Page 40 - January 2007

above minimum code, but seldom provide sufficient deflec- tion to achieve the impact isolation necessary for condo- minium construction. Thicker fiberboard or composite rub- ber mats give better results.
A medium quality composite floor construction, shown in Fig. 2, might consist of 3/4 inch hardwood flooring on a 1 inch thick resilient underlayer, on 3 inch concrete on steel deck on joists. When combined with a double 5/8 inch dry- wall ceiling supported on hat channel and neoprene isolators these constructions can provide IIC ratings in the medium quality range.
Where the appearance of wood or tile is desired, the hard surface can be used in non-walking areas such as within 1 to 2 feet (0.3 - 0.6 m) of a wall with carpet installed where walk- ing traffic occurs. In kitchen and bathroom areas, hard tile over a thick backing material can provide reasonable IIC rat- ings when combined with a point-mounted resiliently sus- pended ceiling. In all cases, engineers should refer to labora- tory tests published by the manufacturers.
Structural deflection
The achievement of a high IIC rating in a given floor- ceiling system does not guarantee that noise will not be a problem or that the sound of walking will not be audible in the units below. The IIC test measures the reaction of a floor system to the impact of a series of 1.1 lb (0.5 kg) weights dropped on the surface. Although this may model the noise of a heel tap, it does not represent the full effect of the load- ing and unloading under the weight of a walker. When peo- ple step or even stand on a floor, it will deflect under the stat- ic and dynamic load of their weight. If the underside of the floor is exposed to the room below, a sound generated by this motion will radiate directly into the receiving space. Noise generated by floor deflection sounds like low-frequency thumps, whereas heel strikes create high frequency clicks.
Three mechanisms are available to improve this con- dition: 1) increase the stiffness of the floor system, 2) increase the structural damping, and 3) increase the vibrational decoupling between the floor and the ceiling. In concrete structures both the stiffness and damping increase with slab thickness. For the 6 inch (152 mm) concrete slab required to achieve an STC of 53-55, structural deflection is rarely a
problem for moderate spans. In wood structures, the most common type of minimum quality construction consists of 1.5 inch (38 mm) lightweight concrete on plywood on joists with ceilings of drywall on resilient channel. This construc- tion can transmit considerable low-frequency noise, since for normal joist lengths the deflection of a resilient channel is not sufficient to overcome the deflection of the joists.
In wood construction, both stiffness and damping can be increased by using the stepped blocking shown in Fig. 3. The blocking works for several reasons. The first is the damping added by the moment connection provided by the glued faces and end nailing. Second, stiffness is increased by building the equivalent of another beam in the middle of the joist system. The third effect is additional load spreading, which distrib- utes a point load among several joists and helps increase the composite floor stiffness. Stepped blocking is more effective than doubling joists or reducing joist spacing, although the two can be combined to good effect.
When prefabricated truss joists are used, a spacer plate must be installed as in Fig. 4. Stepped blocking should be located at the mid-span in joists having a length of between 10 to 18 feet (3—5.5 m) and at the one-third points in joists greater than 18 feet.
Fig. 4. Stepped-blocking in TJI joist systems.
Structural decoupling
If a floor-ceiling system is not a monolithic slab, it generally includes an independently supported ceiling, which may be isolated vibrationally from the structure. In concrete construction the most common support system is hanger wires at 4 ft (1.2 m) on center wrapped around 1 1/2 inch (38 mm) carrying channel (black iron) to which 7/8 inch (22 mm) metal furring channels (hat channels) are wire tied. This system provides some isolation because it uses a point connection rather than a line connection. It can be further improved by utilizing vibration isolators either in the form of neoprene hangers or steel spring isolators cut into the hang- er wires.
In wood structures, the most common type of structural decoupling has been resilient channel. At high frequencies, resilient channel can provide some improvement to the structural isolation; at very low frequencies; however, it is not particularly effective. Neoprene mounts, which include a clip
Fig. 3. Stepped-blocking in 2 x joist systems.
38 Acoustics Today, January 2007