The shofar, which is sounded both on Rosh HaShanah and for the yovel, is a bent ram's horn. All shofarot other than that of a ram are unacceptable.
Even though the sounding of the shofar on Rosh HaShanah is not explicitly mentioned in the Torah [it was derived by our Sages in the following manner]. Concerning the yovel, [Leviticus 25:9] states: "You shall make a proclamation, sounding the shofar... you shall proclaim with the shofar." The oral tradition explains that just as the "sounding" required by the Torah in the yovel requires a shofar, so, too, the "sounding" on Rosh HaShanah requires a shofar.
אַף עַל פִּי שֶׁתְּקִיעַת שׁוֹפָר בְּרֹאשׁ הַשָּׁנָה גְּזֵרַת הַכָּתוּב רֶמֶז יֵשׁ בּוֹ כְּלוֹמַר עוּרוּ יְשֵׁנִים מִשְּׁנַתְכֶם וְנִרְדָּמִים הָקִיצוּ מִתַּרְדֵּמַתְכֶם וְחַפְּשׂוּ בְּמַעֲשֵׂיכֶם וְחִזְרוּ בִּתְשׁוּבָה וְזִכְרוּ בּוֹרַאֲכֶם. אֵלּוּ הַשּׁוֹכְחִים אֶת הָאֱמֶת בְּהַבְלֵי הַזְּמַן וְשׁוֹגִים כָּל שְׁנָתָם בְּהֶבֶל וָרִיק אֲשֶׁר לֹא יוֹעִיל וְלֹא יַצִּיל, הַבִּיטוּ לְנַפְשׁוֹתֵיכֶם וְהֵיטִיבוּ דַּרְכֵיכֶם וּמַעַלְלֵיכֶם וְיַעֲזֹב כָּל אֶחָד מִכֶּם דַּרְכּוֹ הָרָעָה וּמַחֲשַׁבְתּוֹ אֲשֶׁר לֹא טוֹבָה.
Even though the sounding of the shofar on Rosh HaShanah is a decree, it contains an allusion. It is as if [the shofar's call] is saying:
Wake up you sleepy ones from your sleep and you who slumber, arise. Inspect your deeds, repent, remember your Creator. Those who forget the truth in the vanities of time and throughout the entire year, devote their energies to vanity and emptiness which will not benefit or save: Look to your souls. Improve your ways and your deeds and let every one of you abandon his evil path and thoughts.
on the full moon for our feast day. (5) For it is a law for Israel,
a ruling of the God of Jacob;
(2) Utter futility!—said Koheleth—
Utter futility! All is futile!
A human being is only breath and shadow.
Sophocles
כִּי הַוָּי״ו מוֹרָה עַל הַהַמְשָׁכָה מִלְמַעְלָה לְמַטָּה. וְגַם הַמְשָׁכָה זוֹ הִיא עַל־יְדֵי מִדַּת חַסְדּוֹ וְטוּבוֹ וּשְׁאָר מִדּוֹתָיו הַקְּדוֹשׁוֹת, הַנִּכְלָלוֹת בְּדֶרֶךְ כְּלָל בְּמִסְפַּר שֵׁשׁ, שֶׁבַּפָּסוּק: לְךָ ה׳ הַגְּדוּלָּה וְכוּ׳״ עַד ״לְךָ ה׳ הַמַּמְלָכָה וְכוּ׳״, וְלֹא ״עַד״ בִּכְלָל. כִּי מִדַּת מַלְכוּתוֹ יִתְבָּרֵךְ נִקְרֵאת בְּשֵׁם ״דְּבַר ה׳״, כְּמוֹ שֶׁכָּתוּב: ״בַּאֲשֶׁר דְּבַר מֶלֶךְ שִׁלְטוֹן״. וְנִכְלֶלֶת וְנִרְמֶזֶת בְּאוֹת הֵ״א אַחֲרוֹנָה שֶׁל שֵׁם הַוָיָ׳, כִּי פְּנִימִיּוּת וּמְקוֹר הַדִּיבּוּר, הוּא הַהֶבֶל הָעוֹלֶה מִן הַלֵּב וּמִתְחַלֵּק לְה׳ מוֹצָאוֹת הַפֶּה: אחה״ע מֵהַגָּרוֹן וְכוּ׳, וְגַם הֲבָרַת הַהֵ״א הִיא בְּחִינַת הֶבֶל לְבַד, כְּמוֹ שֶׁנִּקְרֵאת: ״אָתָא קַלִּילָא דְּלֵית בָּהּ מְשָׁשָׁא״.
Vav, in shape a vertical line, indicates downward extension. Also, this flow downward is effected through the divine traits of benevolence and goodness and His other sacred traits, included in general terms in the six attributes in the verse, “Yours O G–d is the greatness…” until “Yours O G–d is the dominion…,” until, but not inclusive. His [seventh] attribute, malchut or dominion, is called the “Word of G–d,” as in the verse, “Wherever the word of the king holds sway.” This attribute of dominion is contained and represented in the final hey of the Name of Havaya. The internal aspect and the source of speech is the breath that rises from the heart, then is molded by the five oral articulations—alef, chet, hey, and ayin from the throat, and so on. The hey implies this internal aspect of speech, for it is the pristine unvocalized breath, “a light letter without substance.”
Political language... is designed to make lies sound truthful and murder respectable, and to give an appearance of solidity to pure wind.
George Orwell
The empty vessel makes the loudest sound.
Plato