Multi-Band Acoustic Monitoring of Aerial Signatures

Brief

Mead et al. (2023) describe the acoustic monitoring, omni-directional system (AMOS) deployed in Phase 1 of the Galileo Project at Harvard-Smithsonian Center for Astrophysics. Three off-the-shelf instruments, an Infiltec INFRA20 (0.05–20 Hz), a GRAS 41AC-3 audible microphone (10 Hz–20 kHz), and a Wildlife Acoustics SMM-U2 ultrasonic sensor (16–190 kHz), are co-located on a single tower alongside an ADS-B flight transponder receiver. Theoretical and empirical analysis establishes detectability for 100 dB audible sources at 1 km range below 1 kHz in rural conditions, with infrasonic sources propagating substantially farther and ultrasonic sources attenuating within short distances. The authors document that no prior scientific attempt to study anomalous aerial phenomena has produced publicly available acoustic data, and that internet recordings of purported UAP sound lack the provenance and calibration metadata required for scientific analysis.

Metadata

Category

Hub & Overview

Venue

Journal of Astronomical Instrumentation

Type

Peer-reviewed

Year

2023

Authors

Andrew Mead, Sarah Little, Paul Sail, Michelle Tu, Wesley Andres Watters, Abigail White, Richard Cloete

DOI

10.1142/S2251171723400056

arXiv

2305.18551

Access

Open access

Length

8.9 M

Programs

Galileo Project

Instruments

Infiltec INFRA20 infrasound monitor, GRAS 41AC-3 audible microphone, Wildlife Acoustics SMM-U2 ultrasonic microphone, Wildlife Acoustics SM4BAT ultrasonic recorder, Adafruit ADS-B SDR receiver

Data sources

ADS-B flight transponder data (FAA), IRIS infrasound network, U.S. National Transportation Noise Map (USDOT)

Tags

UAP-physics, acoustic-monitoring, instrumentation, technosignature, multi-modal-sensing

Key points

• AMOS spans 0.05 Hz to 190 kHz using three subsystems with overlapping bandwidths: InfraS (0.05–20 Hz), AudS (10 Hz–20 kHz), and UltraS (16–190 kHz).p.1
• Preliminary analysis finds that 100 dB audible sources at 1 km are detectable, particularly for frequencies below 1 kHz in quiet rural environments; infrasonic sources propagate over much greater distances while ultrasonic energy attenuates rapidly.p.1
• Geometric spreading causes a 6 dB SPL decrease per doubling of distance from a point source, independent of frequency; atmospheric absorption adds a frequency-dependent loss that increases steeply above 1 kHz.p.2
• The UltraS subsystem generates 88.40 GB of 16-bit mono WAV data per 24-hour period at 512 kHz sample rate, against 11.43 GB/day for AudS and just 10 MB/day for InfraS.p.6
• An ADS-B SDR receiver logs flight transponder data, aircraft ID, location, altitude, and ground speed, once per second for all aircraft in range, enabling acoustic signature labeling of known aircraft and ongoing calibration of transmission path variability.p.6
• No prior scientific field study of anomalous aerial phenomena produced publicly available acoustic data; the Yakima Valley and Piedmont studies had ultrasonic microphones installed but presented no recorded acoustic data in any of their reports.p.4
• Historical UAP witness reports are acoustically ambiguous: most describe no sound, while a smaller subset report whistling, hissing, rumbling, buzzing, humming, beeping, or pulsing; AMOS's relatively flat audible response from 3.5 Hz to 20 kHz covers frequencies outside peak human hearing sensitivity.p.3
• Military aircraft within one mile of a lead aircraft are not required to transmit ADS-B signals, creating a systematic gap in transponder-based acoustic ground-truth for certain targets.p.6

Verbatim

• “Even in cases where no discernible sound is reported, it cannot be assumed that the object was not producing sound.”

  p.3

Most interesting

• The three AMOS subsystems generate data at radically asymmetric rates: InfraS produces 10 MB/day, AudS 11.43 GB/day, and UltraS 88.40 GB/day, roughly an 8,800:1 ratio between ultrasonic and infrasonic daily storage demands.
• The trident bat (Cleotis percivali) produces the highest known pure animal tone at 212 kHz, above AMOS's 190 kHz UltraS ceiling, meaning one natural biological source already exceeds the system's upper detection limit.
• A seven-day infrasonic study at Hessdalen in 2011 was compromised by daytime wind noise and produced zero nighttime infrasonic detections correlated with six confirmed optical events, illustrating the difficulty of multi-modal correlation even at a well-instrumented site.
• The GRAS 41AC-3 audible microphone was originally designed for permanent in-field monitoring of overhead aircraft noise near airports, a commercial aviation tool repurposed wholesale for aerial phenomena research.
• Historical UAP witness reports sometimes note animal reactions, including cessation of ambient animal noise, which the authors treat as indirect evidence of infrasonic or ultrasonic emissions, a key motivation for AMOS's full-spectrum coverage.
• The AMOS ultrasonic recorder uses GPS rather than NTP for timekeeping because it is a closed proprietary system, creating a multi-clock synchronization problem that the authors plan to resolve with an on-site transient tone generator until a unified clock solution is implemented.