ADDRESSING HVAC BACKGROUND NOISE IN PROJECTS

Acoustic ceilings can conceal HVAC systems and attenuate noise generated by devices in the plenum. Predicting the amount of sound attenuation provided by a suspended acoustic ceiling is straightforward and described in the ASHRAE Handbook “HVAC Applications” by the American Society of Heating, Refrigerating and Air Conditioning Engineers and the AHRI Standard 885 by the Air Conditioning, Heating and Refrigeration Institute. The foundational study leading to the prediction method used in these standards is ASHRAE RP-755 conducted by the National Research Council Canada.

As stated in the document, “ASHRAE RP-755 was initiated to investigate the transmission of sound through different ceiling types with the intent of providing more reliable design information to deal with sound transmission through ceilings close to HVAC devices.” The following three-step guide, which is based on the ASHRAE/AHRI standards, simplifies the design/prediction method for use by architects and mechanical engineers.

Step 1:
Determine if a suspended acoustic ceiling is part of the building’s design aesthetic and select the ceiling panel based on Noise Reduction Coefficient (NRC), aesthetics, contribution to indoor air quality, environmental/energy impact, cost, or other design criteria.

Step 2:
Determine the maximum background noise level for each room type according to the applicable standard (see Table 1 earlier in this course).

Step 3:
As a best design practice, locate HVAC equipment over unoccupied or noisy areas such as corridors, storage rooms and lobbies. Avoid locating HVAC equipment over normally occupied rooms, especially those that have background noise requirements of NC-35/40-45 dB(A) or lower.

When locating HVAC equipment over occupied rooms cannot be avoided, find the attenuation of the ceiling panels planned for your project in either the ASHRAE Handbook or AHRI standard and add to it the desired background noise level in the room. The result is the maximum loudness of the HVAC equipment in the plenum.

The graph above shows how loud HVAC equipment in the plenum can be when using ceiling panels made of stone wool. As the acceptable background sound level increases from NC 30 to 40, so does the permissible loudness of the HVAC equipment above the ceiling. While this example is specifically for stone wool panels, per ASHRAE and AHRI, the values would hold true for ceiling panels made of other materials since they basically all perform the same.

If these maximum HVAC equipment sound power levels cannot be met, then first consider other HVAC equipment brands or models, or noise control options provided by the HVAC equipment manufacturer. Secondly, contact the ceiling manufacturer for other potential options above the ceiling that can increase the attenuation where the HVAC equipment is located.

Note that per ASHRAE and AHRI, neither Ceiling Attenuation Class (CAC) nor Sound Transmission Class (STC) correlate well to ceiling attenuation performance when the noise source is mechanical equipment located in the plenum above the ceiling. Therefore, CAC and STC should not be included in the ceiling panel specification or used to select ceiling panels.

This process addresses only noise radiated off the HVAC device casing and transmitted through the ceiling below. Noise from the HVAC device may also be duct-borne and emitted into the room through the supply air diffusers and return air grilles. The mechanical engineer should control duct-borne noise, if necessary, with noise control devices such as duct silencers or duct lining. All noise paths/sources should be combined to ensure the background noise limit is not exceeded.

Keep in mind that complying with the background noise levels in the standards and guidelines relates mostly to the design of the building system themselves. However, having sound absorption inside occupied rooms can help to decrease perceived background noise.

SOUND-REDUCING DRYWALL CASE STUDY

Photos courtesy of Rockfon

Project: Orion at Lumino Park
Location: Alberta, Canada
Architect: Casola Koppe Architects
Construction Manager: Kanas Corporation

The Project: Orion at Lumino Park is a 13-unit, 18-story, multifamily high-rise located in Calgary, Alberta, Canada. Kanas Corporation is the project owner, developer, and construction manager for the 100,000-square-foot rental property. Orion is an affordable rental property offering rental rates at 10 percent below market value. What is unusual is that tenants are not short on amenities. Renters enjoy comfortable high-tech, high-quality apartments with low operating costs – at a manageable monthly rate.

The Construction Approach: The exterior walls of this project use ICF (insulated concrete form) construction - cast-in-place concrete sandwiched between rigid insulation to create thicker-than traditional exterior walls. This helps reduce noise from the outdoors resulting in a very favorable Outdoor/Indoor Transmission Class (OITC). “ICF is unusual in the rental world, especially going up 18 stories,” says Jason Whitfield, general manager for Kanas Corporation. He notes that in addition to the acoustical benefits, “This was part of our energy performance objective—creating a super-insulated building.” The energy performance of the ICF building envelope, combined with the superior mechanical and electrical systems, lowers energy costs by more than 50 percent compared to a typical building. For the interior walls between apartments and the corridors, Kanas chose sound reducing drywall. “We wanted to continue to move the needle on the quality of the building by providing a quieter experience for tenants,” says Whitfield.

Innovative Approach to Sound: With sound control a key objective, the drywall manufacturer’s technical team worked together with Kanas and the architect on the best solution to cost-effectively meet acoustic comfort goals. Several lab and field tests were conducted looking at various wall assemblies such as double layers of Type X gypsum panels on both sides, 1 and 2 layers of Type X with 1 layer of sound reducing gypsum board, and several other assembly types. The wall assemblies of Kanas’ past construction projects were also tested, which helped improve the design of Orion.

“After evaluating different types of walls, we chose the sound reducing drywall assembly, confident it would deliver a reduced noise transmission experience for tenants”, says Whitfield. Specifically, Kanas changed from their standard four-layer Type X wall assembly to a two-layer assembly with Type X on one side and sound reducing drywall on the other. He continues, “The approach complemented the way we insulated the apartments against exterior noise, using ICF for the exterior walls.” It was also noted that using sound reducing drywall saved material costs and increased floor space since fewer layers were needed. Those benefits aside, the assembly still achieved the STC performance required. Further, since the selected sound reducing drywall easily scores and snaps just like standard gypsum panels, installation was easier, saving labor time and decreasing jobsite waste.

Sustainability Results: By using sound reducing drywall, less material was used than would otherwise have been used in a multilayer assembly. With Kanas' innovative approach to construction, including its use of sound reducing drywall, they were able to achieve an environmentally sustainable property over the long term, a key objective for the Orion property.