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Figure 5. Picture of a loud mechanical unit near a pipe penetra- tion in the ceiling. This figure shows another example of how sounds from a mechanical unit can easily pass through a hole a pipe passes through that has not been acoustically sealed.
IIC and Structure-Borne Noise
I have on numerous occasions been called in by contractors wanting to prevent footfall from being heard between floors of a home or multifamily dwelling. The contractor did what they believed to be the best action to provide a high IIC for their client and were disappointed, if not horrified, when the installation had inadequate results. Like the STC, the IIC is a single-number rating that identifies the ability of a floor partition to attenuate impact sounds such as from footfalls. Specifically, the IIC is derived from measured values of nor- malized one-third octave band sound pressure levels in ac- cordance with ASTM classification E989. Also like the STC, the preferred minimum IIC is 50.
Typically, this audible footfall is a structure-borne problem, but if there is a major leakage point like a stairwell without insulation and an insufficient STC or uninsulated recessed lights, the sound of a footfall may travel by other routes, and it could be just an acoustic leakage problem or acoustic leak- age and structure-borne. Like water, sound will take the path of least resistance and go through the openings at the perim- eter of doors or electric outlets or where the cut in the floor for a HVAC duct vent cover is greater than the vent itself and is not appropriately caulked and sealed.
These acoustic leakage points can be found with infrared, as used by the Canadian laboratory (Gover and Bradley, 2006), with a patented sound-focusing mechanism (Schnitta and Israel, 2011), or illuminated by shining a bright light into what is a high STC wall, door, vent cover, floor, or ceiling. If light will travel through the partition or under the door, so will sound. For this reason, acoustic leakage paths, as dis- cussed above, in any proposed construction should be ad- dressed so the partitions specified perform up to their engi- neered STC and IIC.
This is a very difficult problem, especially for noise-sensitive clients. I have had a client turn off the heating and refrigera- tor and ask for complete silence in the room so I can hear a barely audible transformer buzz that they find extremely bothersome. It is not that they are hearing something that doesn’t exist; it is that they are simply more sensitive to that particular frequency than most people. In many cases, a per- son will find this small distraction enough of a bother that they cannot be as productive as if that noise were not pres- ent. Typically, these issues are not identified until after con- struction is complete and a client has moved into the home. Often, it is a vibration or a subtle noise that is most notice- able when it is the one thing preventing the desired state of quietude. To a contractor or architect, this often is a result of a misalignment of client expectations and design goals. This is worth noting because communication with the client as to how to achieve quietude goals within their budget is just as important as the design of a space.
These are just a few examples of how even the most well- designed system can have flaws rendering it ineffective or not up to code if the acoustic installation is not tested for the proper seal.
Simple Demonstrations of Structure- Borne Noise and Airborne Noise
The acoustic problem can be both airborne and structure- borne. For the design team to understand the two-part na- ture of the solution set, a demonstration is often helpful. Architects, engineers, and contractors all need to have the language to explain the difference between airborne and structure-borne noise and the available options to address them. Most clients understand this after a simple demon- stration.
The demonstration that sometimes I only need to mention and not even demonstrate is a garage door in operation. In the garage, the motor sound is audible and identifiable, but a few rooms away, this “motor sound” is no longer audible. What is audible is a low-frequency vibration rumble that is not the sound from the motor but the structure-borne sound from the rigid connection of the motor to the ceiling or wall of the garage. This helps to separate the conversation on what is airborne and what is structure-borne noise.
Another demonstration of structure-borne and airborne noise is to place a cell phone in full contact under a table in the middle of that table while on vibrate mode only and have someone call that cell phone. When the phone vi- brates, there is a sound that emanates from the table. Often
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