Rosh Hashanah, Chapter 3, Mishnah 3. This mishnah continues the discussion of which animal, and which type of horn, the shofar blown on Rosh Hashanah must come from.
The text of the mishnah:
"Shofar shel Rosh Hashanah shel ya'el, pashut" - the shofar of Rosh Hashanah is made from the horn of the ya'el, a species of wild goat, and it is "pashut," that is, straight, as opposed to the horn of a ram, which is curved.
It is worth pointing out that further on we find that Rabbi Yehudah disagrees with our mishnah and holds that on Rosh Hashanah one must blow specifically with the horn of a ram, and that is how we rule in practice. The root of the dispute lies in two different perspectives on how a person is meant to stand in prayer on Rosh Hashanah:
The tanna of our mishnah: the horn must be straight, based on the principle "kamah d'pashit inish da'atei tfei ma'ali" - the more a person's thought and mind are straight and unfolded on Rosh Hashanah, the better.
Rabbi Yehudah: the horn must be bent, which is the horn of a ram, based on the principle "kamah d'chayif inish da'atei tfei ma'ali" - the more a person's mind is bent and submissive on Rosh Hashanah, the better. The bent horn testifies to that very bending and submission.
The shofar as it was blown in the Beis HaMikdash:
The continuation of the mishnah describes the shofar as it was blown in the Beis HaMikdash alone:
"U'fiv metzupeh zahav" - its mouth was overlaid with gold. The Gemara clarifies that this does not mean the overlay was on the mouthpiece itself, for then the one blowing would be blowing on gold and not on a shofar, but rather on a strip alongside the mouthpiece, while the mouth of the one blowing rested on the shofar itself and not on the gold.
"U'shtei chatzotzros min hatzad" - and two trumpets at the side. In the Beis HaMikdash two metal trumpets stood on either side of the one blowing the shofar. "Hashofar ma'arich v'hachatzotzros mekatzros" - the sound of the shofar was drawn out beyond the sound of the trumpets, while the sound of the trumpets was shorter, so that at the end of the blast the sound of the shofar alone would be heard, since the mitzvah of the day is with the shofar.
And why is the shofar accompanied by trumpets? Because it says in Tehillim: "With trumpets and the sound of the shofar, let us shout before Hashem" - the trumpets are metal trumpets, and from here we learn that in the Beis HaMikdash the blowing of the shofar is accompanied by trumpets. Outside the Beis HaMikdash, however, this is of course not the practice.
In summary: in this mishnah we learned that according to the tanna the shofar of Rosh Hashanah is from a ya'el and straight, while according to Rabbi Yehudah it is from a ram and bent, and that is the halachah; and that the root of the dispute is the question of whether a person's mind on Rosh Hashanah should be straight and unfolded or bent in submission. We also learned the description of the shofar in the Mikdash: its mouth overlaid with gold, and two trumpets at the side, with the shofar drawing out its sound and the trumpets cutting theirs short, since the mitzvah of the day is with the shofar.