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. Lately I’ve been hearing the hum again. 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 a 30 Hz frequency of the hum (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:
- 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
It’s likely that the Hum is caused by a combination of two or more sources, which would explain why the perceived tone modulates irregularly. For example, have you ever pulled beside another vehicle at a traffic light and felt a pulsing vibration? That’s a phenomenon called a “beat,” when two sound waves with close frequencies interact with each other and create an amplitude modulation.
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
- 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