How Important Are Stc Ratings To Good Acoustical Design?

The U.S. construction industry complies with national model and local building codes. The importance of code was highlighted during the tragic events in Italy in early 2009. A Reuters article blamed less than scrupulous respect for anti-seismic building codes for the collapse of a hospital built in the year 2000 in the region. Similarly, if you compare the number of car crashes on the news daily to the number of fires, and you also get a sense of just how effective and necessary these codes are. Unfortunately, building designs optimized for fire safety often work against good acoustical performance. In other words, fire and sound arent usually building compatible.

When designing for quiet, the industry often references the Gypsum Association 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.

The Gypsum Associations publication references the industrys 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 code dictates an STC Rating of 50 or higher. For designing adequate noise proofing, when it comes to STC ratings, there arent a lot of other measurement options or guidance on the right level to target.

Understanding STC Ratings
But as an industry, we need to consider that much of the foundation for the STC Ratings system is dated. Testing and lab results in the Gypsum Associations manual are more than 40 years old. The STC Ratings system was developed post WWII for typical household noises such as conversation, dishwashing, and music from a radio. It takes into account frequencies down to 125 Hz. As machinery advanced, trucks grew, home theaters bloomed, and subwoofers became ubiquitous, noises below 125 Hz became common and 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 STC value is a calculation based on the ASTM Standard E90. STC is somewhat like how the EPA calculates the miles per gallon performance of vehicles. Even though the sticker may state 28 MPG, results will vary based on speed, driving situation and vehicle condition. If you have one gallon left in your car, and the nearest gas station is 27.5 miles away, chances are, youre not going to risk the drive.

Similarly, STC 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 frequencies 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 will need 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 5 STC points either way doesnt make a huge difference in how people hear noise. Even if youve designed for an STC rating, STC Ratings of 50 in the lab generally get built to 45 in real life. Lets say your design is 3 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.

The most noticeable changes of STC Ratings are greater than 5, and a 10 point difference can be like turning off sound to the human ear.

The Institute for Research in Construction conducted a study that found the point at which sound insulation starts becoming effective is approximately STC 55, or to put it even more simply, STC 55 makes people happy. If we design for an STC 60, it follows that as an industry we will end up with an STC 55 in the field. But STC 60 doesnt happen with traditional wall construction. To get to STC 60, builders need an acoustical consultant or other field expert to help.

Noise Reduction Design Solutions
Experts can help determine the best way that sound transmission control solutions could work for your project. For example, one common method is to add mass; another is to use structurally independent leaves. When working with mass law, each time you double up a layer of similar mass material, you can gain up to 6 STC points. This principle works on that fact that the heavier an object is, the more energy it takes to vibrate it. Movie theaters have used this design process for years, some walls between screens are up to 3 feet thick with many layers of concrete block and then faced with multiple layers of drywall--sometimes 16 to 20 layers on each side of the CMUs . 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 hasnt been used in construction until recently due to many factors. It can be a messy process, end results are not ascetically pleasing, or it could not package in either in an easily usable form or application methods would be difficult. In the past few years, however, solutions that are easy to use, reliable and ascetically pleasing are now available.

Caulking is an absolutely critical component of a high TL wall. During building, a common occurrence happens when the contractor ran out of caulk as he was nearing the end of the job. He may have had every intention of finishing but may have been distracted or forgot to come back to that spot. 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. You now know what STC measures airborne sound transmission. IIC stands for Impact Isolation Class and it measures structure-borne sound transmission.

IIC is calculated similarly to how STC is calculated 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 involve what is often called high heel noise, which occurs at high frequencies. Many experts believe that 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. You can use the features of the surrounding rooms to assist in lowering sound transmission.

How the ductwork runs are done can have a dramatic impact. Sound will take the path of least resistance. 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.

Just like the previous example door orientation can make it easy for sound to travel between rooms. Locating doors away from each other or diagonally opposed will definitely help.

Code Plus Design for True Value
A perceived savings by engineering for value up front could lead to an added liability exposure to all involved. Plans are developed without considering acoustical function of space. Noise issues are not realized until there is occupancy and then there is usually no budget set aside for remediation.

An upfront investment to consider acoustical function can generate a serious return on investment. Noise mitigation costs 10% as much at the design phase rather than the build phase. In other words, delaying the design of noise mitigation until the build phase can increase the cost of implementation by 10 times.

Most all of us are affected by noise or sound in and around our buildings. The best designs incorporate sound management into the overall design solution. Acoustics needs to be not only considered but incorporated at the very start of a design projectjust as other building sciences are utilized during the design phase.

Today, I would urge acoustical designers to use STC Ratings and other codes as a tool to use as a general indicator of goodness but not 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.