Abstract: A propeller for an aircraft or a blower or fan, wherein the propeller blades are intended for operation at speeds characterized by blade tip Mach numbers in the range of about 0.4 to 0.8, comprises at least two pairs of diametrically opposed blades, the blade pairs being arranged at a spacing angle .epsilon. of between about 15.degree. and 50.degree. between them, and the magnitude of the spacing angle being inversely related to the magnitude of the blade tip Mach numbers, thereby to effect a reduction in the level of the rotational sound harmonics by interference. The pairs of blades may be axially spaced from one another, i.e., arranged in respective spaced parallel planes of rotation, shifted, with the rear one of two adjacent blades leading in the direction of rotation.