Page 39 - January 2007

Fig. 1. Types of floor-ceiling assemblies for medium quality STC ratings.
mechanism and dropped sequentially from a height of 4 cm (1.6 inch) onto the surface of the floor at a rate of 10 impacts per second. There are test standards in the United States and in Europe that regulate the laboratory (ASTM E 492 and ISO 140/6) as well as field (ASTM E 1007 and ISO 140/7) test methodologies. Spatially averaged sound pressure levels measured in the room below are adjusted for the absorption in the receiving room and compared to standard values to obtain an overall rating.
Minimum IIC ratings are set to 50 in the UBC with a minimum field tested FIIC of 45 allowed. At this rating, foot- fall noise is quite pronounced and very audible in the unit below. In response some cities and condominium associa- tions have adopted more stringent laws. The City of Redondo Beach, for example, sets a minimum IIC rating of 65 in con- dominiums. Other cities such as Beverly Hills control noise through a property line ordinance. The point at which foot- fall-generated impact noise becomes inaudible is closer to an IIC of 75. The level of quality due a buyer in the control of impact-generated noise is numerically higher than that for airborne noise.
Table 3. Impact Insulation Class vs Level of Construction for Party Floor–Ceiling Construction
Classification
Minimum Code Minimum Quality Medium Quality High Quality
Floor coverings
IIC FIIC 50 45 55 50 65 60 75 70
It is relatively easy to achieve high impact insulation class ratings by using carpet and pad. Medium quality ratings are achievable with a vinyl floor surface on a 1/2 inch rubber mat. When hard surface materials such as quarry tile, marble, or hardwood floors are installed, low impact ratings are not obtained unless relatively thick (1 inch) isolating underlayer- ments are utilized. A number of products are commercially available, which are intended as resilient underlayments for hard surfaced flooring. Thin layers of resilient material such as fiberglass board, cardboard-like materials, and wire mesh mats can raise the IIC ratings modestly, three to five points
Footfall noise
The act of walking across a floor generates noise due to two mechanisms: footfall and structural deflection. Footfall noise is created by the impact of a hard object, such as a heel, striking the surface of a floor. A heel is relatively lightweight and the noise associated with its fall is considered separately from the transfer of weight due to walking. Impact noise can be measured using a standard tapping machine as a source, which leads to an Impact Insulation Class (IIC) rating. The IIC test measures the reaction of a floor system to a series of small hammers dropped from a standard height. Although this may accurately characterize the noise of a heel tap against the floor surface, it does not measure the effect of loading and unloading under the full weight of a walker. Thus the achievement of a particular IIC rating in a given floor—ceiling system does not guarantee that footfall noise will not be a problem, or that the sound of walking will not be audible in the spaces below.
The level of impact noise in the receiving space is prima- rily dependent on the softness of the floor covering, and is best attenuated using a thick carpet and pad. Hard surface floors must be installed on thick resilient underlayerments and used in conjunction with a vibrationally-isolated ceiling to achieve medium quality results.
Impact Insulation Class—IIC
The Impact Insulation Class (IIC) is a laboratory rat- ing much like the Sound Transmission Class; however, it rep- resents the isolation provided by a floor system subjected to a controlled impulsive load. Since there is no standard foot- step, the impulsive loads are generated by a tapping machine consisting of a row of five cylindrical hammers, each weigh- ing a half-kilogram (1.1 lbs), which are raised by a cam
Fig. 2. Isolated-composite floor-ceiling.
Acoustics of Floors in Condominiums 37