Easily-fitted silencer for circular ducts, fitted with a connection that complies with the spiral duct standard. The LD effectively reduces noise in the duct. Two silencers can be used together in installations where noise reduction is a particularly strong requirement. This is very effective. For the most effective noise reduction, the silencer should be fitted immediately behind a fan or bend. The silencer should be used together with an insulated fan where there is a requirement for noise reduction both in the duct and in the surroundings as a whole. Insulation thickness 50 mm.