A horn that has not been specifically designed to have gain. In actual practice, unshaped horns usually have a gain that differs somewhat from 1.0 due to the stud, slots, or small modifications that are used to improve uniformity (e.g., risers and flutes).
Unshaped horns generally have sides that are parallel to the stud axis. All prismatic horns are unshaped. Other horns that are typically unshaped include block horns and slotted cylindrical horns that don't have a face cavity.
See shaped horn.