| 摘要 |
PROBLEM TO BE SOLVED: To prevent noises due to air sound when a rotary polyhedral mirror is rotated at a high speed by providing the inside wall of a cover for covering the circumference of this rotary polyhedral mirror with a plurality of annular peak-shaped projections. SOLUTION: The annular peak-shaped projections 3 which are sound absorption means having a special sound absorption effect are disposed on the inside wall of the cover 3. The peak-shaped projections 13 are formed in a plurality to an annular shape in the circumferential direction of the inside wall of the cylindrical portion 3a of the cover 3. The cross-sectional shape of the respective peak-shaped projections 13 is a peak shape and sound waves repetitively come into collision against the slopes thereof, by which the energy of the sound is successively annihilated on the principle similar to that of pyramidal projects disposed on the inside wall of a so-called acoustic chamber. As a result, the sound absorption effect of the cover 3 is increased and the noises due to the air sound when the rotary polyhedral mirror 1 is rotated at a high speed are lowered. Then, even if the number of revolutions of the rotary polyhedral mirror 1 exceeds 30,000 rpm, there is the sound absorption effect by the inside wall of the cover 3 provided with the peak-shaped projections 13 and, therefore, the scanning optical device of the silent running sound may be obtd. |
