CBP Magazine: Articles

There is more to a quiet building than a sound-transmission-class rating number. Consider the placement of doors, ducts, and even the use of caulk.

When designing for quiet, the U.S. construction industry often references the Gypsum Association's (Hyattsville, MD), fire-resistance design manual to determine the best way to create noise reduction for occupants while still keeping the design in compliance with fire codes. Unfortunately, building designs optimized for fire safety often work against good acoustical performance. Fire and sound are not usually building compatible.

The manual references the industry's most pervasive noise-control requirements, sound transmission class (STC) ratings, as a way to determine the success of noise-damping design. The generally accepted rule is that the higher the STC rating, the better the design. In many cases, model building codes dictate an STC rating of 50 or higher. For designing adequate noise proofing, when it comes to STC ratings, there are not a lot of other measurement options or guidance on the correct levels to reference.

Much of the foundation for the STC ratings system is dated. Testing and lab results in the Gypsum Association's manual are more than 40 yr. old. The STC ratings system was developed post-WWII for typical household noises such as conversation, dishwashing, and radio music. It takes into account frequencies as low as 125 Hz. As machinery advanced, trucks grew, home theaters bloomed, and subwoofers became ubiquitous, noise frequencies less than 125 Hz became common. STC ratings are now less an indicator of quiet.

In order to best understand how to maximize the value of STC ratings, it is helpful to understand how STC is calculated for a particular wall assembly. The value is a calculation based on the ASTM Standard E90. Ratings are not the most precise way to measure sound damping, but it gives a usable scale. However, if you want to address noise in a particular frequency range, i.e., low or high frequencies, it is smart to consult the actual transmission loss (TL) curve to identify exactly how the wall assembly performed in those frequencies. This will let you predict more accurately the results you can expect from the assembly in your project.

STC value calculation ignores TL below 125 and de-emphasizes frequencies 500 Hz and below. If your project needs to control noise at these frequencies, the calculated STC value would not be a particularly valuable criterion for predicting the performance of an assembly for your project.

Another potential complication is that a variable of less than five STC points either way doesn't make a huge difference in how people hear noise. STC ratings of 50 designed in the lab generally are built to 45 in real life. If your design is three STC points short of meeting code, you may change that design to comply with code. The technical requirements will be met, but the owner or occupants may not experience the expected comfort level.

The most noticeable changes of STC ratings are greater than five. A 10-point difference can be like turning off sound to the human ear. A study by the Institute for Research in Construction at the National Research Council Canada, Ottawa, found the point at which sound insulation starts to become effective is approximately STC 55. In other words, STC 55 makes people happy. If we design for an STC 60, we end up with an STC 55 in the field. But STC 60 doesn't happen with traditional wall construction. To get to STC 60, builders need to involve an acoustical consultant or other field expert.

Experts help determine the best way sound-transmission control solutions could work for a project. For example, one common method is to add mass. Another is to use structurally independent leaves.

Each time you double up a layer of similar mass material, you can gain as many as six STC points. This principle works on the fact that the heavier an object is, the more energy it takes to vibrate. Movie theaters have used this design process for years. Some walls between screens are as much as 3 ft. thick, with many layers of concrete block faced with multiple layers of drywall, sometimes 16 to 20 layers on each side.

Structurally independent leaves work by separating the sound control from the structure. Another method uses damped-layer panels. Damping technology is not new. It has been used in planes, trains, and automobiles for years to increase cabin comfort. It has not been used in construction until recently, when easy-to-use, reliable, and aesthetically pleasing solutions became available.

Caulking is an absolutely critical component of a high TL wall. Sometimes, a contractor runs out of caulk near the end of a job. He may have every intention of finishing but forgets to come back to that spot and complete the caulking. By missing that spot, the resulting STC is 10 points lower than specified. The real problem comes into play at the time of occupancy, because at this point the performance is not being tested.

So far, we have focused on wall assemblies and code. It is important to note that controlling noise with floor and ceiling assemblies is another type of challenge. Floor and ceiling assemblies are rated on two independent codes: STC and IIC. STC measures airborne sound transmission; IIC (impact isolation class) measures structure-borne sound transmission.

IIC is calculated similarly to STC calculations with a number of values resulting from tests at various frequencies. However, there is some dissatisfaction with the IIC measurement today. Some of the most common complaints about noise are so-called high-heel noise, which occurs at high frequencies. Many experts believe the current IIC measurement under-weighs high frequencies, so that a high-IIC floor/ceiling assembly will not always solve the problem of high-heel noise. We expect that a growing movement to modify how the calculation is made will soon effect changes that will allow IIC to provide values that more closely fit builders' expectations.

Another, sometimes overlooked, way to help with sound control simply has to do with orientation within the environment. Sound takes the path of least resistance. Take ductwork runs, for example. Some fairly typical designs provide a direct pathway for sound to travel from room to room. A better way would be to run ducts along a hallway and then trunk them in and out of each room. Door orientation can make it easy for sound to travel between rooms. Locating doors away from each other or diagonally opposed will definitely lessen sound transmission.

A perceived savings upfront by engineering for "value" could lead to an added liability exposure for all involved if plans are developed without considering the acoustical function of the space, and noise issues are not realized until there is occupancy. Then there is usually no budget set aside for remediation.

An upfront investment to consider acoustical function can generate a serious return on investment. Delaying the design of noise mitigation until the build phase can increase the cost of implementation by 10 times.

The best buildings incorporate sound management into the overall design solution. Acoustics need to be not only considered but also incorporated at the very beginning of a design project-just as other building sciences are used during the design phase.

Acoustical designers should use STC ratings and other codes as a tool for a general indicator of "goodness," but not as a detailed picture of owner satisfaction. The ultimate proof is in the performance of a wall, and that usually means implementing an overall acoustic design at code-plus.

Dr. Brandon Tinianov is the chief technology officer of Serious Materials, Sunnyvale, CA.

Rick Grass, Calgary, AB
very good article. especially like the TL explanation.

Sound-Rating Perception
Perception	STC
Poor	30 to 39
Good	40 to 49
Better	50 to 59
Excellent	60 to 69

Sound Advice For Acoustical Design