Data Centers Need 4.5-Foot Walls or 1,500-Foot Setbacks to Contain Hum

A facility wall would need to be 4.5 feet thick to absorb the low-frequency hum that data centers typically produce, according to Kasey Fowler-Finn, a biology professor at Saint Louis University who has researched the problem since 2001.

The low-frequency noise emitted by large-scale cooling and HVAC systems travels further and registers as more noticeable than the high-frequency noise that most ordinances target, Fowler-Finn said in an interview last week.

"Data center hums [are] concentrated in single frequencies and they really stand out from background noise no matter how loud or what frequency that background noise is in," Fowler-Finn told Saint Louis Public Radio on Sept. 24.

Last month Fowler-Finn released model noise guidance for data centers, built on two and a half decades of her own research and focused on the specific wavelengths that large HVAC and cooling equipment emits. The physics is unforgiving, she argues.

"A facility can be compliant on paper while still delivering audible — or, for very low frequencies, inaudible but still physiologically relevant — daytime and nighttime noise exposure linked to numerous health concerns," Fowler-Finn writes in the guidance.

All sound loses intensity as it spreads geometrically, she explains. It weakens by 6 decibels each time the distance from the source doubles, and by 30 dB over 3,000 feet. Structures in the sound path absorb energy too — but far less of it at low frequencies, because of the wavelengths involved.

"Low frequencies … pass through walls and windows with little loss, since ordinary building materials are, for practical purposes, ineffective at attenuating them," she says. "Low-frequency sound can also generate secondary noise by inducing vibrations and rattles in nearby structures."

The numbers are stark. A 4-inch wall handles a typical high-frequency source like a vacuum cleaner, which reaches about 1,000 hertz. To mitigate a 63 Hz tone — typical of large-scale HVAC and cooling systems — the wall would need to be 4.5 feet thick. At 20 Hz, it would need to be 14 feet thick, which Fowler-Finn calls "not a remotely practical thickness for a building enclosure."

Her solution is distance and better measurement. Common A-weighted and C-weighted decibel measures work as a starting point, she says, but they must be paired with third-octave band sound pressure testing, which captures levels between 20 Hz and 200 Hz.

"Any tonal or narrow-band component of the facility's noise is independently identifiable [using that testing] rather than averaged away into a single broadband number," she says, "so that a facility cannot pass an overall dBC allowance by averaging a single loud low-frequency band against several quieter ones."

Fowler-Finn calls for setback distances of at least 1,500 feet — well beyond what data centers typically face. Most are permitted under 100- to 600-foot setback requirements written for industrial buildings. "These setbacks are generally written for ordinary or broadband industrial noise," she says.

The litigation record backs her up. Of roughly half a dozen lawsuits filed this year against operational data centers, five listed noise as the main problem. "That holds across very different jurisdictions," Fowler-Finn says. Pending cases sit in Vineland, New Jersey; Dowagiac, Michigan; North Tonawanda, New York; and Hood County, Texas, according to an alert from law firm WilmerHale. In late July, residents sued Microsoft over noise from its Fairwater data center in Mount Pleasant, Wisconsin, which became operational in June.

A September survey by Science for Georgia and Quiet Communities found that more than 80% of people living near a data center can hear the facility inside their homes, more than 70% said the noise interferes with their ability to relax, and almost 70% have considered moving because of it. "Many described avoiding outdoor spaces, experiencing chronic sleep disruption and feeling unable to escape the constant noise," the survey report states.

Fowler-Finn's prescription for building owners is blunt: "Instead of fighting to put these in where people live and work, [building owners] should be working on technology that makes these friendlier to the environment."