Data Centers: Noise in the 21st Century
Data Centers Emit Low-Frequency Noise, Which Can Be Annoying in Quiet Rural Areas
Noise is not a modern problem. It may surprise you that there are numerous accounts of noise complaints in the early days of colonial America, but they were different from the types of noise we associate with cities today: while our cities are packed with cars and constant humming of HVAC systems, Philadelphia in the 18th century was full of horseshoes clomping on cobblestone streets and batting rods beating the laundry dry. 18th century noise was short and impulsive; 21st century noise is long and constant.
And there is perhaps no more iconic noisy symbol of the internet age than the data center. These large campuses full of buildings, full of servers, generate immense heat and require huge cooling either through water or air. Air-cooled data centers (which are the most common) generate high amounts of noise from the hot air blowing out the tops of these buildings. While the fans themselves produce disproportionately low-frequency noise, the noise itself becomes more lowpass filtered due to diffraction effects (the ability of sound waves to bend around edges) over the top of the building and via atmospheric air absorption, which only affects higher frequencies and lets the low frequencies pass through.
Our ears are somewhat less sensitive to low-frequency noise, but only up to a point: as the pressure at low frequencies increases, our ears become more sensitive to it, leading the noise to be perceived as a rattling or humming, depending on the exact frequencies present. As the AI age progresses, the data center buildout is leading to numerous cases of neighbors who are only now realizing what it means to live next to a constant low frequency hum. Since the lowest frequencies have the largest wavelengths, even building a costly wall is not enough in some cases to stop the hum.
It is not beyond our technical means to make quiet data centers, but it will cost more. The regulatory framework hasn't kept pace, and most municipal noise codes fall into one of two camps:
Some do not specify a numerical dB noise limit at all
Others specify a limit only in A-weighted decibels (or dBA)
A value in dBA is useful if you’re looking at speech, or music, or many of the sounds our ears are most sensitive to. But dBA systematically underweights low-frequency energy because it assumes the overall pressure is low enough that our ears will not be sensitive to it. However, when the pressure builds up, reporting only dBA values can hide the true problem because the total sound energy can be much louder than the dBA value.
How Should We Measure Data Center Noise?
This is why for data center projects I recommend reporting C-weighted decibels (or dBC) values. If there’s not much low-frequency energy, the dBA value is a good estimate for the sound’s loudness, but dBC is used in cases where there’s significant low-frequency energy (like for booming subwoofers from NYC nightclubs, for instance). An exact dBC limit may vary somewhat by municipality: in very quiet rural areas, there is not a lot of other noise and our auditory system may be conscious of the low-frequency hum, leading to greater community annoyance. In noisier urban/suburban environments, which may have greater noise in the higher-frequency region of our hearing range, the low frequencies will often not be noticed if there is sufficient masking energy above them. However, if the low-frequency noise is great enough, it can still lead to negative health effects even when it is not heard! These health effects are part of why community opposition to data centers has grown, though not all of it is well-founded. There is a lot we still don’t know about low-frequency noise exposure.
Some data center opposition is pure NIMBYism (“Not In My Back Yard!”), a knee-jerk reaction against building anything new. I am not opposed to data centers! This very article is hosted online, stored in the cloud in a data center. You wouldn’t be reading it if not for data centers. The AI revolution has the potential to transform the modern economy for the better (or not, but that’s a separate problem).
I am in favor of building out more data centers, but we need to do so in a way that does not alienate the surrounding neighbors. Sufficient interior noise absorption, rooftop barriers, and fan retrofits to reduce resonant noise components are all available and can reduce the low-frequency hum to manageable levels. Well-designed mitigation options can reduce the noise by 10-15 dB, which is usually enough to reduce the noise below the threshold of community annoyance. It will be somewhat more expensive, but it will allow the data center buildout to be more sustainable and to exist in harmony with the land it is occupying.
Science for People
I went to graduate school with a great love for physics and a sense of wonder at how our auditory system works to help us understand the world around us. I chose to stay in academia because I wanted the freedom to work on research topics that interested me, without having to kowtow to management. Often this took me in directions which were decidedly non-commercial (understanding the acoustics of Bach’s church or trying to predict the effects of noise transmission during the pandemic). Sometimes these forays into other fields also led back to more basic questions in physics, as new questions necessitate new methods to answer them.
However, having received tenure at American University, I’ve realized that I don’t have to spend all my time on narrow academic questions, and in fact it is refreshing and uplifting to spend time applying what we know about acoustics, noise, and sound perception to real-world problems faced by ordinary people. Just as interdisciplinary work can lead back to new questions for science to answer, so can applied work cross-fertilize between the scientific rigor of the university with the quick-moving solutions of the free market. I will always be an academic first, but I hope here to use science to help people understand sound better and help their world sound better too.