךָ קָד֑וֹשׁ וְלֹֽא־יִרְאֶ֤ה בְךָ֙ עֶרְוַ֣ת דָּבָ֔ר וְשָׁ֖ב מֵאַחֲרֶֽיךָ׃ (ס)
Because Hashem is always amongst us, we have to ensure that we keep our environment pure, so that Hashem shouldn't find something unbecoming with us.
אמר שמואל קול באשה ערוה שנאמר כי קולך ערב ומראך נאוה
What would be called ervah, like is mentioned in the passuk above?
According to the Gemara in Brachos, 24a , Shmuel says that based on the passuk in Shira HaShirim, "For your voice is sweet and your appearance lovely," the voice of a woman is ervah.
What are the sources for these 2 gemarot?
נשדר ליה מר שלמא לילתא א"ל הכי אמר שמואל קול באשה ערוה אפשר ע"י שליח א"ל הכי אמר שמואל
The idea of Kol Isha is also mentioned in the Gemara in Kiddushin 70a, where R' Nachman asked R' Yehuda to send regards to his wife. R' Yehuda then shut him down by saying, "Shmuel says that the voice of a woman is considered nakedness and you can't speak to her." From this we learn that Shmuel holds that the actual speaking voice of a woman is prohibited.
יש ליזהר משמיעת קול זמר אשה בשעת ק"ש הגה ואפי' באשתו אבל קול הרגיל בו אינו ערוה
The Shulchan Aruch, Orach Chayim 75:3 codifies the law differently than Shmuel. It says that you should refrain from hearing the singing voice of a women during zman krias shema- meaning that speaking is ok. Elsewhere, he will say that it is not only during Shema. (The Rama here says that even his wife's voice is ervah when she's singing during this time, but the voice that is regular, i.e. her speaking voice, is not. )
ואסור לשמועה קול ערוה
The Tur in Even HaEzer 21 says, as we already know from the Gemara, that a woman's voice is considered nakedness.
איתא דבערוה אסור לשמוע ולהסתכל, וכן באשתו אסור לקראת ק"ש כנגדו, אבל באשתו אין אסור לשמוע קולה
The ב"ח comments on the Tur here and clarifies, saying that in terms of a man's wife's voice, he cannot listen to her sing in front of him when he is davening or saying Shema, but the rest of the time he is permitted to hear her, singing or not. This is the same idea as the Rama mentioned above.
אבל קול פּנויה או קול אשתו מותר אלא בעת תפלה ודוק' קול ערוה אסור אבל קול דיבור שלה מותר
Furthermore, the Bais Shmuel, a commentary on the Shulchan Aruch, says on the same siman in Even HaEzer: even the voice of a single girl, or a man's wife, is allowed except during davening. He also makes sure to state that the only voice that is ervah is the singing voice, not the speaking voice. This is a new and slightly radical idea because it includes single girls.
אמר שמואל קול באשה ערוה שנאמר כי קולך ערב פירוש לשמוע ולא לענין ק"ש.
The Rosh in Brachos 3:37 says that the original Gemara is not only talking about during Shema or davening, but rather, it applies in every situation.
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