| 摘要 |
A piezoelectric element (120a) is secured to a dust filter (119) as a dust-screening member which is shaped like a plate as a whole and has at least one side that is symmetric with respect to a symmetry axis, the piezoelectric element being driven to produce vibration Z (x, y) at the dust filter, the vibration being expressed as follows: Z x �¢ y = W mn x �¢ y ‹ cos ³ + W nm x �¢ y ‹ sin ³
where Z (x, y) is vibration at a given point P (x, y) on the dust filter, m and n are positive integers including 0, indicating the order of natural vibration corresponding to a vibrational mode, W mn x �¢ y = sin n �¢ À ‹ x + À 2 ‹ sin m �¢ À ‹ y + À 2 , W nm x �¢ y = sin m �¢ À ‹ x + À 2 ‹ sin n �¢ À ‹ y + À 2 , and
³ is +À/4 or ranges from -À/8 to -À/4. LP/LF is 0.5 or more, but less than 1, where LF is the length of the sides to which the vibrating member is arranged in a virtual rectangle (175) having the same area as the surface of the dust-screening member and sides including the one side, and LB is the longitudinal length of the vibrating member of the sides parallel to the one side. |
