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Within The Hum

How Kokomo Found the Machines Behind Its Hum

Kokomo showed how detailed measurements and equipment changes can turn an apparently unexplained hum into a solvable industrial noise problem.

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On this page

  • The complaints that triggered the investigation
  • Cooling towers and compressors identified as contributors
  • What changed after industrial modifications
Preview for How Kokomo Found the Machines Behind Its Hum

Introduction

The Kokomo Hum is one of the few well-known “Hum” cases in which engineers identified specific industrial noise sources through systematic measurement rather than leaving the phenomenon entirely unexplained. Because mysterious low-frequency sounds are sometimes linked to UFOs, secret technology or other extraordinary ideas, the Kokomo investigation is especially significant: it demonstrated how detailed acoustic fieldwork can separate measurable environmental noise from broader claims that are much harder to verify. While the investigation did not resolve every resident’s experience, it showed that at least part of the reported Hum originated from identifiable industrial equipment, providing one of the strongest examples of an apparently mysterious Hum yielding to engineering analysis.[ICBEN]icben.orgMicrosoft WordMicrosoft Word - Cowan.docJune 16, 2008…Published: June 16, 2008

Kokomo Case illustration 1

The complaints that triggered the investigation

By the early 2000s, residents across Kokomo, Indiana, were reporting a persistent low-frequency humming or droning sound. Many described it as resembling a distant diesel engine that was most noticeable at night or indoors. Some residents also associated the Hum with headaches, disturbed sleep and difficulty concentrating, while others living nearby reported hearing nothing at all. These contrasting experiences made the problem particularly difficult to investigate.[OUCI]ouci.dntb.gov.uaOUCIModern-Day Acoustical Scares: From ‘The Hum’ to ‘Wind Turbine Syndrome’OUCIModern-Day Acoustical Scares: From ‘The Hum’ to ‘Wind Turbine Syndrome’

Rather than dismissing the reports, the City of Kokomo commissioned an independent acoustical investigation. The consulting team began by interviewing affected residents, holding public meetings and documenting where complaints occurred throughout the city. According to the investigation, 126 residents were formally identified as Hum hearers, although investigators believed additional people chose not to participate publicly.[ICBEN]icben.orgMicrosoft WordMicrosoft Word - Cowan.docJune 16, 2008…Published: June 16, 2008

The study was deliberately designed to test multiple possible explanations instead of assuming a single cause. Engineers measured:

  • low-frequency airborne sound;
  • ground vibration;
  • electromagnetic fields, because some residents suspected electrical causes; and
  • the timing and locations of reported Hum events.

Ground vibration measurements proved especially important because some people believed machinery was transmitting vibrations through the earth. Measurements at twelve sites found vibration levels more than ten times below recognised human perception thresholds, allowing investigators to rule out ground-borne vibration as the primary explanation.[ICBEN]icben.orgMicrosoft WordMicrosoft Word - Cowan.docJune 16, 2008…Published: June 16, 2008

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Cooling towers and compressors identified as contributors

The breakthrough came from detailed low-frequency acoustic measurements rather than ordinary sound surveys.

Investigators detected two unusually strong tonal signals that repeatedly appeared where residents experienced the Hum:

  • a 10 Hz tone together with harmonics extending to about 60 Hz; and
  • a distinct 36 Hz tone.

These tones stood roughly 20–40 decibels above the surrounding low-frequency background, making them stand out clearly from ordinary environmental noise.[ICBEN]icben.orgMicrosoft WordMicrosoft Word - Cowan.docJune 16, 2008…Published: June 16, 2008

The next challenge was determining where these sounds originated. Conventional listening was not enough because low-frequency sound is difficult for humans to localise. Instead, engineers used a rotating dual-microphone array that compared the phase and intensity of incoming sound from different directions.

This directional analysis traced:

  • the 10 Hz tone to large industrial air compressors at the Haynes International facility; and
  • the 36 Hz tone to a cooling tower at the DaimlerChrysler casting plant.

Both sources generated low-frequency tonal noise that remained detectable over distances exceeding one kilometre under suitable conditions.[ICBEN]icben.orgMicrosoft WordMicrosoft Word - Cowan.docJune 16, 2008…Published: June 16, 2008

Importantly, the investigators did not stop after identifying the first plausible sources. Residents also pointed towards additional industrial sites, including tunnelling operations and ventilation equipment. These facilities were examined as well, but measurements showed they either produced different frequencies or were not operating during the periods when most Hum complaints occurred. That process of elimination strengthened confidence that the compressor and cooling tower were genuine contributors rather than coincidental nearby machinery.[ICBEN]icben.orgMicrosoft WordMicrosoft Word - Cowan.docJune 16, 2008…Published: June 16, 2008

Kokomo Case illustration 2

What changed after industrial modifications

After the acoustic sources had been identified, managers at both industrial facilities agreed to install noise-control measures and modify the equipment responsible for the identified tones.

According to the engineering report, the companies voluntarily implemented silencing and equipment changes intended to reduce emissions at the measured frequencies. The investigation became notable because it did not simply identify a probable source—it resulted in practical engineering interventions designed to test the hypothesis directly.[ICBEN]icben.orgMicrosoft WordMicrosoft Word - Cowan.docJune 16, 2008…Published: June 16, 2008

Reports after the modifications were mixed.

Many summaries of the Kokomo case note that residents reported noticeable improvements following the equipment changes, making Kokomo one of the strongest examples of an environmental Hum being substantially reduced through industrial mitigation.[ResearchGate]researchgate.netResearch Gate(PDF) Deployment of a short-term geophysical field survey to monitor acoustic signals associated with the Windsor HumResearch Gate(PDF) Deployment of a short-term geophysical field survey to monitor acoustic signals associated with the Windsor Hum

However, not every complaint disappeared. Some residents continued to report hearing the Hum, and later commentary argued that the industrial tones alone could not explain every reported experience. Acoustic consultant Jim Cowan himself noted that some reported sensations appeared to involve issues extending beyond the measurable industrial noise identified during the investigation. Subsequent writers have therefore cautioned against presenting Kokomo as a complete solution to every Hum report.[CiteSeerX]citeseerx.ist.psu.eduJournal of Scientific Exploration, Vol. 18, No. 4, pp. 571–595, 2004…

This distinction is important. The investigation successfully demonstrated that genuine, measurable industrial low-frequency noise existed and that it reached residential neighbourhoods. Whether every individual report stemmed from those sources remains a separate question.

The Kokomo Hum: An Acoustical Investigation

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Why the Kokomo case matters

Within discussions of the Hum phenomenon—and particularly in conversations where mysterious sounds become linked to UFOs or secret technologies—Kokomo serves as a valuable counterexample.

Instead of relying on speculation, investigators followed a methodical engineering process:

The Kokomo Hum Is Back

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  1. document complaints systematically;
  2. measure sound rather than relying solely on witness descriptions;
  3. test alternative explanations such as ground vibration;
  4. localise individual frequencies to specific equipment;
  5. modify the suspected machinery; and
  6. evaluate whether residents’ experiences changed afterwards.

That approach showed that at least some apparently mysterious Hum events can arise from ordinary industrial infrastructure whose low-frequency output is difficult to detect without specialised instruments. At the same time, Kokomo also illustrates that identifying measurable noise sources does not necessarily explain every reported perception, leaving room for multiple mechanisms—including environmental acoustics and individual hearing differences—rather than a single universal explanation.[icben.org]icben.orgMicrosoft WordMicrosoft Word - Cowan.docJune 16, 2008…Published: June 16, 2008

Kokomo Case illustration 3

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Endnotes

1. Source: icben.org
Title: Microsoft Word
Link:https://www.icben.org/2008/PDFs/Cowan.pdf

Source snippet

Microsoft Word - Cowan.docJune 16, 2008...

Published: June 16, 2008

2. Source: ouci.dntb.gov.ua
Title: OUCIModern-Day Acoustical Scares: From ‘The Hum’ to ‘Wind Turbine Syndrome’
Link:https://ouci.dntb.gov.ua/en/works/9JJXqQq9/

3. Source: researchgate.net
Link:https://www.researchgate.net/publication/325849648_Deployment_of_a_short-term_geophysical_field_survey_to_monitor_acoustic_signals_associated_with_the_Windsor_Hum

4. Source: researchgate.net
Title: (PDF) Still Humming
Link:https://www.researchgate.net/publication/243750243_Still_Humming

5. Source: researchgate.net
Title: (PDF) Do Unto Others: Extraordinary Acts of Ordinary People
Link:https://www.researchgate.net/publication/242421011_Do_Unto_Others_Extraordinary_Acts_of_Ordinary_People

6. Source: researchgate.net
Link:https://www.researchgate.net/publication/288737394Analysis_On_Cooling_Water_Circuit_Of_A_Compressor_House-_A_Case_Study

7. Source: citeseerx.ist.psu.edu
Link:https://citeseerx.ist.psu.edu/document?doi=e303828cdee01e41ed8ab4e2f2e165379d50a034&repid=rep1&type=pdf

Source snippet

Journal of Scientific Exploration, Vol. 18, No. 4, pp. 571–595, 2004...

8. Source: Wikipedia
Title: The Hum
Link:https://en.wikipedia.org/wiki/The_Hum

9. Source: ira.lib.polyu.edu.hk
Link:https://ira.lib.polyu.edu.hk/handle/10397/111552?mode=full

10. Source: doi.org
Link:https://doi.org/10.1115/1.2841232

11. Source: pure.psu.edu
Title: installation effects on fan acoustic and aerodynamic performance
Link:https://pure.psu.edu/en/publications/installation-effects-on-fan-acoustic-and-aerodynamic-performance/

12. Source: avesis.metu.edu.tr
Link:https://avesis.metu.edu.tr/yayin/28bb9a79-cb2d-4acf-aa8f-13d49990d1a5/local-air-quality-impacts-due-to-downwash-around-thermal-power-plants-numerical-simulations-of-the-effect-of-building-orientation

Additional References

13. Source: advanced-noise-solutions.co.uk
Link:https://www.advanced-noise-solutions.co.uk/case-studies/compressor-noise-reduction/

Source snippet

mpressor Noise Reduction | Industrial Compressor Noise Control | Suppress Low Frequency HumAugust 16, 2024 — Specialist noise solutions...

Published: August 16, 2024

14. Source: kokomoacrepair.org
Link:https://kokomoacrepair.org/

Source snippet

One of the most consequential decisions a Kokomo homeowner faces when an AC system fails is whether to repair the existing system or repl...

15. Source: youtube.com
Link:https://www.youtube.com/watch?v=VB1707GeXq8

Source snippet

The Controversial Sound Only 2% Of People Hear...

16. Source: youtube.com
Title: The HUM Sample (Kokomo Hum | [Taos Hum]({{ ‘taos-hum/’ | relative_url }}) | Bristol Hum)
Link:https://www.youtube.com/watch?v=f3k1Qwx9Y0Q

Source snippet

Still no explanation for the mysterious Windsor hum haunting Ontario city, southeastern Michigan...

17. Source: handwiki.org
Title: Physics:The Hum
Link:https://handwiki.org/wiki/Physics%3AThe_Hum

Source snippet

"↑ Harkawik, Garret (2019). "Doom Vibrations". [https://www.garretharkawik.com/doom-vibrations..."](https://www.garretharkawik.com/doom-vibrations...")...

2

  1. ↑ Cowan, J. P. (October 2003). “The Kok

    Published: October 2003

18. Source: onlinelibrary.wiley.com
Link:https://onlinelibrary.wiley.com/doi/10.1155/2022/5168153

Source snippet

Real‐Time Implementation of Air Audit System for Compressors towards Energy Conservation: An Industrial Case Study - Loganathan - 2022...

19. Source: eppcgs.org
Link:https://eppcgs.org/article/doi/10.26464/epp2018032?viewType=HTML

20. Source: scribd.com
Link:https://www.scribd.com/document/285367254/GPG-Mag-Lev-FullReport

21. Source: evolvewater.co.uk
Link:https://evolvewater.co.uk/project/case-study-magcat-ongc/

22. Source: esshambaugh.com
Link:https://esshambaugh.com/case-studies/school-city-of-mishawaka