Sound deadening for wood floors.

Noise transfer between spaces has been an issue with hard surface wood flooring for many years. New York City was one of the first areas to make specific requirements for sound transfer. These standards were most often applied to multistory apartments. The standard often referred to is Sound Transmission Class (STC) of 50 and Impact Insulation Class (IIC) of 51. STC is the transfer of sound generated within a space through the wall, floor, or ceiling to an adjacent space. IIC is the sound that is generated by a tapping machine directly on to the structure (wall, floor, or ceiling) and sound transferred through the structure to the adjoining space.

NOFMA conducted sound transmission tests in 1982 to address those systems that did or did not meet the standards. The basic systems used were all floating systems of some variety. Woven nylon matting, 1/8" and 1/4" closed cell foam, and asphalt insulation board were used as the sound deadening material. Both wood joists systems and concrete slab systems were tested. Wood joist systems of framing with 1/2" plywood subflooring, sound material, 3/4" floating subfloor, and 3/4" strip flooring; all failed the required standard. The concrete slab system using 1/2" insulation board, 3/4" floated plywood subflooring, and 3/4" strip flooring nailed with 1 1/2" cleats; was the only system that met the standard with STC of 52 and IIC of 55. The slab system using the nylon matting was modified with a suspended ceiling below of 5/8' gypsum in metal channels held by 9" wires and 3 1/2" fiberglass insulation on the gypsum board also met the standard. This system produced STC of 60 and IIC of 61.

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A review of these systems all pointed to isolating the flooring and subflooring from the primary support system to obtain the highest readings. Metal fasteners all transfer noises effectively, so nailing flooring to a subfloor which is also nailed to the slab or framed system will be a noisy system. Items and procedures that will interrupt some of the noise transfer are: floating the flooring subfloor; placing a sound deadening material under the subfloor; isolating the edge of the flooring system from the surrounding structure; gluing the different components together; and/or suspending the underlying ceiling.

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The question is often asked for a nail down flooring system; "Can I put the sound deadening material directly under the flooring and nail through it into the subfloor using longer fasteners?" NOFMA does not recommend this procedure since the sound material will not hold the flooring fastener properly and most materials compress or deflect with point loading. Flooring noises, creaks and squeaks, and movement are the likely result.

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