Do you hear it? When I moved to Ipswich, I began noticing an oscillating low frequency hum when I’m lying in bed at night, similar to a motor idling in the distance. Interestingly, the hum went away during the height of the COVID epidemic in late 2020 and early 2021, but in the summer of 2021, the hum is back, louder than ever and I hear it any time of the day.
My wife thinks it’s in my head and said I shouldn’t write this article. I’ve had high frequency tinnitus for as long as I can remember, but the hum is a very low frequency. I checked all of the obvious possibilities in and around my house, and it’s not a motor, the furnace, the dehumidifier or a transformer vibration.
So I Googled, and learned that “The Hum” is a worldwide phenomenon in which a distant rumbling sound is heard by about 2% of the population, which I guess makes me special. People who hear the hum describe a low-frequency sound inside their houses, around 20-40 hertz (the lower limit of normal human hearing). In industrial acoustics, it is well-established that wooden structures amplify frequencies below 100 Hz, even below 50 Hz. But that doesn’t explain the source of the hum.
I reproduced an 80 Hz frequency which is somewhat close to what I hear but without the modulations) at the Online Tone Generator.
Below is a recording of the hum in Windsor, Ontario.
Even Wikipedia has a page about “The Hum”:
“A name often given to widespread reports of a persistent and invasive low-frequency humming, rumbling, or droning noise not audible to all people. Hums have been widely reported by national media in the UK and the United States, and sometimes named according to the locality where the problem has been particularly publicized.”
The phenomenon has been studied for several decades with no definitive answers, but multiple explanations have been proposed:
- Low-frequency tinnitus oscillating with exterior sound sources, (study by Franz Gunter Frosch)
- The phenomenon is almost always heard inside a house. Some people tried switching off the main breaker but it made no difference.
- A Seismic hum is created by the Earth’s tectonic plate movements and volcanic eruptions.
- Booming sand dunes can create sound during windy weather at a frequency between 70 and 110 Hz.
- Ocean waves especially those located along shelf breaks can resonate with periods of 50 to 300 seconds through the ground and cause vibrations in nearby structures.
- Infrasound, sound waves with a frequency below the lower limit of audibility (generally 20 Hz) must be sufficiently high to hear. Examples are diesel engines and wind turbines.
- Electric hum around transformers by stray magnetic fields cause the enclosure and accessories to vibrate, typically at 50 hz, with harmonic tonal spikes at 100, 200, and 400Hz harmonic frequencies.
- Power lines are known create a humming sound at line voltage frequency (120 / 240 Hz) with harmonic content above 50–60 Hz, aka “Mains Hum.”
- Schumann resonances are continuous natural ELF waves resonating between the ionosphere and Earth’s surface at a frequency of 7.83 Hz, along with progressively weaker harmonics at 14.3, 20.8, 27.3 and 33.8 Hz.
- Auditory perception of microwave pulses, which can create a wave of acoustic pressure conducted by the skull to the inner ear, activating the cochlea.
- ELF (extremely low frequency) transmitters were proposed to communicate with submarines while submerged. Project Sanguine was a U.S. Navy project, proposed in 1968 and would have required a giant antenna covering two fifths of the state of Wisconsin, but was never implemented.
- The VLF Transmitter Cutler in Cutler, Maine provides one-way communication to submarines in the Navy’s Atlantic Fleet at a frequency of 24 kHz and input power of up to 1.8 megawatts, one of the most powerful radio transmitters in the world
- The US government’s High Frequency Active Auroral Research Program (HAARP) transmits RF into the ionosphere.
- In northern latitudes, some people claim to hear the Aurora Borealis.
- And of course, Aliens. But as I mentioned, I’m not crazy.
An example of how a beat is created with two different sound wave frequencies
Feel the beat
Beating is a phenomenon caused by two strong tones or vibrations with frequency close to each other giving the resulting tone an amplitude variation over time. The superposition of two waves of slightly different frequencies but identical amplitudes create a third phenomenon, known as constructive and destructive interference, or “beat.” For example, have you ever pulled beside another vehicle at a traffic light and felt a pulsing vibration? That’s a “beat,” caused by the engines of two vehicles with close frequencies interacting with each other. When the two engine speeds are close, it can result in the phenomenon of beat vibration. When the two engine frequencies change, the beat vibration disappears. The beat is louder and is felt much more strongly than the individual frequencies that create it.
I was reminded of this recently while flying cross country in a Boeing 757. Sound waves from the two jet engines had almost identical frequencies, but it is impossible to make them exactly the same. The result was a loud fluctuating vibration in the entire body of the plane caused by interference as the two waves go in and out of phase with each other. It’s likely that the Hum is caused by a similar interaction of multiple sources, which would explain why the perceived tone modulates irregularly.
Beats occur when waves of similar frequencies are superimposed. The resulting amplitude oscillates with a beat regularity determined by the difference between the two frequencies. If the two frequencies are almost identical, the period between beats is longer but the beat is stronger. In our loud world, multiple sound waves create multiple beats which in turn interact with each other. If this is causing the world-wide hum, the question still remains, what is creating the initial vibrations?
The frequency difference between the two tones corresponds to the beating frequency. Multiple sources with close frequencies result in an irregular beat and variations in the sound amplitude (volume).
In 1996 residents in Hull and Nahant, Massachusetts began reporting the unidentified sound of an engine idling in the distance. More recently, the World Hum Map produced by Dr. Glen MacPherson pinpoints hundreds of hum reports from the UK and the US, including reports from Topsfield, Newburyport, Salem and Gloucester.
Most “hum hearers,” including myself, experience the Hum at a very low volume starting around 10 pm, and we don’t hear it during the day. Have you heard the Hum? I promise you’re not crazy!
References and further reading:
- Wikipedia: The Hum
- Wikipedia: Microseisms
- Global quieting of high-frequency seismic noise due to COVID-19 pandemic lockdown measures
- Denning, David, Journal of Scientific Exploration, Vol. 18, The Hum: An Anomalous Sound Heard Around the World
- Cracking the mystery of the ‘Worldwide Hum’
- Cat in the Shadows, the unexplained: What’s that noise?
- The New Republic: A Maddening Sound
- American Geophysical Union: Relative importance of the day‐night asymmetry in Schumann resonance amplitude records
- Robert Larsson, Vibration characterisation of low frequency engine idle vibrations
- The Atlantic: Why Everything Is Getting Louder
- Manifestations of a low-frequency sound of unknown origin perceived worldwide, also known as “the Hum” or the “Taos Hum”