Prerequisites
Tutorial 2: Chronolocation video: establishing when footage was filmed.
01
Ambient sound is a locating signal, not background noise
Every audio recording carries an acoustic fingerprint. Power-line hum, urban noise profiles, weather patterns, wildlife calls and identifiable infrastructure sounds can place a recording in a region, a season and sometimes a specific environment. The analyst who treats audio as a secondary channel misses evidence the image alone cannot supply.
Audio evidence appears routinely in conflict reporting, atrocity documentation and criminal investigation: voice recordings, intercepted calls, video soundtracks. Investigators have used sound to contradict official timelines, identify execution sites and authenticate or undermine claimed provenance. The technique requires no specialist hardware. A waveform editor, reference recordings and a structured comparison workflow are sufficient for preliminary analysis.
This tutorial covers three analysis streams. Electrical network frequency (ENF) analysis extracts power-grid hum as a temporal locating signal. Acoustic environment profiling compares ambient sound signatures against reference databases. Event-sound identification isolates discrete sounds, such as weapons fire, vehicle types or infrastructure noise, to narrow geographic and contextual range. Each stream produces a probabilistic finding, not a certainty. Used together, they produce a defensible evidential statement.
In the field
In September 2018, BBC Africa Eye published "Anatomy of a Killing," an investigation into the filmed execution of civilians in Cameroon. The investigation combined visual geolocation with audio analysis to corroborate the sequence of events and authenticate the footage.
- Weapon sound analysis. Investigators matched the acoustic profile of weapon reports in the recording to the specific firearms visible in the footage, confirming the audio and video derived from the same event.
- Continuity verification. Spectrogram analysis confirmed no discontinuities in the audio track, supporting the finding that the footage had not been assembled from separate clips.
- Environmental corroboration. Ambient sound was consistent with the claimed outdoor rural environment, providing a secondary check on the footage's claimed context.
BBC Africa Eye · Anatomy of a Killing · 24 September 2018
Learning outcomes
By the end of this tutorial you will be able to:
Extract and interpret ENF signatures from audio recordings to establish or challenge a claimed timestamp
Profile an ambient sound environment against reference recordings to narrow geographic range
Identify discrete event sounds and match them to weapon types, vehicle classes or infrastructure categories
Assess the limitations and false-positive risks of each audio analysis stream
Document an audio analysis chain of custody to evidentiary standard
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