Where Speech Enhancement Hurts Recognition: An Inference Time Polar Projection Diagnosis

Abstract

Speech enhancement (SE) can substantially improve perceptual quality, yet enhanced speech does not necessarily improve automatic speech recognition (ASR). Existing remedies, such as retraining the enhancer jointly with recognizer or interpolating enhanced speech with the noisy input, can mitigate this mismatch, but common explanations such as artifacts and over-suppression remain qualitative and do not localize which enhancement component harms recognition. We propose inference time polar projection, a diagnosis for STFT domain enhancement. Given a mask $M=Ae^{j\phi}$, polar projection forms $M_{\alpha,\gamma}=A^\alpha e^{j\gamma\phi}$, where $\alpha$ controls magnitude strength and $\gamma$ controls phase correction. Sweeping these controls on frozen SE and ASR models turns ASR degradation into measurable magnitude and phase effects. Our projection analysis shows that magnitude strength is the operative axis, while estimated phase correction provides no recognition benefit. The optimal magnitude strength is recognizer dependent: waveform-input wav2vec2.0 favors strong correction, whereas log-Mel-input, noise-robust Whisper prefers weaker correction. Finally, the projection provides a simple mitigation for any SE front end in the STFT mask domain, without retraining either the enhancer or the recognizer, making it directly useful for voice assistants and agents that rely on enhanced speech.