What Spatial Cues Do Multichannel Replay Detectors Rely On? A Physically Interpretable Analysis via Relative Transfer Functions

Replay attacks pose a major security threat to widely deployed voice interaction systems. Multichannel detectors typically exploit spatial information between microphones, but their reliance on specific spatial features remains unclear. We use physically interpretable relative transfer functions (RTFs) as control variables to adjust inter-channel magnitude ratios and phase differences. Without accessing model internals, we select transferable RTF templates using only detection scores from a small set of recordings. These templates can induce replay-to-genuine misclassification on recordings and some models not used in selection, while transcripts and speaker representations remain highly consistent after processing. These findings directly connect model decisions to interpretable physical features across channels, providing insight into multichannel replay detectors and a new perspective on their robustness.