Flow noise in Air duct
In most cases, structure-borne sound travels through the building structure via construction materials, frame and interior elements. It eventually becomes air-borne sound that can be heard at some distance from its source, perhaps several floors away. Air-borne sound can also become structure-borne causing surrounding surfaces to vibrate. Consider a sheet metal duct, which is a virtual wind tunnel through which any noise can travel. It provides a structural and an air-borne path for noise generated by mechanical equipment like fans and chillers. It can also transmit noise from virtually any other source in the building, e.g. people, speakers and machines. This air-borne-to-structure-borne conversion can repeat multiple times until the sound source is switched off.
Elastomeric foam products, such as Armaflex MC and ArmaSound Super Silence, react to sound quite
differently. Unlike fiberglass, which allows sound (in the form of varying air pressure) to freely enter into the spaces between the fibers, closed-cell elastomeric foam is too highly resistive to enable the viscous friction of air inside to develop an effective absorption mechanism. However, instead of simply reflecting the sound like many other types of rigid foam materials, the physical properties of elastomeric foams are such that their structure mechanically responds to the incident sound. This response can be particularly pronounced at certain frequencies, depending on the material’s chemical formulation, elasticity and thickness. In the case of Armaflex MC and ArmaSound Super Silence, a significant proportion of the incident sound energy is converted into movement of the foam and eventually into heat.