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Much Ado About Sound Transmission
Sometimes I get challenged on the topics chosen for this little column. My goal is to write something that is a tad editorial with the goal of stimulating some food for thought in a technical setting while avoiding outright controversy. I usually leave the highly technical stuff to others more capable while I tend to lean more towards the philosophical side of issues – ha!
Anyway, I was recently challenged by an architect to defend the Association's standards and practices for sound control in new built “commodity” based wall and ceiling construction. The architect had just attended an industry promotion for a proprietary product which made a significant claim of improved performance with the use of their product and a particular method of application. The issue at hand presented me with the opportunity to tackle both a technical question nicely combined with a somewhat philosophical dilemma, with a hint of financial reality thrown in just for fun.
The Technical Jargon: The STC or Sound Transmission Class is the wall and ceiling industry protocol for assigning a value to a specific wall system’s ability to reduce the transmission of airborne sound – the higher the assigned value, the higher the wall's ability to reduce airborne sound transmission. STC ratings anywhere in the 50 to 55 range or higher are considered a pretty good wall. The fundamentals of building a quieter wall is to build it according to an actual design and assembly of components as per a specific rated wall system called up by the architect, and then to install it as air tight as possible. Simply put, anything that impedes the airflow between the spaces will enhance the sound performance of the wall.
Much like finding a standard in the in the National Building Code, there are usually several references to complete your research on a topic. The specifications for sound performance can be found in the AWCC 2003 Specifications and Standards Manual Section 9.6 Gypsum Board and further in the Gypsum Association Fire Resistance Design and Sound Control Manual – 19th edition which forms an integral part of the AWCC Manual. The various wall assemblies in the Fire Resistance Design Manual with its corresponding STC ratings tell us how to construct the wall in detail right down to the screws to use. However, there are additional measures that are required to complete the installation, or shall I say, to make the wall function as expected. Remember “build” and “install”. I refer to Section 3, Sound control, page 19, in the aforementioned Fire Resistance Design Manual where it says you must completely seal the entire perimeter of the wall to make it air tight.
The biggest challenge for the wall and ceiling contractor is that every other trade in the project wants to put a hole through his wall. Much like a leaky boat, your wall can only do so much to keep out ambient sound when the wall gets punched full of holes. It is up to the building designer to consolidate the placement of utilities in a way that minimizes errant flanking paths that can defeat the best built wall when it comes to sound control. But regardless of that, there are ways to minimize airborne sound transmission – it’s just a small matter of agreeing whose job it will be.
I concede that there are a lot of different approaches and building system methods in practice when it comes to sound control. In particular the application of acoustical caulk has recently come up with many products available and different application methods abound. What I can tell you is that several independent tests show that two beads of acoustical caulk applied sufficiently behind the GWB on the leg of the track (on both sides) will increase the resulting STC value of your wall by as much as 100 percent. The most common method of application being a single bead under the bottom track.
The architect questioned why we had not adopted this revolutionary practice as our own best practice or standard when such a high claim for greater efficiency could be achieved, largely through a subtle change in an application practice, and of course the use of a particular product. On the surface of the argument I too was initially perplexed at the perceived inconsistency. Without intending to take anything away from the product in question I have discovered that the application method being promoted is common amongst most of the major manufacturers and has been in their literature for some time now.
In defence of the specifications and standards, I would assert that there is plenty of muscle in the existing standards to get the high-end results our architect wants. At the end of the day, your wall will not perform to spec if you don’t seal it up as detailed in this section which includes following the manufacturer directions. The only thing saving you in most cases is that no one ever actually tests the walls. So in this respect, you might say we have a standard of performance that may or may not be met.
The Financial Realities: As usual the discussion quickly digressed to the murky financial reality of the construction industry in general, and more to the point, the relatively low prices for new construction. We both agreed that building better quality features into a project at this time of fierce competitive bidding can be a mug's game. My architect openly admitted that he wasn’t about to price himself out of business by over specifying the sound control requirements of his building project. But neither was he about to give up wanting the wall and ceiling contractor to build him the perfect, “quiet wall”.
The Philosophical question: So how does this all play out in the real world?
- If I was a manufacturer or dealer selling acoustical caulk, I would want to convince you that the additional bead of caulk is good value for the money, and it appears pretty clear that it is.
- If I was an architect firmly bent on improving the STC value for my building, I would include the extra bead of caulk in my bid specs so everyone bids the same thing – remember apples to apples.
- If I was a contractor bidding on a job in this extreme tight market, I wouldn’t want to double my cost for this item unless it was called up in the spec and I was going to get paid for it and not be penalized for my higher pricing.
Changing the way we do things is almost always difficult, and typically requires legislation. We have made significant improvements to the control of smoke in new buildings which is the main cause of death in burning buildings. The manner in which this has happened is that we have seen major improvements to the requirements for smoke control mandated through legislation and the National Building Code. Having a quiet place to sleep isn’t deemed to be a life or death issue whereas having a safe place to sleep is. I think this is a case where we have the knowledge and the technology to make our buildings quieter as well as safer. We just have to get all the parties together to maximize the available resources. Many times I have heard drywall contractors say “Murray – it’s my job to build it, it’s the architect’s job to design it.” Untill next time, good sailing.
Murray Corey is the Executive Director of the BCWCA.
