Technical

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ASC-WallDamp is a construction material that is an add-on or upgrade to standard wall and ceiling construction projects. By locating WallDamp material between the joints in a frame construction project, the rooms in the building take on that solid feeling of lath-and-plaster. With WallDamp in place, any vibration or sonic energy that finds itself inside the walls of the building is quickly removed, before it can be heard. This means that lightweight, cost effective stick frame and gypsum board walls can now provide the performance of much more expensive and heavy construction methods. Another important feature is that the WallDamp construction method uses standard construction materials and practices with the simple addition of a thin sheet of damping material that is slipped into every joint possible.

WallDamp construction is more resistant to the intrusion of external noise such as truck and train rumble, aircraft flyovers, sonic booms and neighborhood heat pumps. It works just as well in reverse. The loud sound from the stereo, home theater system or music practice room stays inside the house and out of the neighbor's yard. Then, there is the sound within the house. The wind can blow the front door closed without jarring through the entire house. The sound power from the stereo or home theater, including the subwoofer thump, stays in the room and doesn't shudder the whole house. Kids can play downstairs and the adults upstairs hear almost nothing. Probably, the best thing that WallDamp contributes to today's living experience is the increased sense of peace and quiet at a time when, day-by-day, more noise seems to be the only option.

What is WallDamp?

WallDamp is a viscoelastic material. It is provided in the form of a thin sheet with adhesive applied to both sides and covered with release paper. The concept of a viscoelastic material can be broken down into two parts:

A) Visco comes from the word viscosity meaning how easily a liquid pours. For example,water is not viscous and pours easily, but honey is a viscous liquid and pours very slowly. A viscous material absorbs energy when it is forced to change shape. It takes work to deform a viscous material. A lump of clay has much viscosity. It takes work to change its shape and then it holds that shape.

B) Elastic means that something can be deformed and it returns to its original shape. A rubber ball is elastic because it returns to its original shape, even after bouncing off the floor. It takes work to deform an elastic material but the material is like a spring, and stores the energy. This is why a ball bounces back.

A) + B) = C) Viscoelastic means it takes work to deform the object and also that the object returns to its original shape. But, it just doesn't spring back like a rubber ball.

Example

If we have three balls, one of each type; visco, elastic and viscoelastic, and then we throw them one at a time on the ground, each behaves differently. The visco ball of clay hits the ground and flattens out like a pancake. The elastic ball hits the ground and bounces right back up off the ground and as round as ever. The viscoelastic ball however takes the middle road, it hits the ground and doesn't bounce up but it also doesn't flatten out, just sits there on the ground, still a completely round ball.

WallDamp Construction Method

When sound or vibration tries to pass through the walls, floor or ceiling of a room, is causes those surfaces and the structure behind each of them to move. When a wall, floor or ceiling vibrates, it changes shape. If WallDamp is located at the joints, between each of the parts that make up the wall, floor or ceiling, then any movement at all by the wall, floor or ceiling causes a distortion of the WallDamp. This distortion then absorbs energy and any vibration is quieted right down.

To improve the ability of WallDamp to extract energy out of the micro movement of a structure, the viscoelastic sheet is coated with a strong adhesive on both sides. This means that even the slightest movement at the joint between members of a structure produces distortion of the WallDamp material. The most common application for WallDamp today is to glue it to each face of the wall studs and then nail or screw the gypsum board down as usual. It is also used between the first and second layers of double layer gypsum board.

What is fantastic about WallDamp construction is that it not only keeps sound and vibration from passing through the surfaces of the room but that it actually absorbs it. By comparison, a concrete room is well known for keeping sound from getting out, but it's bad for the person left in the room, the listener. If sound can't get out of a room it has to stay in the room and reverberate. Carpet, draperies and furnishings may provide some acoustic friction inside the room to attenuate sounds in the treble range but they supply little to no absorption for sounds in the bass range. That is why concrete rooms are very boomy sounding. Only WallDamp provides for both a soundproof and an energy-absorbing wall.

The best WallDamp construction system is the Iso-WallDamp system developed at Acoustic Sciences Corporation. In the Iso-Wall system, a damped double layer gypsum board wall is built on one side of the stud and a suspended damped double layer gypsum board wall is placed on the other side of the studs. The suspended wall rides on standard resilient channels, flexible metal strips, which are attached to the sides of the studs or for the ceiling, to the bottom sides of the joists. The pressure fluctuations from low frequency sound can still move the walls ever so slightly but now, when the walls move, they no longer directly push the studs or joists. Since movement of the wall doesn't push the studs, it cannot transfer sonic pressure pulses into the structure of the house.

Standard isolation wall construction methods have been providing some measure of sound isolation for nearly 50 years. However, powerful low frequency sound generators, the subwoofers found in today's Hi-fi and home theater systems have only been popular for about 5 years. With standard isolation construction methods, once the walls begin shaking with the sound, they continue shaking even after the sound stops. This isn't much different than the concrete room except that this time the reverberant vibration is not in the air but in the shuddering walls. Here is where WallDamp goes to work, absorbing the energy out of the shuddering vibrating walls. With WallDamp we have the best of all, lightweight and cost effective walls that sound better than concrete walls.

How Does WallDamp Actually Work?

A wall, floor or ceiling is constructed using a complex set of objects, each touching the other in some fashion. When any one piece moves, it pushes on the adjoining pieces and so on. The reason WallDamp works is that it is located between all the pieces so that every time one piece pushes against another, WallDamp is there absorbing energy out of the push. Actually WallDamp does not absorb much energy when one piece pushes or pulls squarely on another piece. It does absorb a lot of energy when one piece tries to slide past the other. Fortunately when an object pushes on another object, there is a strong tendency for the two to slip or slide a bit at their contact point. This is where WallDamp extracts the energy - from the shifting surfaces. (See Footnote below for an explanation of how WallDamp extracts energy.)

The second thing about WallDamp is that it has been made to extract energy from microscopic movements. That's good because very small movements are involved in the making of sound, especially when it comes to surfaces as big as walls, floors and ceilings. There are many kinds of viscoelastic materials. The most familiar is the soft type that is put into the heels of runner's shoes to absorb the shock of the footfall. The deformation of this soft material is measured in fractions of an inch. WallDamp is relatively hard but that is because the deformation it has to process is measured in thousandths of an inch or less. The hardness corresponds to the force and the deformation involved. WallDamp is made for structural damping.

Another thing to know about viscoelastic materials is, if they get too hot, they get soft and lose their energy absorbing power. Similarly, if they get too cold, they harden up and lose their energy absorbing power. Each viscoelastic material is blended to operate in a certain temperature range. WallDamp is made for the temperatures found in the walls, floors and ceilings of homes, apartments, duplexes and offices, places where people live and work.

WallDamp may look simple but what it does is complex and well suited to its application in damping out the free vibration of walls, floor and ceilings. WallDamp is setting the new standard in frame construction. And, once you hear the difference, you'll know what you've been missing.

How Does WallDamp Extract Energy From the Walls?

Viscoelastic materials absorb energy when one molecule of the material is forced to move past another molecule. The molecules involved are not smooth but rough shaped and when one moves past another, they sort of plug into each other. As they continue to move past, they are forced to unplug but this leaves both molecules twanging in the aftermath of being unplugged. These twanging molecules become "heat" which conducts away, warming up other molecules of WallDamp and eventually the parts that were connected to the WallDamp. Molecular friction absorbs the energy of distortion. This is the visco part of the material. Other molecules in WallDamp are connected tightly together to form a spring that becomes stretched when the WallDamp is distorted. These molecular fiber springs then act to pull the material back into shape once the distorting force is removed. Without these shape restoring molecular springs to bear the load, the visco molecules would simple squish out of the joints like grease under pressure and the energy absorbing benefit of the material would be lost.

Engineering Report - WallDamp in Sheer Walls

Arthur Noxon, Acoustical Engineer at ASC, August 15, 2005

WallDamp, a self-adhesive structural vibration damping shim stock is used between studs and interior sheetrock. Architects and builders are interested in apply WallDamp between studs and sheer wall sheathing, both inside and outside walls. This report reviews the basis for sheer wall design and concludes that a WallDamp interlayer does not interact with or compromise the mechanics of sheer wall design.

A sheer wall is created when nails or screws are installed through a sheet of plywood, into the framing members below. Typically, 2 x 4 framing will be made rigid by nailing a sheet of ½” plywood nailed to the face of the studs and plates. WallDamp can be added as an interlayer between the stud face and the sheathing of a sheer wall to dampen exterior wall vibration. This is especially useful for quieting the low frequency (20 to 40 Hz) sympathetic vibration of outside walls due to low rev truck engine noise from the street.

When a wall vibrates, it moves in and out, perpendicular to the plane of the wall. When a sheer wall is stressed, any tendency for movement is in the plane of the wall, a completely opposite direction from that movement due to the vibration motion of the wall. WallDamp is intended to dampen bending wave type motion is plane surface assemblies. However, the strength and performance of a sheer should not be compromised by adding WallDamp.

Sheer Wall Design

This type of sheer wall construction is based on the mechanical principles of a pinned connection. The face of the frame is mounted with a set of pins (screws or nails) whose shank is in intimate, side-load compressive contact with the thickness of the plywood sheathing. Any tendency for the frame to change position, due to forces being applied in the plane of the frame causes increased compressive load at the contact surface between the thickness of the plywood and the side of the pin which counteracts the tendency for the frame to distort. The compressive loads imparted into the plane of the sheet of plywood due to frame distortion puts the plywood into sheer stress, which then acts to resist further frame distortion.

The head of the pin acts primarily to restrain the plywood from exhibiting the Euler effect, the lateral bending of a long thin structure under axial compressive load, in this case, a compressive load in the plane of the plywood sheet. Essentially, the head of the nails or screws keep the plywood sheet from bending away from the framework under load, which otherwise would work and enlarge the contact hole or worse, lift completely off the pins. In any case, movement by parts of the plywood perpendicular to the plane of the plywood reduces the stiffness of the plywood/frame assembly.

A small additional benefit to the strength of the sheathed frame assembly can be realized if the head of the screw or nail is well seated. This causes a small circle of intimate compressive contact between the surface of the plywood and the face of the frame member directly under the nail or screw head. The friction coefficient combined with the compressive force creates a secondary mechanism by which sheer stress can be transferred from the distorting frame to the plywood sheet. However, due to the vagaries of infield nail or screw sets, this form of shear stress transfer is not depended upon in the design of sheer walls.

WallDamp as a Shim Stock

Adding 1/20” thick self adhesive shim stock (WallDamp) between the face of the framing members and the plywood sheathing does not degrade the mechanical properties of the pinned frame/sheet assembly of a sheer wall. The pins remain in place, the heads of the pins continue to keep the sheet tight against the frame and any movement of the frame continues to transfer compressive loads through the side of the pins into the sheathing. The WallDamp material is not a rubber type material and does not laterally flow under pressure. Its adhesive surface along with it’s high density (1/3rd that of steel) actually enhance the grip between the face of the frame and the face of the sheathing.

WallDamp type materials have been used for over 40 years in the construction industry as an interlayer between structural elements to dampen the vibration of the structure. Other common brand names for this type of constrained layer damping material are DeciDamp and dB-Rock.

Conclusion

WallDamp is a self adhesive, hard shim stock that is located between the stud and the sheer wall sheathing. It does not affect or otherwise interact with the stress transfer mechanism that exists in sheer wall design and assemblies.