Page 56 - 2016Fall

Residential Quietude
this sound can be quite loud. Once that cell phone is pulled just a 1⁄2 inch away from the bottom of the table, that sound disappears. If this demonstration is repeated with the ring volume on, it is not as impressive but shows that without the treatment of the airborne ring, the noise is still audible. This demonstrates the distinct but equally important aspects of how airborne sound and structure-borne sound are both part of the solution set in any installation but especially in residential design. The structure-borne aspect is often very difficult to eradicate once a vibration enters a structure, par- ticularly with mechanical units.
Another wonderful example is to take two wood blocks that are each about a half meter long and 50 mm × 100 mm. Place them on the floor with two edges touching. Place one foot on one of the blocks. Tap the edge of that block with a hammer, not touching the other block. The wood with your foot on it will not move, but the other will move away, showing how energy can transfer from one structure to another.
Examples of Structure-Borne Noise
One of the most common examples of structure-borne noise is when the noise from mechanical equipment such as a con- denser unit or pump in a room or closet is disturbing to an adjacent room even when the STC is sufficient to stop the sounds from the mechanical unit. Typically, this involves making certain that there are no rigid connections and that certain flexible connectors or spring isolators are properly installed as well as ensuring that the ceiling of the mechani- cal room has a sufficient STC with no points of acoustic leakage. This is where the demonstration of the cell phone on the bottom of the desk helps to explain the problem.
A common type of structure-borne vibration is experienced with footfall. There are many approaches to solve this prob- lem. The floor can be floated or placed on a resilient materi- al, such as recycled tires. When the treatment is on the floor side, sometimes a simple rug with an acoustic carpet pad un- derneath solves the problem. There are other times when the floor cannot be treated and the ceiling is then floated or at- tached using a connector that has some flexibility to it, such as resilient channels or a neoprene strip. Even though the solution is engineered to inhibit the structure-borne footfall from entering a room below, there are secondary parts to the engineered solution, such as where the floor or ceiling meets the wall. As engineers, we are taught to make certain that our acoustic design is not short circuited by a rigid connec-
tion. An example of that would be including proper resilient strips where the floor or ceiling meets the wall or replacing a resilient channel with a clip that includes neoprene that pre- vents the attaching nail from reconnecting to the structure. Despite the inclusion of this aspect of the design, there are many occasions that I have found where the floor or ceiling is not properly decoupled as was shown in the drawings.
Invisible speakers, or any sort of sound-amplifying device installed into a wall or ceiling, are a common source of airborne and structure-borne noise. The airborne portion of the problem is readily addressed by speaker backs with an STC of, but often the structure-borne noise is forgotten in the installation. By connecting a speaker directly to the channel which is connected to the drywall, there is noth- ing preventing the vibration from entering the structure of the room and traveling throughout the house even when used at a moderate volume. If the ceiling is constructed of drywall with regular paint, this may not be a problem. Vi- bration of the channel, however, is a serious problem if the ceiling is plastered, especially with a Venetian plaster. This is because over time the channel vibrations will cause cracks in the plaster, damaging the ceiling. A simple resilient strip that has a rated deflection, such as a simple antivibration pad made of neoprene or rubber, solves that problem by decou- pling the speaker from the channel.
Elevators are a common source of such structure-borne problems if they are installed without proper isolators on the supporting structures or motors. When a motor is installed without proper isolators, this allows for vibration from the motor to travel into the I beam on which the motor is in- stalled, which is resting on a wall that may be shared with an apartment or a room intended for quietude, such as a bed- room. This vibration is very difficult to eradicate because to just separate or decouple the walls from the elevator frame with an airspace does not address the aspect of what struc- ture-borne piece is going into the floor as well as what is go- ing into the ceiling. This may require changing the speed of the elevator in combination with decoupling the vibration that is going into the wall. If the wall of the elevator is not in direct contact with the floor, such as a concrete elevator shaft with a small airspace between it and an adjacent room structure, sometimes the acoustic solution to this structure- borne noise can be as simple as decoupling where the stud connects to the concrete wall.
54 | Acoustics Today | Fall 2016