טור אורך חיים קכד,ה
ושמעתי מאבא מורי שהיה אומר על כל ברכה וברכה שהיה שומע בכל מקום ברוך הוא וברוך שמו וזהו שאמר משה רבינו כי שם יהוה אקרא הבו גודל לאלהינו ועוד אפי' כשמזכירין צדיק בשר ודם צריך לברכו שנאמר זכר צדיק לברכה
For every blessing that a person hears in any place, one says, "Blessed is [God] and Blessed is [God's] Name."
9.Its obvious that if one is in a place where he cannot interrupt that he may not interrupt to say amen, see siman 56 (deals with what places and for who one may interrupt for).
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If you are are reciting prayers where an interruption is forbidden, you should not say Baruch Hu Uvaruch Shemo. Similarly if you hear a berachah which you must hear to fulfill your obligation, so that it be considered as though you recited it yourself, as for instance the berachah over the Shofar, or the berachah over the Megillah, you should not say Baruch Hu Uvaruch Shemo because this constitutes an interruption in the middle of the berachah. (See Dagul Merevavah Chapter 124). The laws of answering Amein when you are reciting prayers where interruption is prohibited, will be explained, God willing, later in special chapters. (Chapter 14 and Chapter 16).
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