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A suspected vishing call used an AI-cloned executive voice to authorize a wire transfer
Security operations received a voicemail that sounds like the CEO but the tone seems off
Incident response needs to determine whether a recorded phone call contains synthetic speech
Fraud investigation requires forensic proof that audio was AI-generated
Red team exercises use voice cloning and blue team needs detection capability
Do not use for text-based phishing (email/SMS); use email header analysis or URL detonation tools instead.
Detection Gaps & Validation
Liveness and voiceprint spoofing: speaker-verification/voiceprint systems can be fooled by high-quality clones, and a passing voiceprint match is not proof of authenticity. Never treat "the voice matches" as identity confirmation for high-risk actions like wire transfers.
Codec masking: phone-network compression (G.711, AMR, Opus over VoIP) attenuates the high-frequency artifacts the detector relies on. A clean-sounding low-bitrate call can hide vocoder banding — analyze at the highest available fidelity and account for the codec when scoring.
Spectrogram artifacts to look for: an energy cutoff at the vocoder ceiling (~7-8 kHz), periodic banding at fixed intervals in the mel spectrogram, abnormally low pitch jitter/shimmer, and missing formant transitions at consonant-vowel boundaries.
False negatives on strong clones: fine-tuned models (VALL-E, ElevenLabs) trained on 30+ minutes of target audio can evade basic MFCC/spectral features; short clips (<3s) produce unreliable statistics.
How to confirm: the authoritative check is out-of-band verification — call the purported speaker back on a known-good number, or use a pre-agreed challenge phrase/code word, before acting on the request. Corroborate the ML verdict with spectrogram review and, where available, a genuine reference corpus for the individual. A confidence score alone is not sufficient grounds to authorize a transaction.
Prerequisites
Python 3.9+ with librosa, numpy, scikit-learn, and scipy installed
Audio samples in WAV, MP3, or FLAC format (mono or stereo, any sample rate)
Reference corpus of known genuine voice samples for the targeted individual (optional but improves accuracy)
FFmpeg installed for audio format conversion (librosa dependency)
Minimum 3 seconds of audio for reliable feature extraction
Workflow
Step 1: Audio Preprocessing
Normalize and prepare audio samples for feature extraction:
import librosa
import numpy as np
# Load audio, resample to 16kHz mono
y, sr = librosa.load("suspect_call.wav", sr=16000, mono=True)
# Trim silence from beginning and end
y_trimmed, _ = librosa.effects.trim(y, top_db=25)
# Normalize amplitude to [-1, 1]
y_norm = y_trimmed / np.max(np.abs(y_trimmed))
Audio preprocessing ensures consistent feature extraction across different recording conditions, microphones, and codec artifacts.
Step 2: Extract Spectral Features
Extract the feature set that distinguishes real from synthetic speech:
MFCCs capture the spectral envelope of speech, representing how the vocal tract shapes sound. Deepfake audio often shows unnatural smoothness in higher-order MFCCs because neural vocoders approximate but do not perfectly replicate the acoustic resonance of a physical vocal tract.
Missing or attenuated formant transitions during consonant-vowel boundaries
Step 3: Build Feature Vector and Classify
Aggregate frame-level features into a fixed-length vector and classify:
from sklearn.ensemble import RandomForestClassifier, GradientBoostingClassifier
from sklearn.model_selection import cross_val_score
defbuild_feature_vector(y, sr):
features = []
mfccs = librosa.feature.mfcc(y=y, sr=sr, n_mfcc=20)
for coeff in mfccs:
features.extend([np.mean(coeff), np.std(coeff), np.min(coeff), np.max(coeff)])
for feat_fn in [librosa.feature.spectral_centroid,
librosa.feature.spectral_bandwidth,
librosa.feature.spectral_rolloff,
librosa.feature.zero_crossing_rate]:
feat = feat_fn(y=y, sr=sr) if feat_fn != librosa.feature.zero_crossing_rate else feat_fn(y)
features.extend([np.mean(feat), np.std(feat), np.min(feat), np.max(feat)])
contrast = librosa.feature.spectral_contrast(y=y, sr=sr)
for band in contrast:
features.extend([np.mean(band), np.std(band)])
return np.array(features)
Classification uses an ensemble approach: Random Forest for robustness and Gradient Boosting for accuracy, with a voting mechanism to reduce false positives.
Step 4: Temporal Artifact Analysis
Examine time-domain artifacts that neural vocoders leave behind:
Real human speech exhibits natural pitch jitter (micro-variations in fundamental frequency) and shimmer (amplitude perturbations). Deepfake audio generated by Tacotron 2, VALL-E, or ElevenLabs typically shows reduced jitter and shimmer compared to genuine speech.
Visual inspection reveals banding artifacts in mel spectrograms, unnatural energy cutoffs above the vocoder's frequency ceiling, and periodic noise patterns in the high-frequency range that are characteristic of neural speech synthesis.
Step 6: Generate Forensic Report
Compile findings into an actionable report:
DEEPFAKE AUDIO ANALYSIS REPORT
================================
File: suspect_executive_call.wav
Duration: 47.3 seconds
Sample Rate: 16000 Hz
Analysis Date: 2026-03-19
CLASSIFICATION RESULT
Verdict: LIKELY DEEPFAKE (confidence: 94.2%)
Ensemble Score: RF=0.91, GBT=0.97, Avg=0.94
FEATURE ANOMALIES DETECTED
- MFCC variance in coefficients 13-20: 62% below genuine baseline
- Spectral contrast (4-8 kHz): 0.23 (genuine avg: 0.41)
- Pitch jitter: 0.8 Hz (genuine avg: 2.4 Hz)
- Zero-crossing rate std: 0.003 (genuine avg: 0.011)
SPECTROGRAM ARTIFACTS
- Energy cutoff above 7.8 kHz (consistent with neural vocoder ceiling)
- Banding pattern at 50ms intervals in mel spectrogram
- Missing formant transitions at 12.4s, 23.1s, 35.7s timestamps
RECOMMENDATION
High confidence of AI-generated audio. Recommend out-of-band
verification with the purported speaker. Preserve original audio
file with chain of custody documentation for potential legal action.
Key Concepts
Term
Definition
MFCC
Mel-Frequency Cepstral Coefficients; representation of the short-term power spectrum on a mel (perceptual) frequency scale
Spectral Centroid
Weighted mean of frequencies present in the signal; indicates perceived brightness of a sound
Spectral Contrast
Difference in amplitude between peaks and valleys in the spectrum across frequency sub-bands
Vocoder
Signal processing component that synthesizes audio waveforms from acoustic features; used in TTS and voice cloning
Pitch Jitter
Cycle-to-cycle variation in fundamental frequency; natural in human speech, reduced in synthetic speech
Vishing
Voice phishing; social engineering attack conducted via phone calls, increasingly using AI-cloned voices
Formant
Resonant frequencies of the vocal tract that define vowel sounds; transitions between formants are difficult for AI to replicate perfectly
Tools & Systems
librosa: Python library for audio analysis providing MFCC, spectral feature extraction, and spectrogram generation
scikit-learn: Machine learning library used for Random Forest and Gradient Boosting classification
Resemblyzer: Speaker embedding library for comparing voice identity between known genuine and suspect samples
Speechbrain: Deep learning toolkit for speech processing with pretrained deepfake detection models
Praat: Phonetics software for detailed pitch, jitter, and shimmer analysis of speech samples
FFmpeg: Audio format conversion and preprocessing utility required by librosa
Common Scenarios
Scenario: Executive Impersonation Wire Transfer Fraud
Context: CFO receives a phone call appearing to be from the CEO requesting an urgent wire transfer of $2.3M. The call came from an unknown number but the voice sounded identical to the CEO. IT security was able to obtain a recording of the call from the phone system.
Approach:
Extract the audio from the phone system recording and convert to WAV at 16kHz
Run MFCC and spectral feature extraction on the suspect audio
Compare against known genuine CEO voice samples from recorded meetings
Analyze pitch jitter and shimmer against human speech baselines
Classify using the trained ensemble model and generate confidence score
Produce forensic report with spectrogram evidence for legal/compliance
Pitfalls:
Phone codec compression (G.711, AMR) degrades audio quality and can mask deepfake artifacts
Short audio clips (under 3 seconds) produce unreliable feature statistics
Background noise from the call environment can reduce classification accuracy
Highly sophisticated voice cloning (e.g., fine-tuned VALL-E with 30+ minutes of training data) may evade basic feature analysis
Genuine speech transmitted through VoIP may exhibit spectral artifacts similar to deepfakes
