How Sustainable Design Choices Affect Noise Control
Sustainable design has brought both benefits and challenges to mechanical noise control efforts. From equipment selection to HVAC path sound attenuation, and mechanical trends to architectural trends, these changes are influencing mechanical designs and the associated acoustical results.
“Engineers do not have control over some building design decisions, such as architectural choices, and must adapt in order to meet the acoustics goals,” said Mandy Kachur, P.E., Member ASHRAE, and principal consultant for Soundscape Engineering.
Incorporating good acoustic design into sustainability efforts positively affects building occupants. Blending the two fields can reduce energy consumption, increase worker efficiency and satisfaction, create healthier environments and deliver spaces that perform their intended uses, she said. Neglecting acoustics can have a detrimental effect on outcomes.
For example, an elementary school could be lauded as an energy-efficient school building, but it does not serve its occupants’ needs if the young students cannot hear their teacher. “Young children are not as skilled as older children in determining the meaning of a sentence should they miss a word or two because the background sound level is too high in the room or at their seating location. This also applies to second language and special education classrooms,” said Kachur.
High background sound levels in spaces also challenge hearing-impaired people by making communication difficult in spaces such as retirement community residences and dining facilities, said Kachur. This can lead to social withdrawal, which has been linked to poor health.
“As we get older, it is more difficult to hear speech when it is noisy, and it is the engineer’s job to make sure that the HVAC system is not contributing to this difficulty,” she said.
Architectural Trend Challenges
Dealing with Ducts
Smaller areas being allocated for ducts is an architectural trend Kachur said she has seen impacting HVAC noise. This squeeze reduces the duct size, which increases the velocity of the air in the ducts and consequently the aerodynamically induced noise. While radiated noise can be reduced with duct lagging, heavier gauge duct, or high ceiling attenuation class (CAC) lay-in ceiling panels, these treatments do not attenuate the duct-borne noise that propagates into the space through the diffusers and inlets.
Kachur said the “Noise and Vibration Control” chapter in the ASHRAE Handbook—HVAC Applications includes a table that lists recommended maximum air velocities for different ceiling configurations such as exposed ductwork and different ceiling materials.
Selecting quiet equipment will require less noise control treatment to meet a space’s background sound requirements and is one way to increase energy efficiency, according to Kachur. For example, a designer can save energy if they can reduce the pressure drop across a duct sound attenuator by making it shorter or possibly eliminating it, she said.
Attending to Duct Path Attenuation
Duct sound attenuators are particularly helpful in reducing low frequency sound within a short duct length. Their use will add to the pressure drop in the system though, which is contrary to sustainable design, said Kachur. Selecting attenuators that are larger than the duct size and casings that extend beyond the duct dimensions to increase the internal free area offer a few options to reduce the impact of sound attenuators within the system.
Kachur said she sees severe angle transitions to attenuators in the field, and “This can cause turbulent airflow into the attenuator, which then unnecessarily increases the system pressure drop beyond the catalog rating. Correcting this during the submittal phase will save energy for the life of the system.”
If there is not enough duct length for transitioning to a larger size attenuator, Kachur suggests considering an extended casing model. “Attenuators can become more sustainable by extending the casing outside of the duct dimensions to allow the acoustical media to be placed further outside the airstream, thus reducing the pressure drop across the attenuator,” she said. Though there is a higher initial cost for the extended casing, the energy savings in the long term may be worth the investment.
The acoustical media can be encapsulated in a polymer film if an engineer or owner is concerned about exposed fibers, according to Kachur.
Acoustical duct liner is another sound reduction strategy for ducts. “If closed-cell foam is preferred to fiberglass or open-cell foam for lining, the designer needs to be aware that its absorption is quite different than the more traditional porous absorbers. This performance reduction must be taken into account.”
Kachur pointed to ASHRAE Research Project Report 1408, “The Effect of Lining Length on the Insertion Loss of Acoustical Duct Liner in Sheet Metal Ductwork,” which addresses the performance of various acoustical duct linings. The research project quantifies the sound attenuation of acoustically lined sheet metal ducts of varying lengths and materials to better predict attenuation and improve the tools available in the “Noise and Vibration Control” chapter in the ASHRAE Handbook .
Another architectural design trend Kachur’s firm has been seeing is more daylighting in spaces, which has resulted in increased glazing area, increased room height and the elimination of finished ceilings.
“More hard surfaces and larger volumes result in a longer reverberation time, which contributes to sound buildup in the spaces,” she said. “Add radiated noise from exposed equipment, such as terminal units and heat pumps, and the accumulated sound is louder than [in] the days when acoustical panel ceilings were the norm.”
Kachur urges caution when using manufacturers’ noise criteria (NC) ratings to estimate room sound levels in reverberant spaces. The NC calculation includes a standardized sound power-to-pressure conversion allowance which may not be applicable to the reverberant space in question. As a result, the final sound level in the room could be higher than the NC ratings might indicate. She recommends using the octave band sound power data to predict noise levels, which enables the inclusion of the specific architectural characteristics of the room under analysis. She said sound path modeling software is helpful to do this calculation.
If a designer sees a design direction that negatively affects acoustics, such as a reverberant space or equipment located within occupied spaces, they should call these challenges to the attention of the architect or the owner, Kachur encourages.
Sometimes natural ventilation is a design option. Kachur said engineers must consider the resulting sound transmission. Noise can come in from outside the building, and air passages within the building can transfer sound from one space to another, she said.
“Acoustically lined transfer ducts are an option for these situations,” she said. Kachur said these ducts can retain some of the architectural sound isolation.
Designers should be conscious of quiet HVAC systems in spaces where speech privacy and disturbance reduction are important, Kachur said. These spaces include offices, medical exam rooms and libraries.
“Underfloor air distribution and chilled beam systems often result in a quiet office, allowing conversations to be heard for a longer distance away,” she said. “In these cases, an electronic sound masking system may be required for occupant productivity and comfort through reduction of distractions. Variable [air] volume systems do not produce a consistent amount of sound and cannot solely be relied upon for adequate sound masking.”
Some occupants may wear earbuds to solve concentration issues, but other occupants might not want to do that. Kachur said it is the building designer’s responsibility to provide an environment where all occupants can be as efficient as possible and can work comfortably without having their personal preferences compromised.
“One thing I want to point out is that sound masking is only effective when you play the masking signal at the listener location since the goal is to decrease the signal [voice] to noise ratio at the listener’s ear,” she said.
Kachur advises to not use sound masking in rooms where speech intelligibility is important, like in a conference room.
Equipment Selection Trends
While selecting fans at their highest efficiency results in the quietest performance, they still generate sound. One option that often decreases the sound level and maintains unit efficiencies is a fan array. The higher rotational speeds of smaller fans shift the blade passage frequency to a higher range, which is easier to attenuate. Smaller fans rotating at higher speeds can also reduce vibration energy transmitted to the structure, according to Kachur.
Another noise control option is using an electronically commutated motor (ECM) with slow-starts, which decrease noise levels during start-up and operation.
It is also an option to surface mount sound-absorbing panels in the units in some situations. “With the previous style enclosure panels that used fiberglass, the lining could just be perforated for added absorption within the unit. That can’t be done anymore because closed-cell foam is used in the insulating panels, and it’s much less absorptive,” she said. “So you need to actually surface mount acoustical panels inside the unit for attenuation.”
If a designer is concerned about having absorptive material inside a fan or plenum section, it can be encapsulated with a polymer film, she said.
Water-Source Heat Pumps
With water-source heat pumps, the noise-generating compressor is now in the same space as the occupants. Engineers might need to specify a mass-loaded vinyl sound blanket wrap around the heat pump to reduce radiated noise. Kachur suggests always using vibration isolators to reduce sound transmitted to the structure.
Kachur said chilled beams typically produce lower sound levels, and consequently the space might need a sound masking system depending on the chilled beams system’s use. She said designers should make the team aware of this condition.
Chillers and Cooling Towers
Options for noise reduction for these pieces of equipment are variable frequency drives (VFD) and low-noise fans, said Kachur. Whether the VFD controls a fan or a compressor, the reduction in speed generates less sound.
“And as always, position these pieces of equipment away from sensitive locations. That’s been a noise control technique for a long time,” she said. If that cannot be done, sound-blocking panels from the manufacturer can be installed on cooling towers or screen walls designed for chillers.
Kachur said to be careful when locating cooling towers above grade. “Higher deflection vibration isolators may be needed for lower rotational speeds,” she said. She recommends referring to the vibration isolation table in the ASHRAE Handbook—HVAC Applications’ “Noise and Vibration Control” chapter.
Underfloor Air Distribution
Underfloor air distribution systems are another quiet system that has come into favor, said Kachur. She recommends against placing floor diffusers near demising partitions on both sides of the wall because it provides a flanking path under the wall and transmits sound from one space to another.
For sound sensitive rooms, Kachur said designers might have to continue the partition down to the structural floor or use other sound-blocking constructions under the raised floor with a transfer duct to retain the wall’s sound isolating performance.
As a reminder, she said, “UFAD systems might require sound masking in order to get the proper acoustical environment for the occupants.”
A Sustainable Future
Kachur said sustainable design initiatives overall have had a positive effect on building acoustics despite some of the challenges posed.
Possibly the biggest benefit has been the U.S. Green Building Council inclusion of a prerequisite and several elective credits for acoustics in their LEED rating system. Prior to its inclusion, acoustics might have been of secondary importance or completely forgotten in sustainable design projects, she said.
For example, she has seen this benefit K-12 students. Unit heaters historically are loud pieces of equipment often used in classrooms. However, when ANSI Standard S12.60, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools was released in 2002, manufacturers designed quieter equipment that meets the standard’s requirements, she said. LEED® for Schools has since integrated the ANSI standard into their credits.
While the LEED documents may be a primary resource for engineers who design sustainable, quiet spaces, “The ‘Noise and Vibration Control’ chapter in the ASHRAE Handbook—HVAC Applications, which I’ve mentioned several times already, is an invaluable resource for all aspects of noise control,” Kachur said. •