a trumpet: the long, straight metal trumpet blown by the priests, not a ram's horn.
This word names one specific instrument: the straight metal trumpet, made to order by human craftsmen. The law commands two of them to be beaten out of silver so the priests can sound signals for the whole nation. That craftsmanship is the point of the word. It is not just any noise-maker but a manufactured instrument, and it belongs in the hands of the priests.
Its jobs are practical and public. A blast on these trumpets could call the people together, signal the camp to move, sound the alarm for war, or ring out over the sacrifices at festivals and new moons. In the later worship of the temple, whole ranks of priests blew them together with the singers and the other instruments, so the word turns up again and again in the accounts of grand ceremonies in Chronicles.
The reason this word needs its own entry is a near-twin. Hebrew has a second, more famous trumpet, the ram's horn, and English usually calls both of them "trumpet." This one is the crafted metal instrument of the priests; the ram's horn is a natural horn used more widely, including by ordinary people. Several passages sound them side by side, which shows the writers kept the two apart even when our English translations blur them together.