Floor Acoustics in Multi-Unit Residential Projects

Why Proflex Floor Acoustics products are needed under your floors.

The condominium in Boston was comfortable, about 1600 square feet, and the builder had a reputation
for quality. Unfortunately, even with a $700,000 price tag, the noise problems were considerable. Movements of people upstairs, both footfall on the hard surface floors and walking on carpeted
floors, were loud enough to wake them in the morning. When toilets were flushed upstairs, it sounded like a waterfall running through their walls. The closing of doors, cabinets, and drawers could all be clearly heard. Walking in the upstairs hallway was plainly audible. Situations such as these are increasingly common in Boston, where the city building department and elected officials
refuse to enforce the State’s building codes on noise. Even if they were enforced, they are so anemic that minimum code compliance does not yield a quiet living environment. In fact when the owner complained to the builder, subsequent acoustical tests showed the building to be in compliance
(barely) with Massachusetts minimum code standards. As a growing population is extruded into highly urbanized areas, multifamily dwellings have become the most common way of packing more people into less space. Pressures of population and cost force people together, and noise and noise
transmission between occupied spaces are significant concerns. People want their apartment and condominium homes to be quiet and free from intrusions, just like a single-family
residence, but the reality is quite different. Floor ceiling noise transmission
The most common noise and vibration problem in condominiums is transmission through floor–ceilings and it falls into four categories: 1) airborne, 2) footfall, 3) structural deflection, and 4) floor squeak.


Airborne noise
Airborne noise is created in one space and is transmitted through the air and through an intervening partition into the adjacent space. Sources might include TV, stereo systems, or simple conversation. The isolation of airborne noise such as speech is well characterized by the Sound Transmission Class (STC) rating. The STC rating is the result of a test performed in accordance with ASTM E 90 or ISO 140 laboratory test standards. To do the test a partition is built into a heavy wall separating two concrete test chambers in a laboratory. A standard sound is created in one room, called the source room, and transmitted through the partition and into the receiver room. The difference between
the levels in the source and receiver room, adjusted for the area of the partition and the absorption in the receiver room is the transmission loss in decibels. The measured transmission loss values in 16 third-octave frequency bands are compared to a standard curve (ASTM E413) to determine the STC rating. The higher the STC rating, the better the partition is at stopping airborne noise. Under field (F) conditions the measured FSTC rating is about five points lower than the laboratory rating, and this difference is acknowledged in the building codes. Thus if an STC 50 is the required laboratory rating, a field test of 45 is equivalent. In New England the test procedure has been diluted by eliminating the inclusion of the correction for the absorption in the receiving room.

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 primarily 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.

The achievement of adequate isolation between dwelling units is becoming more routine with the recognition of the factors influencing noise transmission and with the introduction
of products on the market that provide vibrational separation between floor-ceiling components. While the use of each of the techniques cited above does not guarantee a perfect result, the Proflex RCU 250 has used them to improve the isolation between spaces in multifamily dwellings and achieve results in line with the standards recommended in this article. Where dwelling units are separated by design, good results can be achieved without complicated construction techniques. For example, in multifamily dwellings a townhouse plan is preferred over stacked units to avoid common floor-ceilings. When multi-story units are necessary, a plan that stacks similar rooms, one above another, avoids incompatible uses such as a bathroom located above a bedroom. Closets and other non-sensitive spaces can be located on party walls to provide additional shielding. Modern buildings are constructed from lightweight materials, usually wood or light gauge steel studs, and the sound transmitted between spaces can be relatively high. In the older masonry and concrete structures, the mass law of building acoustics insured that sound isolation would be very good. The exigencies of cost and time have pushed building construction towards lighter and cheaper materials, and hence to greater sound transmission. Given these very real constraints, it is incumbent upon architects and engineers to find ways of providing adequate sound isolation in residential structures using commonly available and affordable materials such as Proflex 90, Proflex RCU 250, or Proflex RCU 500. Finally, we should not be lulled into a sense of complacency by compliance with minimum code standards. These standards are so poor that they do not yield a quality result. Instead, we should design to the reasonable expectation of a buyer. When better buildings become available they will command a higher value in the marketplace and reward a builder for his or her efforts.