Hello - I wanted to share a small project I worked on test whether picking one voice out of background chatter could separate humans from machines, in the spirit of an audio CAPTCHA.

Setup

• •40 synthetic clips: one target sentence read by a synthetic voice, mixed over multiple background voices reading random short fragments (staggered starts, 4 dB attenuation)
• •146 human transcriptions from 18 sessions, unlimited replays allowed
• •8 model configurations, one pass per clip: Whisper base/small, Vosk, Wav2Vec2, Qwen3-ASR 0.6B, Parakeet CTC 0.6B, and Gemini 3.8 Flash with a naive prompt and with an instruction to focus on the main speaker
• •Primary metric: word accuracy (1 − WER, floored at 0); default passing threshold = word accuracy ≥ 0.5

TL;DR: humans are better at the task but the gap is too narrow to be a reliable CAPTCHA

Results

• •Humans 0.71 mean word accuracy vs. 0.54 for instructed Gemini (and 0.55 for Qwen3-ASR)
• •Pass rate: 77% humans vs. 55% for Gemini
• •Note: no threshold from ≥0.5 to ≥0.9 gets near my initial rough mental target: humans > 80% and machines < 20%
• •Paired same-clip gap vs. Gemini: +0.13 (95% CI +0.03 to +0.23, session-level bootstrap); AUC 0.62
• •The instruction prompt alone lifted Gemini from 0.34 to 0.54
• •Replays didn't measurably help humans; people replayed the clips that were hardest for the models too
• •Misses rise toward the end of the sentence for humans and every model alike

Limitations: small sample (18 sessions, mostly native English speakers), a single synthetic setup, no source separation, and no large or commercial ASR beyond Gemini.

I dusted off my dormant blog and did a write-up (https://www.kelvinjiang.com/2026/10/can-machines-follow-voice-through-crowded-room.html) and also shared the code, data, and audio corpus: https://github.com/cloudkj/crosstalk

Feedback on the method is very welcome, especially ideas for audio that stays easy for people but is consistently hard for models!

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