Creating A
Soundproof Office
A thoughtful, low-cost workshop
conversion provides staff in an abutting office with a quiet
work environment.
by Paul Fisette and Daniel Pepin - © 2002
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.
Design Challenge
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.
Baseline Data
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:
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.
Sound Readings
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.
Table 1 Sound Readings
| |
In Shop (pre-const.) |
In
Office(pre-const.) |
In
Office(no gaskets) |
In
Office(with gaskets) |
| Jointer |
120 db |
82 db |
57 db |
54 db |
| Radial Saw |
114 db |
81 db |
51 db |
43 db |
| Planer |
122 db |
88 db |
58 db |
51 db |
* 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.
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Contact Information:
Dave Damery, Director
Building Materials and Wood Technology
120 Holdsworth Natural Resources Center
University of Massachusetts, Amherst, MA 01003
Tel: +1 (413) 545-1770
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