Building codes provide a reasonable level of safety and occupant comfort. For example, buildings are never fireproof, but they’re built to provide a practical fire buffer. The same can be said of acoustics. Although never soundproof, buildings must meet International Building Code standards for noise transfer between residences.
Some noise should be expected in a multifamily living arrangement, though, and the code recognizes this. However, some incoming residents to senior living communities might not expect that. Instead, they’ll likely anticipate the same quiet environment they’ve experienced in a single-family home. Children have long since moved out, and prospective residents may even live alone. The only sounds they hear are their own.
Rarely will anyone explain to residents entering a new community the reality of what they might hear from their neighbors, but expectations should be better managed in order not to disappoint new move-ins. The walls, ceilings, and floors between units will never be soundproof, but buildings can be constructed to manage noise at an acceptable level.
The basics
Current building codes address both airborne and structure-borne sound. Simply stated, airborne means hearing your neighbor’s voice or TV, and structure-borne is hearing your neighbor’s footfalls above you or their garage door open.
To address airborne sound in particular, the wall, floor, and ceiling assemblies between dwellings must meet a sound transmission class (STC) of 50 to reduce transfer from one side of the wall to the other, a level that far exceeds standard wall construction found in older building stock or in a single-family home. STC ratings are a fairly sensible predictor of how a wall will perform and use a range of 125 to 4,000 hertz, which is the range of frequencies associated with human speech.
A wood stud wall with a layer of wallboard on either side has an STC of about 34. To get to about 50-54 STC (as measured in a laboratory), a wood stud wall must have two layers of wallboard on one side of the studs, the stud cavity filled with fiberglass insulation, and another layer fastened with resilient metal clips to offset the board from the stud by one-half inch. These clips allow one side of the wall to vibrate independently from the other side, so the noise energy doesn’t translate from one side of the clip to the other at 100 percent.
This example wall is also, in effect, 50 percent more dense than the standard wall. Building codes also require that all openings, cracks, and seams be sealed with acoustical caulk and that penetrations, such as electrical outlets, be staggered between studs to avoid having two across from each other within the same cavity opening.
So overall, code calls for the standard wall to be about 15 to 20 STC points higher than it would have been. That seems pretty good, but how good? A rule of thumb is that for every 10 STC you add to an assembly, the sound coming to the other side is roughly half as loud. So an added STC of 15-20 will roughly slash airborne noise by one-third or one-quarter, but doesn’t eliminate it altogether.
Floors will be very similar to walls, except a typical assembly “sandwich” would be made up of wood joists and fiberglass insulation topped with plywood subfloor and gypsum poured topping. The bottom would have several layers of wallboard attached to metal clips on the underside of the joists.
Additionally, a floor needs to perform well in terms of structure-borne noise, such as footfalls. Typically, this is addressed with an acoustical mat placed between the gypsum topping and the plywood in the assembly to provide additional resilience. The mats can also be placed right below flooring.
Adding and subtracting
When considering going beyond code, it’s important to measure diminishing returns. For example, 5 STC is clearly noticeable, 3 STC is just barely perceptible, and 1 STC is almost imperceptible. Mass is important to acoustical performance, so if the thickness of a membrane was doubled with two layers of wallboard instead of one, the STC rating will increase by about 5. Installing insulation in a wall also adds about 5 STC. An STC of 60 is considered a “luxury” level, but even then some loud noises will be heard faintly through the walls.
Although wood construction is most often used in residential settings, metal studs perform a bit better due to their flexible characteristics, as they can soften or decouple some of the noise vibrations from one side of a wall to another. Additional considerations in acoustical dissipation include mass, airspace between, and resiliency (or materials that flex and decouple). Any time two or more of these properties can be added to an assembly, improvements will be realized.
During pricing exercises on a building or renovation project, designers and owners often must collaborate to make concessions and reduce costs. Some of those decisions may affect acoustical comfort, including replacing cast iron waste pipes with PVC pipes, reducing or changing types of insulation on pipes, providing alternate wall assemblies or alternate finishes that may be less absorptive of sound, or choosing HVAC and mechanical items that perform worse acoustically.
All of these choices can degrade the acoustical comfort present in a building during its initial design, so it’s important to note the added cost of addressing acoustical issues after construction is complete and adequately weigh the ramifications of those decisions.
Peace and quiet
Prospective residents should always be alerted that their future home won’t be soundproof, even in new construction. Whether they’re moving into an apartment or semi-detached unit, it’s often a housing product that’s new to them, so differences between single-home and community living should be outlined. In most cases, adhering to the building standards above and proactively managing expectations will provide desired acoustical comfort and satisfaction.
James Mehaffey is senior project manager at RLPS Architects (Lancaster, Pa.). He can be reached at [email protected].
