bullhorn
loud hailer
loud-hailer

Then a firm, calm voice through a loud hailer.

We were calling all the time through the loud hailer.

He thumbed the switch and raised the loud hailer to his mouth.

He raised the loud hailer to his mouth and, as he talked, started to walk toward the house.

Allen sighed but reached for the loud hailer and raised it to his lips.

A loud hailer could also be used along with a radio warning.

The rating turned and spoke to someone behind him, then once more faced John with the loud hailer.

It was an effort to lift his own loud hailer.

He restored the loud hailer to its cradle under his seat.

John retrieved his loud hailer and directed it at the lieutenant.