11The hum: machines, transformers and mystery noises

The short version: Most city hums are electrical or mechanical. Transformers and other magnetic equipment hum at twice the grid frequency - about 120 Hz in North America, about 100 Hz in Europe and most of the world - because their iron cores flex twice in every cycle of the current. Cooling fans, chillers and backup generators add steadier drones, and data centers have drawn organized complaints and new local limits in Virginia and Arizona. A few hums have no agreed source at all; the Windsor Hum in Ontario is the rare case where researchers named a likely culprit, a steel plant across the river in Michigan, and in mid-2020, as the plant was being idled, the lead researcher said he had not heard it for almost a year.

Why does electrical equipment hum?

What you hear: A low, steady, slightly buzzy tone from a transformer box, a substation, a fluorescent fixture or a power adapter, the same pitch day and night.

Power grids deliver alternating current that reverses direction 50 or 60 times a second.²⁰³ In a transformer, motor or other piece of magnetic equipment, that current drives a magnetic field through an iron core. The field reaches a peak twice in every cycle - once in each direction - so the force on the core pulses at twice the grid frequency.²⁰³

On top of that, the core steel itself changes shape very slightly when magnetized, an effect called magnetostriction. As the field rises and falls, the laminated steel sheets stretch and relax, and the whole core vibrates like a small loudspeaker.²⁰³ The result is mains hum: a fundamental tone at twice line frequency, with strong harmonics above it that give it a buzzy edge rather than a pure tone.²⁰³

Because the pitch is locked to the grid, mains hum does not drift. A sound that holds exactly the same note for hours is a strong hint that something electrical is involved.

Why is the hum 120 Hz in North America and 100 Hz in Europe?

What you hear: The same transformer drone, pitched slightly lower in Europe than in North America.

The hum follows the grid. Doubling the line frequency gives:

Source for the frequencies: EPFL Graph Search, "Mains hum."²⁰³ The country examples are a general guide to which grid each place uses.

Japan is split down the middle: eastern Japan runs a 50 Hz grid and western Japan a 60 Hz grid, because Tokyo Electric Light bought a 50 Hz German AEG generator in 1895 and Osaka Electric Light a 60 Hz General Electric generator two years later.²⁰⁴ The two halves trade power through frequency converter stations, the first of which, a 300-megawatt station at Sakuma, was completed in 1965.²⁰⁴

The same source notes that aircraft, which run on 400 Hz power, produce a correspondingly higher hum.²⁰³

What is the 60 Hz buzz in my speakers?

What you hear: A low drone from a stereo, guitar amp or PA that gets louder when you plug in another cable.

This one is different. Hum that leaks into audio equipment, often through a ground loop, sits at the line frequency itself - 60 Hz in North America, 50 Hz in 50 Hz countries - rather than at double it, according to the same EPFL summary.²⁰³ So a hum that pitch-matches about 60 Hz in your own sound system points at the wiring of your gear, while a hum near 120 Hz from outside points at magnetic equipment.

Why do transformers and power lines hum?

What you hear: A drone from a green pad-mounted box on the sidewalk or a fenced substation; sometimes a crackle or sizzle from high-voltage lines, louder in damp weather.

Transformers are the classic source, for the magnetostriction reasons above.²⁰³ High-voltage power lines can also hum through corona - electrical discharge into the air around the conductor.²⁰³ For how electricity gets from power plants to your street, see How a City Works: power.

Substation hum falls off quickly with distance. For a utility substation in Arlington, Washington, with two 28-MVA transformers, a 2016 acoustical analysis predicted up to 50 dBA at the substation's property lines and 23 to 26 dBA at homes more than 1,000 feet away, against limits of 60 dBA and 57 dBA.²⁰⁵

What about rooftop units, chillers and generators?

What you hear: A whooshing fan roar from a building roof or a back alley, cycling on and off; or a diesel engine that starts up, runs at a steady pitch for a while and shuts down.

HVAC equipment - rooftop air-handling units, condensers, chillers, cooling towers - makes a mix of fan noise and motor or compressor hum. Cooling towers can reach up to 70 dBA within 400 feet (about 120 meters), according to the Environmental and Energy Study Institute (EESI), though its figure is for cooling towers at gas-fired power plants rather than data centers.²⁰⁶

Generators are louder. On construction sites, the FHWA measured a generator at 81 dBA Lmax at 50 feet (see Construction and leaf blowers).¹⁹¹ EESI puts industrial diesel generators at up to 105 dB (the article does not give the weighting or distance), so those two figures cannot be compared directly.²⁰⁶ Backup generators at data centers are tested at least monthly, which neighbors may notice as a recurring roar on a schedule.²⁰⁶

Why are data centers so loud?

What you hear: A constant whooshing drone at the edge of a neighborhood, strongest at night when everything else goes quiet, plus a periodic diesel roar.

The noise is mostly cooling. EESI reports that cooling systems use about 40% of a data center's electricity, and the fans, chillers and cooling towers run around the clock.²⁰⁶ Much of the sound energy sits at low frequencies. Kyle Hart of NPCA told EESI that low-frequency content is hard to measure with standard meters.²⁰⁶ A-weighted meters (dBA, explained in Sound and decibels) turn down low frequencies by design, so a reading that looks modest can understate a drone that residents find intrusive.

Proximity makes it worse. A 2024 review by Virginia's Joint Legislative Audit and Review Commission (JLARC), cited by EESI, found nearly one-third of Virginia data centers sit within 200 feet of residentially zoned property.²⁰⁶

Chandler, Arizona is one of the most cited cases. Residents of the Brittany Heights neighborhood complained of a nighttime hum after a data center arrived in late 2014; the city passed a zoning amendment in 2022, and in 2025 the council voted against a new center.²⁰⁶ According to the Prince William Times, a Chandler ordinance from January 2023 requires resident meetings, a paid noise liaison and a pre-construction acoustics baseline, and says post-opening noise may not raise neighborhood levels above what existed before.²⁰⁷

On the industry side, EESI reports that Amazon is retrofitting acoustical shrouds at sites in Northern Virginia.²⁰⁶

Who sets data center noise limits?

What you hear: The same hum, judged very differently depending on which side of a county line you live on.

Limits are local and vary widely. As reported by the Prince William Times in February 2023 (weighting not stated in the article):²⁰⁷

Source: Prince William Times, 2023-02-27.²⁰⁷

Because the article does not say whether these are dBA, they should not be compared with dBA limits elsewhere in this guide. Warrenton's band-by-band approach is aimed at the problem described above: a single A-weighted number can miss low-frequency hum.²⁰⁷

What is "the Hum"?

What you hear: A low drone or throb, often described as like a distant idling engine, that you hear indoors at night - and that the person next to you may not hear at all.

"The Hum" is the name given to persistent low-frequency sounds reported by some residents in particular places, which others in the same place cannot hear. Most reported hums remain unexplained, and proposed causes range from industrial sources to effects inside the listener's own hearing. There is no single agreed explanation, and different researchers argue for different causes.

Three cases come up most often, as summarized by Live Science in 2013.²⁰⁸ In Taos, New Mexico, residents began complaining of a low rumble in spring 1991; researchers from Los Alamos and Sandia national laboratories and the University of New Mexico could not identify the source. In Bristol, England, about 800 people reported a steady thrumming in the 1970s, which was eventually attributed to vehicle traffic and factories running 24-hour shifts. In Kokomo, Indiana, a 2003 study funded by the city found two industrial sites, one a DaimlerChrysler plant, producing noise at specific frequencies, yet some residents kept complaining after noise-abatement measures.²⁰⁸

The Windsor Hum

What you hear: A low rumble that residents of Windsor, Ontario reported for years, traced by researchers to the other side of the border.

Residents of Windsor began reporting a hum around 2011.²⁰⁹ A study by the University of Windsor and Western University pointed across the border to Zug Island in River Rouge, Michigan, home to U.S. Steel blast furnaces.²⁰⁹ The work was funded by Canada's Department of Foreign Affairs and International Trade.²¹⁰

U.S. Steel scaled back operations on Zug Island from April 1, 2020. In July 2020 researcher Colin Novak said, "I haven't heard it for almost a year."²¹¹ The final report came out in July 2021. Novak said the blast furnaces were "really the only type of noise emitter that could do that," and that the hum was not expected to directly harm health but could annoy people or disrupt sleep.²¹⁰

Not everyone agreed. Researcher Glen MacPherson doubted Zug Island was the sole source and suggested some reports reflect a wider "world hum."²¹²

The Windsor case shows how hard tracing a hum can be: it took a multi-year, cross-border study, and the strongest evidence came when the suspected source shut down.

How to track down a hum

What you hear: Something low and steady that you cannot place, and you want to know whether it is a machine, a neighbor or your own ears.

These steps follow from what the sources above describe. None replaces a professional acoustic survey.

1. Note the pattern. A hum that holds exactly one pitch around the clock suggests electrical equipment locked to the grid.²⁰³ One that rises and falls with heat or time of day suggests cooling equipment. One that appears on a schedule for a while, then stops, may be a backup generator test - data center generators are tested at least monthly.²⁰⁶
2. Estimate the pitch. A free tone-generator or tuner app can help you match it. Near 120 Hz in North America (100 Hz in 50 Hz countries) points to transformers and magnetic equipment; near 60 Hz (or 50 Hz) inside your own sound system points to your audio wiring.²⁰³
3. Walk it. Check whether it gets louder near a transformer box, substation, rooftop unit, cooling tower or generator enclosure. Industrial sources can carry a long way: in Windsor, the likely source was on the other side of an international border.²⁰⁹
4. Ask a neighbor to listen. If others hear it, it is more likely an external source. If only some people hear it, that is the pattern reported in many unexplained hum cases.
5. Do not trust a dBA reading alone. A-weighted meters, including most phone apps, discount low frequencies, and low-frequency content is hard to measure with standard meters.²⁰⁶ A low reading does not mean the hum is imaginary.
6. Report it to the right office. Equipment noise from a utility, a building's mechanical system or a facility usually falls under your city's noise code. Who makes the noise rules explains who handles what.

Elsewhere in the world

The physics travels, but the pitch does not: in 50 Hz countries such as the UK, the EU states and Australia, transformer hum centers on 100 Hz rather than North America's 120 Hz.²⁰³ The best-documented traced hum in our sources sits on the US-Canada border, where Canadian residents and researchers pointed to a US plant.²⁰⁹

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