Siman 587
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א הַתּוֹקֵעַ בְּתוֹךְ הַבּוֹר אוֹ בְּתוֹךְ הַמְּעָרָה, אוֹתָם הָעוֹמְדִים בְּתוֹךְ הַבּוֹר וְהַמְּעָרָה יָצְאוּ; וְהָעוֹמְדִים בַּחוּץ, אִם קוֹל שׁוֹפָר שָׁמְעוּ, יָצְאוּ, וְאִם קוֹל הֲבָרָה שָׁמְעוּ, לֹא יָצְאוּ; וְכֵן הַתּוֹקֵעַ לְתוֹךְ חָבִית גְּדוֹלָה וְכַיּוֹצֵא בָּהּ, אִם קוֹל שׁוֹפָר שָׁמַע, יָצָא, וְאִם קוֹל הֲבָרָה שָׁמַע, לֹא יָצָא.
1 Law of one who sounds the shofar in a pit, 3 Seifim: If one sounds the shofar in a pit or cave, those standing in the pit or cave fulfill their obligations. Concerning those who stand outside, if they hear the sound of the shofar, they fulfill their obligation. If they hear the echo, they do not fulfill their obligation. Similarly, if one sounds the shofar into a large barrel or something similar, those who hear the sound of the shofar fulfill their obligation and those who hear the echo do not.
ב אִם הִתְחִיל לִתְקֹעַ בַּבּוֹר וְעָלָה חוּץ לַבּוֹר וּגְמָרָהּ, יָצָא, שֶׁכָּל מַה שֶּׁשָּׁמַע בֵּין בִּפְנִים בֵּין בַּחוּץ הָיָה קוֹל שׁוֹפָר. הַגָּה: וְכֵן אוֹתָם שֶׁהָיוּ בַּבּוֹר בִּתְחִלַּת הַתְּקִיעָה (גְּמָרָא).
2 If one begins sounding the shofar in the pit and then ascends out of the pit and finishes the sounding, they fulfill their obligation, as everything that was heard, inside and out, was the sound of the shofar. Rema: The same applies to those who were in the pit at the beginning of the sounding (Gemara).
ג הַשּׁוֹמֵעַ מִקְצָת תְּקִיעָה שֶׁלֹּא בְּחִיּוּב, וּמִקְצָתָהּ בְּחִיּוּב; אוֹ שֶׁאָמַר לַתּוֹקֵעַ כְּמִתְעַסֵק: כַּוֵּן לְהוֹצִיאֵנִי יְדֵי חוֹבָתִי, וְתָקַע וּמָשַׁךְ בָּהּ שִׁעוּר תְּקִיעָה, לֹא יָצָא; וְיֵשׁ אוֹמְרִים שֶׁיָּצָא אִי אִיכָּא שִׁעוּר תְּקִיעָה בְּחִיּוּב; הַגָּה: וְהוּא הַדִּין אִם שָׁמַע מִקְצָת הַתְּקִיעָה קוֹל הֲבָרָה, כְּגוֹן שֶׁהָיָה הַתּוֹקֵעַ בְּבוֹר וְהוּא עוֹמֵד בַּחוּץ וּבְאֶמְצַע הַתְּקִיעָה יָצָא לַחוּץ (טוּר).
3 One who hears part of the soundings in a non-obligated fashion and part of it in an obligated fashion, or one who tells somebody unintentionally sounding a shofar "Intend to fulfill my obligation" who then proceeds to blow for the minimal length, neither of these fulfills their obligation. There are those who say that they fulfill their obligation if there is a minimal length of sounding in a state of obligation. Rema: The law is the same if one heard part of the sounding via echo, like if the blower was in a pit and the hearer was outside and in the middle of the sounding, the blower exited (Tur).