Acoustic Expectations

Building codes provide a reasonable level of safety and occupant comfort. Buildings are never “fire proof”, but they are built to provide a practical fire buffer, whether it be one, two or three hours. The same can be said of acoustical separation in buildings. Acoustics are a relative newcomer to building regulations, but the International Building Code does regulate how much noise should be allowed to transfer between residences. However, some noise should be expected in a multi-family living arrangement, and the Code recognizes this.

I grew up in a semi-detached home, then a dorm room and then lived in a town house until I was 27. None of these close quarter living units was even close to complying with ‘modern’ code-required sound requirements. For almost 30 years I expected to hear my neighbors come in the door, hear their garbage disposal and generally, hear a bit of their everyday life. But as long as they kept their Pavaratti album to a normal level, this was ok because this is what I had come to expect.

However, this is not what some incoming residents to retirement living may expect. They often anticipate the same quiet environment they experienced in a single family home, with no children, sometimes living completely alone. The only sounds they hear are made by themselves.

Rarely will anyone explain to prospective residents the reality of what they can expect to hear from their neighbors. This is a disservice to the residents and to the administrators who must somehow address the complaints. Expectations need to be managed to avoid frustration and disappointment for new move-ins. The walls, ceilings and floors between units will in most cases never be “sound proofed”, but we as designers should do what is possible to manage and dissipate sound to acceptable levels.

Current building codes address both air-borne and structure-borne sound. Simply stated, air-borne sound is hearing your neighbor’s voice or TV. Structure-borne noise is hearing your neighbor’s footfalls above you or their garage door open. So at a minimum the wall, floor and ceiling assemblies between dwelling units must meet sound transmission class (STS) of 50 to reduce air-borne noise from one side of the wall to the other. This far exceeds standard wall construction found in older building stock or a single family home.

To get technical: a wood stud wall with a layer of wall board 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 wall board on one side of the studs, the stud cavity is filled with fiberglass insulation, with another layer fastened with resilient metal clips which 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 and as such the noise does not translate from one side of the clip to the other at 100%. The fiberglass insulation works as an attenuator, reducing the strength of the signal, so to speak. This wall is also in effect 50% more dense than the standard wall. The building codes also require that all openings, cracks and seams must be sealed with acoustical caulk and that penetrations, such as electrical outlets, must be staggered between studs, to avoid having two across from one another within the same cavity opening.

So in a sense, a standard party wall is about 15 to 20 STC points higher than it would have been prior to the regulations in the building codes. That seems pretty good, but how good? A rule of thumb is 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 air-borne noise to somewhere between one third and one quarter of the original source. Notice it does not eliminate the noise 100%. STC ratings are a fairly sensible predictor of how a wall will perform. However, SCT uses a range of 125 to 4000 Hertz, which are the range of frequencies associated with human speech. This range does not really consider very high or very low frequencies, such as those produced by machines, air handlers or electrical transformers. It isn’t perfect but the STC rating system is the standard by which designers live.

In terms of diminishing returns, 5 STC is clearly noticeable, 3 STC is just barely perceptible, and 1 STC is almost imperceptible. Mass is also important to acoustical performance. If you double the thickness of a membrane, such as 2 layers of wall board in lieu of one, the STC rating will increase by about 5, which is clearly noticeable. Installing insulation is a wall also adds about 5 to the STC. An STC of 60 is considered a “luxury” level, but even at that level, some loud noises will be heard faintly through the walls.

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 wall board attached to the same kind of metal clips on the underside of the joists. Additionally, a floor needs to perform in terms of impact noise, i.e. footfalls, which is a structure-borne noise. Typically this is addressed with an acoustical mat placed between the gypsum topping and the plywood in the assembly. These mats provide additional resilience to the floor make up. The mats can also be placed right below flooring as well.

The above examples are in terms of wood construction because that is fairly standard in low to mid-rise residential construction. But actually metal studs preform a bit better than wood studs. This is due to their flexibility characteristics as they a can soften or decouple some of the noise vibrations from one side of the wall to another. In terms of acoustical dissipation, the big three are: Mass, Airspace between and Resiliency (or materials that flex and decouple). Any time you can add two or more of these properties to an assembly, improvements will be realized.

During the pricing exercises on a building or renovation project, designs are often modified to reduce costs. Many decisions can affect acoustical comfort. These items include replacing cast iron waste pipes for PVC pipes, reducing or changing types of insulation on pipes, providing alternate wall assemblies, alternate finishes which may be less absorptive of sound, alternate HVAC and mechanical items that may perform worse acoustically or alternative door hardware. All of these issues can degrade the acoustical comfort in a building. So be sure the possible sound transmission ramifications of these changes are considered since it’s much harder and likely more costly to address issues after construction.

How does this all translate to the expectations of prospective residents? They should be alerted that their future home will not be totally sound proofed even in new construction. Whether an apartment or semi-detached home, they are often moving into a new type of housing product, so it must be explained that there is an inherent difference between living in a single home versus within a community. In most cases, adhering to the standards outlined above and proactively managing expectations will provide the desired acoustical comfort and satisfaction.

Jim Mehaffey, AIA, is a senior project manager and code coordinator at RLPS. He contributes to office-wide quality control and code analysis for projects throughout the design process and as part of construction document review.