Low-Vibration HVAC Noise Control

September 29, 2005
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By the time HVAC noise complaints cross Tim Kuski's desk, they are pretty bad. As general manager of Centrifugal and Vane Axial Products for Greenheck, Schofield, Wis., Kuski has heard his fair share of noise complaints - and he would appreciate the pun.

"Sound and vibration control is becoming more important in HVAC," he said. "Sound and vibration are very closely related. The relationship between the two has to do with the frequency at which a vibration or pulsation occurs." The lowest he deals with are centered around 63 Hz (cycles per second) and go up to 8,000 Hz. The lower frequencies, he said, have been more difficult to deal with than those at the upper end.

With the use of lighter weight building construction materials, and the common placement of air-handling units (AHUs) on roofs instead of in subgrade equipment rooms, there also are more opportunities for AHU component vibration and noise to reach the building's occupants.

"There is a continuum from vibration to sound," Kuski said. "Below 20 Hz you can't hear it, but you can feel it. Sound and vibration work together. Low-frequency sound touches onto vibration.

"It's one of the biggest challenges in our industry, to make products that don't have low-frequency sound. Sound attenuators can't attenuate 60- to 125-Hz sound.

In the old days, you had an equipment room in the basement," he continued. "Down in the basement if you put in some fans, they were on something solid.

"Nowadays a lot of the stuff comes prepackaged, the crane comes and plops it up on the roof. Roofs aren't that sturdy! If you get something shaking that roof up and down, there's the transmission path that starts shaking the CEO's desk." Hanging light fixtures and even the ceiling grid shake and generate noise. "That's how low-frequency vibration turns into noise."

He compared high-frequency fan noise to a jet engine, which generates a high-pitched hum. Low-frequency fan noise, on the other hand, sounds more like a helicopter cutting the air. Higher-efficiency filtration may improve air quality, he added, but it also results in higher external fan static pressure.

"Fans operate at higher rpm to build pressure. The result is higher sound."

Fan Designs

In the design of fans, two things are considered for noise and vibration:

1. Low-frequency vibration.

2. Most of the energy a fan creates has a tone at the blade pass frequency. (Blade pass frequency equals fan rpm multiplied by number of blades in the rotor divided by 60.)

"Vane axial fans have lots of blades in there, they rotate at a high rpm and create 500-Hz tones," Kuski said. These would be like the jet engine type of high-pitched tones. "A lower-frequency fan would be a centrifugal fan, with fewer blades, slower rpm, and creating a lot of lower-frequency energy. It would be like a helicopter noise."

Some manufacturers, such as Greenheck, have started putting more blades into centrifugal fans, he said, making them easier to attenuate with conventional sound attenuation. These new fans generate smoother, high-quality sound that is less tonal, Kuski said.

Noise Diagnosis

Occupants in general are becoming less tolerant of noise and vibration, he said.

Normal 60-Hz appliance hum are a part of our daily lives. "The hum from a lighting ballast is 120 Hz," Kuski said. "We see it a lot in single-phase motors. It's a problem up over an occupied space; it annoys the hell out of people. If you've got a cheap bathroom fan, that noise you hear is 120 Hz."

When sound/vibration problems are being studied in the field, the first thing to ask yourself is, can problem be felt (vibration) or heard (noise)? If the vibration is felt but not heard, it could indicate a low-frequency problem.

Frequency analysis is then performed to identify the source.

"Let's talk sound," Kuski said. "You do the analysis with a sound analyzer, a piece of equipment where the cheapest ones are several thousand dollars. A mic measures sound in those eight distinct octaves in the NC chart. A sound consultant will walk into the room with the instrument, measure dB in the eight octave bands, and plot it on his NC chart. It helps him start to diagnose what the problem is.

"If there's a fan tone due to the blade pass frequency, he goes after the fan guy. If it's low frequency, maybe he needs to put in vibration isolation to break the path. There also are things called duct flex connections, where on each end of the fan there might be a fabric connection that keeps the vibration from getting into the ductwork."

When you get to the lower-frequency vibrations (20 Hz and less), the analyzer uses an electrical device called an accelerometer that measures the physical displacement of that piece of equipment.

"Vibration measurement is a very big industry," Kuski said. "It's like putting an EKG on a person; knowing the frequency helps you diagnose and solve the problem."

Noise Solutions

One of the first solutions is to break the transmission path of the sound/vibration problem. (Typical transmission paths have already been pointed out.) Economical solutions may include:

  • Vibration isolators applied right to the fan.

  • Sound attenuators applied within the duct themselves, very close to the fan.

  • Sound barriers, typically fiberglass that changes vibration into heat and drywall, which prevents sound from going through. When there is duct vibration to be stopped, drywall may be attached right to the sheet metal duct, as long as there is little risk of duct condensation.

  • Changes to equipment (i.e., replacing equipment, put in a new fan, changing fan rpm to get off of a bad frequency).

    New fan technology can also help reduce noise and vibration during the design phase, Kuski said. Mixed-flow fans (for instance, the Greenheck Model QEI) offer an alternative to conventional vane axial and tubular-centrifugal inline fans. The manufacturer's Web page (www.greenheck.com/products/centrifugal/mixed_flow.php) offers an audio demonstration of mixed-flow vs. typical vane axial fans.

    Centrifugal fans (Model QEP plenum fans) also offer high-efficiency/low-noise wheel designs, Kuski said. "The increased number of blades and airfoil blade shape reduce the tonal sound quality and generate five to seven dB less sound power in the hard-to-attenuate 63- and 125-Hz octave bands," he said.

    Finally, sound enclosures "can be used on inline mixed-flow fans to reduce radiated casing sound in applications where the fans are in close proximity to occupied spaces."

    In Kuski's world there are no common noise problems. "They're usually pretty bad when they cross my desk," he said. "However, troubleshooting is something we have expertise in."

    Publication date: 10/03/2005

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