Creating a Soundproof Office
A thoughtful, low-cost workshop conversion provides staff in an abutting office with a quiet work environment.
There are benefits having your business office in the same location as your workshop. It’s nice to discuss projects with clients in the comfort of a well-organized office and at the same time be able to slip into the production facility to show off your work. There’s an economy associated with running your operation at one site. It is easier, more convenient, and less expensive. But trying to communicate ideas over the scream of a radial arm saw can be impossible. There’s no need to shout. You can create a quiet attached office with a modest investment.
Our situation was somewhat unusual. The office/shop combination was located in an institutional building. However, the sound-control strategy we used works for many structures. We began with an open workshop area that measured 36’0” x 45’0”. There was an additional 14’0” x 18’0” storage room attached to the northeast corner of the shop (see floor plan). The problem began when the storage room was converted into office space. The workshop is used daily for a variety of noisy woodworking projects. Unfortunately, the common wall separating the office from the workshop was not built with any special sound attenuating details. Office staff quickly learned they could not conduct a normal conversation while carpenters ran oak trim through the 16-inch industrial jointer in the abutting workshop. A design fix was put on the fast track.
The workshop and office had to coexist in this location. The first thing we explored was to develop a staggered-use schedule. But that didn’t work. The shop must operate during office hours. Next, we studied the shop layout to see if we could build a sound barrier separating the shop and office. As we played with the floor plan, we realized that our shop equipment was not organized efficiently. The assortment of woodworking tools was scattered over a broad area. We could compress the layout, improve the workflow, and build sound control into a new design plan.
Before developing a construction plan, we wanted to know what we were up against. So we took a series of readings using a hand-held sound meter. We took readings in the shop (sender location) and inside of the office (receiver location). The measurements were taken with all equipment turned off (ambient), with individual pieces of equipment simply turned on, and with individual pieces of equipment cutting wood. The readings showed 3 pieces of equipment were clearly most offensive. The 24-inch Baxter Whitney surface planer; 16-inch, 4-cutter Newman jointer; and the 16-inch Dealt radial arm saw really howled. Ambient sound in the shop and office with all equipment turned off was about 50 decibels (db). This is roughly equivalent to the sound of a normal conversation. But holy earplug! These tools generated about 120 db in the shop while cutting wood. This is roughly equivalent to the sound of jet taking off 300 feet away. The sound meter recorded 85 db inside the office, equivalent to a loud stereo. Surprisingly, the 16-inch Baxter Whitney table saw was more than 20 db quieter. Our design goal was to keep sound transmitted into the office under 60 db.
Sound Control Theory
Sound is a vibration of some “thing” that causes the layer of molecules or air particles next to it to vibrate. The particles transmit the vibrating motion to the next layer of molecules and then to the next and so on. A popular demonstration used to illustrate this concept is to drop a stone into water. The ripples of water in this analogy correspond to the motion of sound waves. Keep in mind; water ripples along 2 dimensions of the water’s surface. Sound waves radiate in 3 dimensions, like a sphere. Sound pressure of the source can be measured, but the range of pressures is so broad that a logarithmic scale is more useful. Sound-level units called decibels are used to express the ratio of sound between the source of sound (sender) and what you hear (receiver) at another location. One decibel equals the smallest detectable change in sound intensity. A 5-decibel increment is noticeable. And each 10-decibel increment is perceived as a doubling of loudness. The science related to sound control is complicated, but at a practical level, it can be distilled into source, path, and receiver.
The sound in our workshop was not structure-borne like footfalls on a floor or water hammer in a pipe. Controlling the transmission of airborne noise, generated by woodworking tools, was our primary goal. The floors, walls and ceilings are all possible pathways. The sound in the shop causes the surfaces of these building assemblies to vibrate. And, in turn, these vibrating elements excite air molecules waiting on the other side of the assembly ready to carry sound into the office. We had to minimize vibration of the building assemblies. We also needed to seal all air leaks connecting the office to the shop to block sound transmitted by direct air leakage.
Overall, the best approach to block airborne sound is to:
- increase the weight of building materials
- design discontinuous construction details
- add sound absorbing materials to structural cavities
We were lucky. The floor and ceiling was poured concrete. Heavy materials like poured concrete reflect sound and resist vibration. Concrete is simply too dense and difficult for air pressure to set into motion. Poured concrete is also continuous and airtight. We aimed to build a wall between the shop and office that was airtight, resisted vibration, and absorbed sound that leaked into the wall cavity. The sound exclusion of the wall assembly could be predicted to some extent.
The ability of a wall, floor, or ceiling to resist the transmission of airborne sound is expressed by its Sound Transmission Class (STC) rating. For example if the sound on one side of a wall is measured at 100 decibels and drops to 60 decibels on the other side, the wall blocks 40 decibels of sound and earns an STC rating of 40. STC ratings are given to a variety of wall assemblies based on acoustical testing. Construction details that show how these walls should be built are available in many sound attenuation handbooks. Keep in mind that many different frequencies of sound can be generated by a source. Building assemblies do not block all frequencies equally well. United States Gypsum (USG) invested a lot of time and money developing an MTC rating system designed to predict a wall’s ability to impede the frequencies transmitted as result of machinery and music. They found that a given wall might have an STC rating of 60, but an MTC rating of only 50. The idea did not catch on and USG has stopped promoting the system.
The Construction Process
Our existing floor plan was reorganized into a more compact and efficient layout. We positioned a barrier wall along the entire 45-foot length of the original workshop area and left an 8-foot wide hallway between the office and shop. The redesigned shop was shrunk to 28’ x 30’ providing us with enough space to build a new and much needed 28’ x 15’ storage room along the north side of the shop. There were a couple of considerations guiding the plan.
The workshop ceiling was 11’6” high. However, an 8-foot wide section of the ceiling along the entire length of the shop abutting the office (over new hallway) was 2’6” lower. The space above this drop ceiling was used as a utility chase. Unfortunately, this chase would also function as a flanking path. Sound would travel from shop to office through this pathway. So we decided to build an 11’6” tall wall to the shop side of the dropped ceiling forming a continuous seal along the entire length of the shop (see cross section detail). The office staff enjoyed the added privacy and dust reduction provided by the hallway. The new storage room would serve as a sound buffer. However, we were convinced the critical sound-blocking element would be the new hallway wall. We then reviewed a variety of construction options.
United States Gypsum (http://www.usg.com/ 312-321-4000) provides a wealth of very good information and guidance for anyone building sound control into a structure. A visit to their web site is a must. We used USG High Sound-Attenuation Steel Framed Systems technical directive to design our walls. The plan called for 20 gauge steel studs spaced 16” o.c. Resilient channel was screw-attached to the shop-side of the wall. The wall would have a double layer of 5/8-inch type X drywall fastened to resilient channel on the shop side and a single layer of 5/8-inch type X drywall fastened directly to the studs on the hall side of wall (see section diagram). It earned an STC rating of 56 and a 2-hour fire rating.
The steps in the construction phase were straightforward. First we snapped lines on the floor and ceiling to locate wall plates. Then cut and dry fit the track. We predrilled 1/4-inch holes through the track into the concrete floor and ceiling at 16-inch centers to receive the 1 1/2-inch long anchor pins that would hold the track in place. Before the track was permanently fastened with the anchor pins, we ran a double bead of Tremco Acoustical Sealant (http://www.tremcosealants.com/ 800-321-7906) under the entire length of the track to form an air seal. Tremco was also applied behind the end studs of the wall too. A word of warning: Tremco is affectionately called Black Death! You’ll need mineral spirits to remove any misplaced globs. If you cut the nozzle on the tube too small, the sealant comes out like molasses and will blow out through the back of the tube. Cut a healthy 3/8-inch diameter hole and you’ll develop a good steady flow. Once the track was permanently secured, the studs were fastened 16-inches o.c. using self-tapping panhead screws. We used a small pair of vice grips to hold the studs in place while the screws were driven. Otherwise, the screws tend to deflect the stud and roll around while you are trying to drive them through. Next we installed resilient channel (RC-1) horizontally to the shop side of the wall with self-tapping panhead screws at 24-inch centers.
Resilient channel is a product that minimizes contact area between members in a building assembly. Resilient channel is U-shaped and made of steel. It has a 2-inch wide face that drywall is attached to and a small, 1/2-inch offset flange that extends back from its face. You screw the channel to the studs through this flange (see photo or illustration). As a result of its shape, the connection area between the drywall and the stud is interrupted. The pathway is reduced to a 1/2-inch wide spot every 24 inches vertically and 16-inches horizontally. As a result, sound transmission through the assembly is reduced. Top and bottom channels were held off the ceiling and floor by 2-inches to disconnect the wall from floor and ceiling assemblies. If you have not worked with resilient channel you are in for a surprise. It is laid onto the wall frame with the fastening flange located along the bottom edge. The channel hinges down, away from the frame and toward you, under the weight of the drywall. At first this seems wrong, but when you think about it, this makes sense. This hinge action opens the space required to separate the channel from the frame.
Sound attenuation batts soak up sound and can improve the STC rating of a wall. We carefully installed 3-1/2 inch thick sound attenuation batts (Owens Corning http://www.owenscorning.com/ 1-800-438-7465) in all stud cavities after the resilient channel was fastened to the wall. We purchased batts that were sized for steel studs. These larger batts extend into and completely fill the hollow profile of the steel studs. Language on the package claims they can improve partition STC ratings by up to 10 decibels.
A multiple layered wall system was built. One layer of 5/8-inch type X drywall was installed vertically to the hallway face of the wall. We left a 1/4-inch gap around the perimeter of the drywall attached to the wall and filled this gap with Tremco sealant to block air leakage. On the shop side we applied a double layer of 5/8 inch type X drywall vertically across the resilient channel already fastened to the wall. The idea here is that the added density provided by a double layer of gypsum board resists more of the vibration caused by air pressure.
The seams in the first of the double layers installed to the shop side of the wall were taped and mudded before the second layer of drywall was applied. The seams in the second layer on the shop side were offset from the seams in the first layer. All panels were installed vertically to minimize the amount of crack length. A 1/4-inch gap was maintained around the perimeter of this face and was filled with Tremco. The nice thing about fastening the vertical drywall to horizontally run resilient channel is that you do not have to “hit” a stud with the seams. There’s less cutting and less waste. It’s useful to mark the location of the channel on the end-cap walls so that you know where they are when you screw the drywall in place. We built the wall separating the shop from the new storage room last, following the exact same procedure used to construct the hallway wall. Now the walls were built, but we still had door openings to deal with.
Doors can be a difficult. The standing rule is to avoid using them in sound control partitions when possible. Research shows that hollow core doors are terrible sound barriers. Solid core doors with tight-fitting perimeter gaskets and thresholds are best. Doors in hallways should not be placed directly across from each other. And the swing of adjacent doors should be arranged so sound will not be reflected between them. We needed to provide walking access into the new storage room and wide access into the shop from the hall. So we installed a double, solid core birch fire door leading into the shop. A single-door version was installed in the storage room. Both were outfitted with knocked down metal, slip-jamb frames. The space around the frame was sealed with pieces of attenuation batts and Tremco. The doors were sealed with face-mounted bulb-type perimeter gaskets manufactured by National Guard Products (http://www.ngpinc.com/ 800-647-7874). A self-sticking gasket was used on the astragal of the double door. Installing a threshold was out of the question because we are always moving material into and out of the shop. So we did the next best thing. We installed retractable door sweeps, also sold by National Guard Products (220NDKB). You can adjust the height of the sweeps to fit tightly against the floor when the door is closed. They retract as the door is opened and don’t drag on the floor. It was now time to see how well the system worked.
The project was a tremendous success. Sound measurements and happy office workers are proof. Our sound readings were taken in the shop and inside the closed office in 3 stages: The first readings were taken before construction began. The second set was taken when the walls were built and the doors were installed with no gaskets or sweeps. Then a third set was taken with gaskets and door sweeps in place. Materials for the entire project cost $3,000. The critical readings are outlined in Table 1.
|In Shop (pre-const.)||In Office(pre-const.)||In Office(no gaskets)||In Office(with gaskets)|
* all measurement we taken while machining 2-inch oak boards.
This project involved construction in an institutional space, but it was similar to many projects. Convert a garage to an office/workshop combination and you will deal with the same issues. True, the structure may be wood-framed, but the floor is a concrete slab. Detailing a wood-frame ceiling is not that different. Just seal the flanking paths around the top of the wall carefully add and you should get similar results. These ideas are useful for other space conversions as well. Many old industrial buildings are being subdivided into office space, artist studios and retail stores. Sound attenuation in these spaces is important and affordable.
Last updated: June 4, 2009