Within Sensors
When a Silent UFO Is Not Silent
Some events seem silent because the strongest signal may fall below or above normal hearing rather than inside ordinary audio.
On this page
- What audible microphones miss
- Why infrasound and ultrasound add context
- How multi band data changes witness claims
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Introduction
Many UFO reports describe an object that appeared close enough to see clearly yet seemed completely silent. In older accounts, silence was often treated as a mystery in itself. Modern acoustic monitoring has changed that assumption. A silent sighting is no longer automatically interpreted as an absence of sound. Instead, investigators ask whether the strongest acoustic energy may have existed outside normal human hearing, either below the audible range as infrasound or above it as ultrasound. Multi-band sensor systems were designed specifically to test that possibility. [World Scientific]worldscientific.comSection 2 presents the technical specifications of the AMOS…Read more…
This shift matters because witness testimony alone cannot determine whether an event generated low-frequency pressure waves, ultrasonic emissions, or only conventional sound. A person may hear nothing while nearby instruments record substantial acoustic activity. In UFO investigations, that distinction changes the question from “Was it silent?” to “What frequencies were actually present?”
What Audible Microphones Miss
Human hearing covers only a narrow slice of the acoustic spectrum, generally from about 20 hertz to 20 kilohertz. Many natural and technological processes produce energy outside that range. Traditional audio recordings therefore capture only part of the picture. [CTBTO]ctbto.orgCTBTOInfrasound monitoringAcoustic waves with very low frequencies are called infrasound…. infrasound monitoring is a useful technique…
A witness can accurately report silence while still standing inside an active acoustic environment. Several factors contribute to this:
- Infrasound below 20 hertz can create sensations of pressure, vibration or unease without producing a recognisable audible sound.
- Ultrasound above 20 kilohertz may be generated by mechanical systems, electronics or aerodynamic effects while remaining inaudible to most people.
- Distance effects can alter how frequencies propagate, allowing some components of a signal to travel much farther than others.
- Atmospheric filtering can suppress parts of a sound spectrum before it reaches an observer. [CTBTO]ctbto.orgOpen source on ctbto.org.
This creates a recurring problem in UFO reports. Witnesses often describe a visible object accompanied by unusual bodily sensations, faint vibration, or a feeling of pressure despite reporting little or no conventional noise. Such reports do not prove an unusual source, but they illustrate why audible-range microphones alone are an incomplete test.
Why Infrasound Changes the Meaning of “Silent”
Infrasound has become one of the most important tools for analysing apparently silent aerial events because low-frequency waves can travel extraordinary distances with relatively little loss. The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) operates a global infrasound monitoring network originally built to detect atmospheric nuclear explosions. The same sensors routinely detect meteors, volcanic eruptions, rocket launches and other large atmospheric disturbances. [CTBTO]ctbto.orgCTBTOCTBTO Infrasound Stations Detect Russian Meteorite BlastPeople cannot hear the low frequency waves emitted by the explosion but they…
The 2013 Chelyabinsk meteor provided a striking example. Many people observed the bright fireball visually before hearing major sound effects, while infrasonic waves from the explosion were recorded across continents by the CTBTO network. The signals travelled thousands of kilometres and allowed researchers to estimate characteristics of the event that ordinary witnesses could not perceive directly. CTBTO [ScienceDirect For UFO investigations]sciencedirect.comThe farthest station recorded signals was 15,000 km away in Antarctica. The origin time…Read more…, the lesson is methodological rather than sensational. A report that describes a silent luminous object does not automatically imply an acoustically silent event. If the object produced low-frequency atmospheric disturbances, conventional hearing might reveal little while dedicated sensors recorded a measurable signature.
Distance Can Mislead Witnesses
Sound does not reach every observer in the same way. Wind, temperature layers and atmospheric structure can reshape acoustic signals during propagation. Recent infrasound studies have shown that atmospheric conditions can selectively filter frequency content, meaning observers in different locations may experience the same event differently. [arXiv]arxiv.orgarXivMulti-Band Acoustic Monitoring of Aerial Signaturesby A Mead · 2023 · Cited by 10 — It uses three distinct systems with overlapping…
As a result:
- One witness may hear a sharp boom.
- Another may perceive only vibration.
- A third may hear nothing at all.
- Sensors may still detect a coherent infrasonic signal.
That variability complicates attempts to treat witness silence as decisive evidence either for or against an unusual aerial event.
Why Ultrasound Adds Another Layer
Ultrasound receives less attention in public UFO discussions, but multi-band monitoring systems increasingly include ultrasonic microphones. The rationale is straightforward: some mechanical and aerodynamic processes produce high-frequency emissions that are difficult or impossible for humans to hear. [arXiv]arxiv.orgarXivMulti-Band Acoustic Monitoring of Aerial Signaturesby A Mead · 2023 · Cited by 10 — It uses three distinct systems with overlapping…
Modern drone research illustrates the value of this approach. Acoustic detection systems identify unmanned aircraft by analysing characteristic sound signatures produced by motors and propellers. Some diagnostic information appears in frequency regions that are less obvious to casual listeners yet remain measurable with specialised equipment. [OSL Technology]osltechnology.comOSL Technology How Acoustic Detection of Drones EnhancesOSL TechnologyHow Acoustic Detection of Drones Enhances…March 19, 2025 — 11 May 2023 — Acoustic sensors use precision audio capture te… MicrodB This does not mean UFO reports are secretly ultrasonic. Rather [microdb.fr]microdb.frUAV Detection from Acoustic Signature: Requirements and…This paper reviews the state of the art in this field and presents possible te…, it demonstrates that aerial objects can generate useful acoustic information outside ordinary hearing. If investigators limit themselves to audible microphones, they may discard potentially important data before analysis even begins.
How Multi-Band Systems Test Witness Claims
The strongest argument for multi-band monitoring is not that it finds exotic sounds. It is that it creates a way to check claims against measurements.
The Galileo Project’s Acoustic Monitoring Omni-directional System (AMOS) was designed around this principle. Rather than recording only conventional audio, it simultaneously monitors infrasonic, audible and ultrasonic frequencies. The system covers roughly 0.05 hertz to 190 kilohertz through overlapping sensor bands. [Galileo]galileo.hsites.harvard.eduGalileoMulti-Band Acoustic Monitoring of Aerial SignaturesThe AMOS system spans infrasonic frequencies down to 0.05Hz, all of audible, an…
When an aerial event is observed, investigators can compare:
- Camera footage.
- Acoustic recordings.
- Environmental conditions.
- Timing information.
- Other sensor outputs from the same moment. [ResearchGate]researchgate.netResearch Gate(PDF) The Scientific Investigation of Unidentified AerialResearchGate(PDF) The Scientific Investigation of Unidentified Aerial…May 29, 2023 — (Abridged) The Galileo Project aims to investigat…
A witness might report:
The object was directly overhead and completely silent.
Multi-band data can then test several possibilities:
- No acoustic signal was detected.
- Only normal aircraft frequencies were detected.
- An infrasonic disturbance was detected.
- An ultrasonic component was detected.
- The event matched a known acoustic profile such as an aircraft, drone or atmospheric phenomenon.
The value lies in discrimination rather than confirmation. Multi-band sensors help separate genuinely unusual observations from ordinary events that only appeared unusual because part of their acoustic signature fell outside human perception.
The Problem of False Mysteries
One recurring risk in UFO analysis is treating missing perception as evidence of extraordinary technology. Multi-band monitoring often pushes in the opposite direction.
A seemingly silent object may turn out to be:
- A distant aircraft whose higher frequencies dissipated before reaching observers.
- A meteor generating powerful infrasound but limited audible sound at ground level.
- A drone whose acoustic signature was partially masked by environmental noise.
- An atmospheric event that produced measurable pressure waves outside the hearing range. [ScienceDirect]sciencedirect.comThe farthest station recorded signals was 15,000 km away in Antarctica. The origin time…Read more… [2EurekAlert!]eurekalert.orgnews releasesNothing escapes The Global Ear: Nuclear tests, volcanoes…Jun 17, 2015 — Twenty of the CTBTO's infrasound monitoring stations detected…
This is why acoustic researchers emphasise classification and signature libraries. The goal is not merely to detect sound but to compare detected patterns against known phenomena. AMOS researchers explicitly describe building datasets of aircraft and other aerial signatures so that future detections can be assessed against established acoustic profiles. [arXiv]arxiv.orgarXivMulti-Band Acoustic Monitoring of Aerial Signaturesby A Mead · 2023 · Cited by 10 — It uses three distinct systems with overlapping…
In practice, many apparently mysterious cases become less mysterious when multiple sensor bands are available.
When Silence Remains Interesting
Multi-band systems do not eliminate unexplained reports. They simply narrow the range of possibilities.
An event becomes more noteworthy when several conditions occur simultaneously:
- Visual observation is strong.
- Environmental conditions are well documented.
- Multi-band acoustic sensors record little or nothing.
- Other known explanations fit poorly.
Even then, silence alone is not evidence of an unconventional craft. It is only one data point among many. Acoustic propagation is complex, and a lack of recorded sound can result from distance, weather, terrain, instrument placement or simple signal weakness. [arXiv]arxiv.orgarXivMulti-Band Acoustic Monitoring of Aerial Signaturesby A Mead · 2023 · Cited by 10 — It uses three distinct systems with overlapping…
The most important contribution of multi-band monitoring is therefore methodological. It transforms silence from a subjective witness impression into a measurable question. Instead of asking whether people heard something unusual, investigators can ask which frequencies were present, which were absent, and whether the resulting pattern matches known aerial phenomena.
How Multi-Band Data Changes Witness Testimony
Before modern sensor networks, witness descriptions of UFO noise existed largely as isolated narratives. One person reported humming, another reported vibration, and a third reported complete silence. There was often no independent way to evaluate those differences.
Multi-band monitoring changes the role of testimony. Witness accounts remain important because they identify when and where an event occurred, but acoustic instruments provide an external record that can support, refine or contradict perception. [World Scientific]worldscientific.comSection 2 presents the technical specifications of the AMOS…Read more…
That shift has a subtle but important consequence for UFO research. The question is no longer whether a witness was correct about hearing nothing. The witness may have been entirely correct. The more useful question is whether the event generated acoustic energy beyond the limits of human hearing.
Modern infrasound and ultrasonic monitoring were developed precisely because nature repeatedly produces significant signals that people cannot hear. Applying those same tools to unusual aerial reports does not prove extraordinary explanations. It provides a way to determine whether a supposedly silent event was truly silent, or merely silent to human ears. CTBTO [Galileo]galileo.hsites.harvard.eduGalileoMulti-Band Acoustic Monitoring of Aerial SignaturesThe AMOS system spans infrasonic frequencies down to 0.05Hz, all of audible, an…
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Further Reading
Books and field guides related to When a Silent UFO Is Not Silent. Use these as the next step if you want deeper reading beyond the article.
Infrasound Monitoring for Atmospheric Studies
Directly addresses detection beyond normal auditory perception.
Fundamentals of Acoustics
Explains frequency ranges and sound physics relevant to 'silent' events.
The Believing Brain
Provides context on perception and interpretation of unusual experiences.
Introduction to Sound
Helps explain why people may miss significant acoustic activity.
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Endnotes
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Link: https://www.ctbto.org/our-work/monitoring-technologies/infrasound-monitoringSource snippet
CTBTOInfrasound monitoringAcoustic waves with very low frequencies are called infrasound.... infrasound monitoring is a useful technique...
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Source: arxiv.org
Link: https://arxiv.org/pdf/2305.18551Source snippet
arXivMulti-Band Acoustic Monitoring of Aerial Signaturesby A Mead · 2023 · Cited by 10 — It uses three distinct systems with overlapping...
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Source: arxiv.org
Link: https://arxiv.org/abs/2602.00421 -
Source: ctbto.org
Link: https://www.ctbto.org/our-work/international-monitoring-system -
Source: ctbto.org
Link: https://www.ctbto.org/news-and-events/news/ctbto-infrasound-stations-detect-russian-meteorite-blastSource snippet
CTBTOCTBTO Infrasound Stations Detect Russian Meteorite BlastPeople cannot hear the low frequency waves emitted by the explosion but they...
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Source: sciencedirect.com
Link: https://www.sciencedirect.com/science/article/pii/S209099771630075XSource snippet
The farthest station recorded signals was 15,000 km away in Antarctica. The origin time...Read more...
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Source: eurekalert.org
Title: news releases
Link: https://www.eurekalert.org/news-releases/914564Source snippet
Nothing escapes The Global Ear: Nuclear tests, volcanoes...Jun 17, 2015 — Twenty of the CTBTO's infrasound monitoring stations detected...
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Source: microdb.fr
Link: https://microdb.fr/en/scientific-publications/uav-detection-from-acoustic-signature-requirements-and-state-of-the-art/Source snippet
UAV Detection from Acoustic Signature: Requirements and...This paper reviews the state of the art in this field and presents possible te...
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Source: researchgate.net
Title: Research Gate(PDF) The Scientific Investigation of Unidentified Aerial
Link: https://www.researchgate.net/publication/371163445_The_Scientific_Investigation_of_Unidentified_Aerial_Phenomena_UAP_Using_Multimodal_Ground-Based_ObservatoriesSource snippet
ResearchGate(PDF) The Scientific Investigation of Unidentified Aerial...May 29, 2023 — (Abridged) The Galileo Project aims to investigat...
Published: May 29, 2023
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Source: arxiv.org
Title: arXiv Multi-Band Acoustic Monitoring of Aerial Signatures
Link: https://arxiv.org/abs/2305.18551Source snippet
arXivMulti-Band Acoustic Monitoring of Aerial SignaturesMay 29, 2023...
Published: May 29, 2023
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Source: ctbto.org
Link: https://www.ctbto.org/our-work/civil-and-scientific-applicationsSource snippet
Civil and Scientific ApplicationsIMS infrasound stations can detect the ultra-low frequency sound waves emitted by volcanic eruptions and...
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Technology Workshop 2024 (ITW2024)Rocket launches are a source of infrasound detectable at infrasound [arrays]({{ 'arrays/' | relative_url }}) in thousands of kilometers d...
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Source: ctbto.org
Title: Listening without sound
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CTBTO engineers refine a...17 Jun 2010 — The Trafelberg facility is a proving ground for infrasound, which is able to detect sounds belo...
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Video: How To Detect A Secret Nuclear Test by Minute PhysicsA monitoring system that can detect pretty much any nuclear explosion that ta...
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AUDRON: A Deep Learning Framework with Fused...by R Chatterjee · 2025 · Cited by 1 — This study introduces AUDRON (AUdio-based Drone Rec...
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Initial results from the first field expedition of UAPx to study...23 Jan 2026 — The acoustic monitoring, omni-directional system (AMOS)...
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Helicopter Location Using Infrasound ArraysThe acoustic monitoring, omni-directional system (AMOS) in the Galileo Project is a passive, m...
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Link: [https://www.worldscientific.com/doi/10.1142/S2251171723400056?srsltid=AfmBOooaTOWOmv5l9CIm70-RTcMtA6iDdH6y2Ytpx797i1areA2iiAW](https://www.worldscientific.com/doi/10.1142/S2251171723400056?srsltid=AfmBOooaTOWOmv5l9CIm70-RTcMtA6iDdH6y2Ytpx797i1areA2iiAW)Source snippet
Section 2 presents the technical specifications of the AMOS...Read more...
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GalileoMulti-Band Acoustic Monitoring of Aerial SignaturesThe AMOS system spans infrasonic frequencies down to 0.05Hz, all of audible, an...
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Title: OSL Technology How Acoustic Detection of Drones Enhances
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OSL TechnologyHow Acoustic Detection of Drones Enhances...March 19, 2025 — 11 May 2023 — Acoustic sensors use precision audio capture te...
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Link: https://galileo.hsites.harvard.edu/publications/scientific-investigation-unidentified-aerial-phenomena-uap-using-multimodalSource snippet
Scientific Investigation of Unidentified Aerial Phenomena...A primary objective of the Galileo Project is to build an integrated softwar...
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Title: ctbtos monitoring capabilities continue improve
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CTBTO's Monitoring Capabilities Continue to ImproveThe IMS uses a combination of seismic, hydroacoustic, infrasound, and radionuclide sta...
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Additional References
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Link: https://drone-warfare.com/counter-uas/acoustic-detection/Source snippet
Counter-UAS 101 – Acoustic Drone DetectionAcoustic drone detection exploits propeller noise to identify threats passively. Ukraine's 1400...
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Link: https://www.semanticscholar.org/paper/10b6cce675d167ed15ed3c0485cf369b44b2c332Source snippet
[PDF] Multi-Band Acoustic Monitoring of Aerial SignaturesThe Galileo Project's acoustic monitoring, omni-directional system (AMOS) aids i...
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Source: ldeo.columbia.edu
Link: https://www.ldeo.columbia.edu/~richards/my_papers/CISAC_CTBT_Monitoring.pdfSource snippet
columbia.edu2 CTBT Monitoring CapabilityThe IMS system will establish a global array of infrasound sensors to enable routine monitoring o...
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Link: https://aiforgood.itu.int/about-us/un-ai-actions/ctbto/Source snippet
Nuclear-Test-Ban Treaty Organization (CTBTO)The CTBTO works to ban nuclear tests globally with a comprehensive [verification]({{ 'verification/' | relative_url }}) system, inclu...
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Source: thedebrief.org
Link: https://thedebrief.org/did-microphones-recently-capture-the-mystery-sound-of-unidentified-anomalous-phenomena/Source snippet
Did Microphones Recently Capture the 'Mystery Sound' of...10 Nov 2023 — The acoustic monitoring, omni-directional system (AMOS) of the G...
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Link: https://pubs.aip.org/aip/adv/article/15/12/120701/3373725/From-classical-approaches-to-recent-advancements-ASource snippet
holistic review of acoustic detection for unmanned aerial...2 Dec 2025 — UAV acoustic signal datasets typically encompass sound signatur...
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Source: phys.org
Title: 2013 02 russian fireball largest ctbto infrasound
Link: https://phys.org/news/2013-02-russian-fireball-largest-ctbto-infrasound.htmlSource snippet
Russian fireball largest ever detected by CTBTO's...19 Feb 2013 — Infrasound has been used as part of the CTBTO's tools to detect atomic...
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Source: dsiac.dtic.mil
Title: DSIACWhat is an acoustic drone detection system?
Link: https://dsiac.dtic.mil/primers/what-is-an-acoustic-drone-detection-system/Source snippet
DSIAC - dtic.mil8 Sept 2025 — An acoustic drone detection system uses the sound of a drone's motors and blades to determine the type of d...
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Source: space.com
Title: 19860 russia meteor explosion largest detected
Link: https://www.space.com/19860-russia-meteor-explosion-largest-detected.htmlSource snippet
Russia Meteor Blast Largest Seen by Nuclear Detection Grid19 Feb 2013 — There are currently 45 infrasound stations in the CTBTO's network...
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Link: https://www.nationalacademies.org/read/10471/chapter/5Source snippet
y detecting the presence of a characteristic bubble pulse...
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