# Ultrasonic Content Scanner Scan the inaudible band of your recordings for hidden high-frequency tones: tracking beacons, flyback whine, aliasing images - with frequencies, durations and classifications. > Canonical page: https://elysiatools.com/en/tools/audio-ultrasonic-content-scanner - **Category:** Media - **Keywords:** ultrasonic, inaudible tones, beacon detection, cross-device tracking, silverpush, 18 khz, 20 khz, flyback whine, 15734 hz, aliasing images, nyquist, privacy scan, stft ## Overview Just because you cannot hear it does not mean it is not there. This tool scans the ultrasonic band - 15 kHz up to the Nyquist limit of your file - for narrowband high-frequency content that survives inside ordinary recordings, using a short-time Fourier sweep with per-bin prominence detection (a tone counts only when it stands clearly above its local spectral neighborhood, so wideband hiss does not false-positive). What it looks for, with the frequencies the literature actually names: ultrasonic TRACKING BEACONS in the 18-20 kHz band - the cross-device tracking channel documented by Arp et al. (EuroS&PW 2017), where TV commercials and in-store audio embedded inaudible identifiers picked up by phone apps carrying the SilverPush, Lisnr and Shopkick SDKs (Google Play removed such apps in 2017) - a steady tone here in a broadcast capture is worth knowing about; the classic 15.734 kHz NTSC flyback-line whine (and the 15.625 kHz PAL sibling) that CRT-era and badly-shielded gear leaves on recordings; ALIASING IMAGES hugging the Nyquist limit, the fingerprint of a careless sample-rate conversion; and short transient HF pings. Each detected tone is reported with its frequency (10 Hz resolution), start and end time, prominence in dB, duty share and a classification. The privacy angle is real: if your upload contains 18-20 kHz identifiers, that recording can link devices and locations across time. The tool needs a source sampled at 32 kHz or higher to have an ultrasonic band at all; 48 kHz sources cover the full beacon range, and heavily lossy encodes (which low-pass around 16-20 kHz) may have already erased the evidence - a negative result on an MP3 is not proof of a clean original. ## Inputs - **Audio recording** (file) - **Detection sensitivity** (select) ## When to use - Auditing television, radio, or podcast recordings for hidden ultrasonic tracking beacons embedded between 18 kHz and 20 kHz. - Checking digitized archival audio or video soundtrack captures for CRT flyback interference at 15.734 kHz (NTSC) or 15.625 kHz (PAL). - Verifying that audio sample-rate conversions or processing chains have not introduced aliasing images near the Nyquist frequency. ## How it works - You supply an audio file sampled at 32 kHz or higher and select your detection sensitivity threshold (15 dB, 20 dB, or 25 dB prominence). - The scanner computes a short-time Fourier transform (STFT) across the ultrasonic band between 15 kHz and the file's Nyquist frequency. - Prominence algorithms evaluate whether narrowband spectral spikes stand significantly above the local noise floor at a 10 Hz resolution. - Detected signals are classified as tracking beacons, flyback whine, aliasing images, or transient pings, alongside timestamps, duration, and prominence metrics. ## Use cases - Privacy researchers auditing retail or broadcast audio captures to determine if cross-device tracking SDK beacons are present. - Audio mastering engineers checking master tracks for digital foldover and aliasing caused by faulty downsampling plugins. - Archival audio restoration specialists cataloging electrical interference and monitor whine in historical microphone recordings. ## Frequently asked questions ### Why does my audio file need a sample rate of at least 32 kHz? By Nyquist theorem, an audio file can only capture frequencies up to half its sample rate. A 32 kHz file caps at 16 kHz, while 48 kHz or higher is required to inspect the 18–20 kHz tracking beacon band. ### Can I detect tracking beacons in compressed MP3 files? Lossy compression encoders often apply low-pass filters that cut out audio above 16 kHz to 20 kHz, which may discard ultrasonic tones before the scanner can analyze them. ### How does the tool prevent background hiss from triggering false positives? It measures per-bin spectral prominence against surrounding bins, ignoring broad noise floors and only flagging sharp, isolated narrowband tones. ### What is the difference between standard, relaxed, and strict sensitivity? Sensitivity controls the prominence threshold required to flag a tone: Relaxed requires 15 dB above the floor, Standard requires 20 dB, and Strict requires 25 dB. ### What does a 15.734 kHz detection signify? A persistent frequency near 15.734 kHz is typically horizontal flyback coil radiation from legacy NTSC cathode-ray tube (CRT) equipment picked up by unshielded audio gear. ## Related tools - [Audio Digital Release Packager](https://elysiatools.com/en/tools/audio-digital-release-packager): Package an album for digital distribution: distributor-profile compliance checks, cover-art validation, MP3 previews, FLAC archive copies and a delivery ZIP with a README and machine-readable manifest. - [Ground Loop Diagnoser](https://elysiatools.com/en/tools/audio-ground-loop-diagnoser): Diagnose electrical hum contamination in recordings: 50/60 Hz mains family with harmonic profile, ground-loop vs PSU-buzz classification, and a recommended repair chain. - [Podcast Chapter Marker Builder](https://elysiatools.com/en/tools/podcast-chapter-marker-builder): Build every podcast chapter format from one timecoded list: Podcasting 2.0 JSON + RSS tag, ID3v2.4 CHAP+CTOC burned into an MP3, Vorbis comments, mp4chaps, YouTube timestamps and SRT, with a per-player support matrix. - [Forensic Speech Enhancer](https://elysiatools.com/en/tools/audio-forensic-enhancer): Speech-focused cleanup for noisy evidence and interview recordings: documented FFmpeg filter chains with presets for evidence clarity, broadcast, telephone band and transcription prep. - [Audio Kick-Bass Phase Aligner](https://elysiatools.com/en/tools/audio-kick-bass-phase-aligner): Compare kick and bass timing/phase in the low band, report the offset and polarity, and export the corrected track with sample-accurate alignment. - [Bulk Images Resize for Instagram](https://elysiatools.com/en/tools/bulk-images-resize-for-instagram): Resize multiple images for Instagram with preset dimensions for different content types (posts, stories, reels, profile photos) - [Drum Replacer](https://elysiatools.com/en/tools/drum-replacer): Detect kick, snare, and hi-hat hits in a drum track or full mix, then layer or replace them with your own samples (or built-in synthesized ones). - [Sub-Bass Mono Maker](https://elysiatools.com/en/tools/audio-sub-bass-mono-maker): Fold everything below your crossover to mono while keeping the stereo image above it — the elliptical-EQ move that protects vinyl cuts and mono-sub club systems. ## Samples - [Copyright-Free FLAC Audio Samples](https://elysiatools.com/en/samples/flac-samples): Lossless FLAC audio samples for testing and development, mirrored from MP3 set with nature sounds and meditation music - [Copyright-Free MP3 Audio Samples](https://elysiatools.com/en/samples/mp3-samples): Collection of royalty-free audio samples for testing and development purposes including nature sounds, meditation music, and ambient audio - [Copyright-Free WAV Audio Samples](https://elysiatools.com/en/samples/wav-samples): Uncompressed PCM WAV audio samples for testing and development, mirrored from MP3 set with nature sounds and meditation music - [WebRTC Real-Time Communication Samples](https://elysiatools.com/en/samples/webrtc-samples): Comprehensive WebRTC samples for peer-to-peer audio/video communication, data channels, screen sharing, and signaling server implementation