Types of rectangular attenuators: DZ-2 for the most effective attenuation at frequency range 250 Hz with splitters type K-2 , fitted with alternating belts of absorption material and steel membranes DZ-3 for the effective attenuation at frequency range 500 to 4000 Hz with splitters type K-3, which have the entire surface made of absorption material		DZ-2
Types of circular sound attenuators: ODZ-1 to be mounted in round ducts; sizes from diameter 250 to diameter 1250 mm MDZ-1 to prevent sound from spreading between rooms; sizes from diameter dN/diameter DN 80/180 to 355/400 mm		ODZ
Acoustic louvres are visually similar to protecor grilles. Absorption material, fastened to the blades attenuates the sound. Acoustic louvres are made of steel or aluminium sheet. References		JAR-1