This silencer absorbs noise and is easy to install in a round air duct. If desired, you can place several silencers behind each other for extra noise reduction. To reduce noise from ventilation systems, such as the noise of a heat recovery unit, you use one or more silencers. The silencer tackles the noise as close to the source as possible. Silencers are made of polyurethane flake foam. The silencer has a dust-resistant skin on one side. The material is flame-extinguishing according to EN ISO 11925-2:2002. By pushing one or more plugs out of the silencer (or putting them back in), the best ratio between sound attenuation and airflow can be determined. The best result is achieved by placing two silencers with an interval of +/- 10 cm with all plugs removed one after the other in the air duct.