4 Way Diffusers Grilles White Finish 150mm to 600mm. Square and rectangular multi-cone diffusers have been designed to meet a wide range of modern applications and include sizes for ceiling tile replacement. They are supplied as standard with a spring loaded removable centre core for ease of cleaning and adjustment. All diffusers are supplied with a safety cord between the diffuser core and frame. Size Range From 150 x 150mm to 600 x 600mm nominal neck size. For 600 x 600 T bar ceilings please specify 450mm square neck with 595 x 595 dimensions across face of diffuser.