A hardwood floor underlay with high density reduces movements and risk of squeaks in the hardwood floor. In addition, you get a underlay with very good audio characteristics.
Maxima - For wooden subfloors
Hardwood floor underlay with a high density, like Maxima, will retain its original shape when it’s installed under the hardwood floor. Choosing this type of underlay will therefore help the hardwood floor to retain its original features and the risk of unwanted sound from the hardwood floor is reduced. Underlay like Maxima also has good sound characteristics. Sonority within the room and transmission between floors is reduced.
Boengard - For concrete subfloors
Usually you have to use a plastic moisture barrier if you are going to install your hardwood floor on to a concrete subfloor but, with BoenGard there is no need. With BoenGard you get a combined moisture barrier and underlay in a single product, so it makes your job a lot easier and saves time when installing on a concrete subfloors. BoenGard also has the same great sound qualities as Maxima.