Science of sound: Sunbury City Council hears expert on data center noise ahead of Nov. 3 ballot vote

SUNBURY — Sunbury City Council held its third and final special meeting on data centers Oct. 22, ahead of a Nov. 3 vote on a proposed city charter amendment that would prohibit their construction within city limits.

Noise, sound, acoustics, frequencies and decibels were all discussed by professional acoustics engineer Rob Stevens from HGC, a noise, vibration and acoustics engineering firm.

During his presentation, Stevens explained how people perceive sound, outlined safe and potentially harmful decibel levels, distinguished between sound, noise and vibration, and discussed methods industries use to control noise.

Most relevant to residents’ concerns was Stevens’ discussion of how data centers can reduce noise from cooling fans.

Residents have previously raised concerns about the low-frequency hum associated with data centers during City Council and Planning and Zoning Commission meetings.

Stevens explained that three types of fans manufacturers use in equipment: standard axial fans, low-noise fans and ultra-low noise fans.

Most HVAC systems use the inexpensive-to-produce axial fan, because Stevens explained that low-noise fans and ultra-low noise fans are much more expensive to manufacture.

Ultra-low noise fans, he said, are commonly found in commercial air conditioners. They can be as large as 30 feet in diameter and are very effective in noise mitigation. However, when they need to be replaced, they can cost upwards of $1,000.

Stevens also explained how variable speed drives (VSDs) regulate the speed of cooling fans used in data centers.

“Fans are designed to meet a cooling load, a maximum heat dissipation time, which is generally a hot summer day,” Stevens said. “So that means at all other times, fans are actually over-designed. We can take advantage of this when it comes to wanting quieter sound levels at night.

“Generally we want them to be five to 10 decibels quieter at night than during the day because there is a greater need and expectation for quiet at night.”

Because temperatures tend to drop at night, Stevens said the cooling load drops and allows the fans to decrease their workload by around 50%. Slower fans means less sound.

He also explained how sound barriers can come into play in terms of blocking sound from a source, as demonstrated in one of his slides below:

Stevens also explained inline silencers, which allow the flow of air through but minimize the amount of sound coming out.

To continue using the commercial air conditioner example, units typically have several “hoods” that contain these silencers at various points around the unit.

Stevens said that fiberglass or mineral wool are typically the mediums inside the hoods that “soak up the sound.”

Highlights from council’s questions

Mayor Joe St. John asked whether a sound level meter designed to measure frequencies audible to humans could detect the humming or droning noise residents fear a data center might produce.

He also asked if city staff could purchase one (or several) for parts of the city around the data center, should the proposed Amazon facility be constructed.

Stevens said a lot of decibel readers have very high quality microphones and other technology that make them expensive. He also said there are many different things the reader can read, so it can be difficult to refine exactly what kind of data or sound someone is trying capture.

Councilwoman Cindi Cooper asked if there are consultants out there who can help council with this, and Stevens said an organization like the Institute of Noise Control Engineering (INCE) would be a good resource.

Council and Stevens also discussed what a maximum frequency level or maximum decibel level could be in Sunbury, something council could put into effect or edit via legislation during a meeting.

City Manager Daryl Hennessy said the city plans to consult HGC about the city’s zoning code in terms of what an appropriate level may be in the future.