Summary: A precise hearing and audiology study has clarified the biological mechanisms behind “The Hum” — a mysterious, low-frequency buzzing reported by people in many coastal and densely populated cities worldwide. Researchers investigated whether the sound stems from exceptionally sensitive low-frequency hearing, measurable emissions from the inner ear, or an unmeasurable, internally generated auditory phenomenon.
The study found that while a very small number of people do have unusually acute sensitivity to some low frequencies, most people who report hearing The Hum are experiencing a rare, low-frequency form of subjective tinnitus generated within their own auditory system rather than from an external sound source.
Key Facts
- The global mystery of The Hum: First widely recorded in Bristol, England in the 1970s, reports of a low-frequency humming have since emerged in many countries, often in coastal or densely populated towns. The sound is usually described as a low, steady vibration that is most noticeable indoors at night. For some it causes sleep disruption or illness, while others in the same room hear nothing at all.
- NTNU-led research: To separate external, infrastructure-related noise from internal biological causes, Professor Markus Drexl and colleagues at the Norwegian University of Science and Technology (NTNU) examined 28 people in Germany who reported persistent, unexplained humming.
- Not explained by hypersensitive hearing: The team tested whether sufferers had unusually low-frequency hearing thresholds. With only two exceptions showing better-than-average sensitivity at particular low frequencies, the group’s hearing thresholds were normal, undermining the idea that most Hum listeners simply have extraordinary low-frequency hearing.
- Otoacoustic emission testing: The cochlea can produce weak sounds (otoacoustic emissions) in the 500–5,000 Hz range as a byproduct of its amplification. Sensitive ear-canal microphones can detect these emissions in some people. The researchers tested for spontaneous low-frequency otoacoustic emissions but found none that could account for the reported humming.
- Subjective tinnitus is the most likely explanation: After excluding measurable external low-frequency sources and otoacoustic emissions for most participants, the investigators concluded that the majority of reported cases are best explained by subjective low-frequency tinnitus — an internally generated, unmeasurable percept that sufferers initially interpret as an external hum or vibration but find travels with them wherever they go.
- Remaining knowledge gaps about infrasound: Public concern has risen about technical sources of low-frequency noise (roughly 20–250 Hz) and infrasound (below 20 Hz) from wind turbines, traffic, and heat pumps. Modern audiology still lacks a comprehensive understanding of how the auditory system and brain process very low frequencies and infrasound. The researchers emphasize the need for deeper study of how sensory systems handle the bottom end of the acoustic spectrum.
Source: NTNU
Some people find the sound merely annoying; others experience real distress or sickness from the low-frequency vibration.
The hum is often difficult to hear outdoors but becomes prominent indoors and typically at night when background noises are reduced. People commonly check for nearby motors or machinery, yet find nothing visible. At the same time, others in the same space often hear nothing.
Origins and early reports: first noticed in coastal cities
The Hum first garnered widespread attention in Bristol, England in the mid-1970s, when many residents wrote to the local paper about an unexplained buzzing. Early speculation pointed to industrial sources such as warehouse fans, but the sound persisted even after suspected machinery was removed. Since then, reports have appeared across the UK — often in coastal towns — and later in other countries including the United States, Canada, Australia, New Zealand, South Africa and multiple European cities. The phenomenon is most often reported in relatively densely populated areas.
Individual accounts vary: some people experience the sound only in specific places, while others report it moving with them. The World Hum Map and Database Project, launched in 2012 by Canadian Glen MacPherson, collects many of these reports from around the world.
A range of possible causes has been proposed
Explanations have ranged from man-made low-frequency noise (ventilation systems, heat pumps, traffic, wind turbines) and natural sounds (wave action, wind) to various speculative theories. Because low-frequency sounds have long wavelengths and travel far, pinpointing a source can be difficult. The Hum has therefore attracted interest from hearing scientists and audiologists seeking to distinguish physical sources from internal perceptual phenomena.
What the researchers tested
The NTNU team focused on two testable hypotheses: (1) that listeners have unusually sensitive low-frequency hearing able to detect external sound waves that others miss, and (2) that listeners are hearing their own spontaneous otoacoustic emissions produced by the cochlea. To evaluate these ideas, the researchers performed high-resolution low-frequency threshold measurements and otoacoustic emission tests on 28 complainants and matched controls.
Results showed that most complainants did not have atypical low-frequency thresholds or measurable low-frequency otoacoustic emissions. While fine-grained variations in hearing sensitivity (microstructure) could allow a narrow band of heightened perception in a tiny minority, conventional testing did not support hypersensitivity or inner-ear emissions as the general cause.
A subjective, internal source explains most cases
The investigators conclude that subjective low-frequency tinnitus — an internally generated auditory percept without an external acoustic origin — is the most plausible explanation for the majority of reported Hum cases. Such tinnitus can be especially pronounced in quiet indoor settings at night when environmental masking is minimal, making internal phantom sounds appear louder and more intrusive.
Why this matters for public health and acoustics research
Understanding the Hum has practical implications: distinguishing measurable environmental noise from subjective tinnitus informs public health responses, environmental monitoring, and how communities address complaints. The researchers stress that fuller knowledge of how the entire auditory system processes low-frequency sound and infrasound is needed to assess both real external exposures and internally generated perceptions.
Frequently asked questions
A: Quiet indoor environments remove competing sounds that normally mask internal auditory percepts. If the perception is a form of low-frequency tinnitus, the lack of background noise makes internal phantom frequencies stand out, appearing loud, persistent, and distressing.
A: The cochlea emits very weak otoacoustic signals as a byproduct of its amplification processes. While most people do not notice these emissions, they can sometimes be detected objectively and, in rare cases, be perceived by the listener. In this study, however, low-frequency otoacoustic emissions were not found to explain The Hum.
A: True external sources can be recorded with microphones and remain localized to a physical area. If a percept cannot be objectively measured and follows an individual when they move to different environments, this behavior indicates an internal origin such as low-frequency tinnitus.
Editorial notes
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full.
- Additional context was added by editorial staff.
Research details and citation
Author: Nancy Bazilchuk (NTNU coverage)
Source: NTNU
Contact: Nancy Bazilchuk – NTNU
Image credit: Neuroscience News
Original research: Bonifaz Baumann, Andrej Voss, Carlos Jurado, and Markus Drexl. “On the potential sources of a low-frequency sound percept that only a few can perceive.” PLOS One. DOI: 10.1371/journal.pone.0326818 (closed access).
Abstract (summary)
A small subset of people report persistent low-frequency humming or pulsing percepts (LFSPs) that others nearby do not hear. This study tested two main hypotheses — heightened low-frequency sensitivity and perception of spontaneous otoacoustic emissions — using high-resolution threshold and emission measurements in 28 complainants and control subjects. Most complainants did not show unusually sensitive low-frequency hearing or measurable low-frequency otoacoustic emissions. While physical external sound sources are not entirely ruled out in some cases, subjective low-frequency tinnitus is suggested as the most frequent explanation for these percepts.