Summary: A precision hearing and audiology study has clarified the biological mechanisms behind “The Hum” — an enigmatic, low-frequency buzzing reported by people in coastal and densely populated cities worldwide. The research investigated whether the phenomenon arises from unusually sensitive low-frequency hearing, measurable emissions produced by the ear itself, or a purely internal auditory perception. The results indicate that while a very small number of people do have exceptional low-frequency sensitivity, most who perceive the Hum are experiencing a rare, low-frequency form of subjective tinnitus generated entirely within the auditory system.
Key facts
- The global enigma: First widely reported in Bristol, England in the mid-1970s, The Hum has since been noted in coastal and urban locations across the UK, the United States, Canada, Australia, New Zealand, South Africa and parts of Europe. The sound is typically described as a low-frequency vibration, most noticeable indoors and at night, that can disrupt sleep and cause discomfort for some people while others nearby hear nothing at all.
- Study framework: To separate environmental noise from internal biological causes, Professor Markus Drexl and an international research team at NTNU carried out a focused study of 28 individuals in Germany who report persistent, unexplained humming.
- Hyper-sensitive hearing ruled out for most: The team tested whether sufferers had exceptional low-frequency thresholds. Apart from two outliers with somewhat better-than-average sensitivity at specific low frequencies, most participants showed normal hearing thresholds, refuting the idea that the majority simply have hyper-acute low-frequency hearing.
- Oto-acoustic emissions tested: The cochlea naturally produces weak oto-acoustic emissions (typically between 500 and 5,000 Hz) as a by-product of inner-ear amplification. Sensitive ear-canal measurements tested whether participants were hearing spontaneous low-frequency oto-acoustic emissions; no measurable low-frequency emissions were found in these subjects.
- Subjective tinnitus explanation: After excluding measurable external sources and low-frequency oto-acoustic emissions for most participants, the researchers concluded that the predominant cause is subjective low-frequency tinnitus — an internal, unmeasurable auditory percept that sufferers often initially mistake for external machinery or environmental vibration.
- Knowledge gaps on low-frequency processing: Public concern about technical low-frequency noise (approximately 20–250 Hz) and infrasound (below 20 Hz) has grown alongside developments like wind turbines, heat pumps and heavier urban noise. Drexl emphasizes that the auditory system’s processing of the lowest frequencies is still incompletely understood, and that advancing this understanding is essential to address phenomena like The Hum.
Some people report the sound as merely annoying and manageable; others experience nausea, sleep disruption, or significant distress. The humming is often faint or absent outdoors but becomes prominent indoors at night, when background environmental sounds drop and the auditory system is left unmasked by other noise. Looking outside for a visible motor or machine frequently yields nothing. In the same room, some people can hear the Hum while others perceive nothing.
First discovered in coastal cities
The Hum first attracted attention in Bristol in the mid-1970s, when local newspapers received many letters from residents describing an unexplained low-frequency noise. Early theories blamed nearby industrial equipment, but the reports continued even after suspected sources were removed. Over subsequent decades similar reports emerged in coastal towns across the UK (Hythe, Plymouth, Southampton, Swansea) and in other countries — notably Taos, New Mexico and Kokomo, Indiana in the United States — and then globally in populated regions.
Some responders have tracked their own experiences geographically: for example, a Canadian teacher noticed the hum while living on the west coast and found it disappeared after moving to a different city in the region. These sporadic, localised occurrences led to crowd-sourced mapping projects and renewed scientific interest.
Many different theories
Proposed explanations have ranged from ordinary environmental and industrial low-frequency sources — ventilation, heat pumps, traffic, wind turbines, waves and wind — to more speculative ideas. Because low-frequency sound waves have long wavelengths and can travel long distances, identifying and localizing such sources is technically challenging. The Hum therefore attracted the attention of hearing researchers attempting to distinguish true external noise from internal auditory phenomena.
Sounds that can be measured
The research team tested two main hypotheses: first, that the Hum corresponds to measurable low-frequency environmental sound that some people hear and others do not; and second, that the perception stems from internal auditory sources such as spontaneous oto-acoustic emissions.
Low-frequency environmental sounds can be recorded with suitable microphones and instruments, but localization is difficult because these waves propagate far and are easily masked by other noise. In past cases, objective recordings of low-frequency sources have been documented, but they do not explain the majority of Hum reports.
Extra good hearing?
High-resolution threshold testing showed that most people reporting the Hum do not have broadly enhanced low-frequency sensitivity. Two participants did show better-than-average sensitivity at particular low frequencies, suggesting that a small subset of listeners could detect external low-frequency sound that others miss. However, conventional testing may miss very narrow frequency sensitivities that could allow perception of a 50 Hz tone, for example, so rare microstructure differences cannot be fully ruled out by standard clinical tests.
The ear can produce sounds itself
The cochlea produces weak oto-acoustic emissions (OAEs) as a normal consequence of its amplification machinery, typically at frequencies between about 500 and 5,000 Hz. These emissions are measurable with a sensitive ear-canal microphone and, in some cases, can become perceptible to the individual and experienced as tinnitus. The study specifically tested for low-frequency spontaneous oto-acoustic emissions but found no evidence that such emissions caused the Hum in the participants examined.
Sounds that cannot be measured
When no external source can be found and objective measurements remain negative, the remaining explanation is subjective tinnitus: an internally generated auditory percept not traceable to outside sound. Many people with tinnitus initially interpret the sound as external; over time they may realize it moves with them and persists across locations, identifying it as internal. Based on the tests and current understanding of auditory physiology, the NTNU team concluded that the dominant explanation for most reported Hum cases is low-frequency subjective tinnitus, although isolated instances of true external sources remain possible.
Must understand the entire auditory system
Professor Drexl notes that most auditory research and clinical practice focuses on mid-to-high frequencies relevant to speech and everyday listening. Less is known about how the ear and brain detect, process and sometimes generate low-frequency sound and infrasound. Growing public concern about low-frequency technical noise underscores the need for improved measurement methods and a deeper understanding of low-frequency auditory processing to reliably separate external sources from internally generated perceptions.
Key questions answered
Q: Why does The Hum seem louder and more disruptive at night indoors but often vanish outdoors?
A: Quiet indoor environments reduce competing sounds that normally mask internal auditory perceptions. If the Hum is subjective tinnitus in the low-frequency range, silence lets those internal frequencies stand out, making them seem louder and more intrusive.
Q: Can the human ear really generate audible sounds from the inside?
A: Yes. The inner ear produces very weak oto-acoustic emissions as a biological by-product of cochlear amplification. Most people do not perceive them, but they can occasionally enter conscious awareness. The NTNU study found no evidence that low-frequency oto-acoustic emissions explain the global Hum reports.
Q: How can researchers tell whether a town’s vibration is real infrastructure noise or mass tinnitus?
A: Objective measurement is key. External sources can be recorded and localized and remain anchored to a location. If the perceived sound cannot be recorded and continues to follow individuals after they relocate, it strongly suggests a subjective tinnitus origin.
Editorial notes
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by staff.
About this auditory neuroscience and tinnitus research news
Author: Nancy Bazilchuk
Source: NTNU
Contact: Nancy Bazilchuk – NTNU
Image: Credit to Neuroscience News
Original research: Closed access. Title: “On the potential sources of a low-frequency sound percept that only a few can perceive” by Bonifaz Baumann, Andrej Voss, Carlos Jurado, and Markus Drexl. Published in PLOS One. DOI: 10.1371/journal.pone.0326818
Abstract (summary): A small percentage of people report persistent low-frequency humming or pulsing percepts that others around them do not detect. This study tested two main hypotheses: unusually high low-frequency sensitivity and perception of spontaneous low-frequency oto-acoustic emissions. High-resolution low-frequency threshold and OAE measurements in 28 complainants revealed that most did not have unusually sensitive low-frequency hearing and no low-frequency spontaneous oto-acoustic emissions were detected. While physical external sources are not ruled out in individual cases, the findings support the conclusion that many of these low-frequency sound percepts are best explained by subjective tinnitus originating within the auditory system.