Understanding Frechet Speech Distance for Synthetic Speech Quality Evaluation

TL;DR

This paper evaluates the effectiveness of Frechet Speech Distance and its variants as objective metrics for synthetic speech quality, comparing them with human judgments and other measures across various embeddings and conditions.

Contribution

It provides a comprehensive analysis of FSD and SMMD, identifying optimal embeddings like WavLM Base+ and validating their use as cost-effective, reproducible alternatives to subjective evaluations.

Findings

WavLM Base+ features align best with human ratings.

FSD and SMMD are useful supplementary metrics.

Metrics cannot fully replace human listening tests.

Abstract

Objective evaluation of synthetic speech quality remains a critical challenge. Human listening tests are the gold standard, but costly and impractical at scale. Fr\'echet Distance has emerged as a promising alternative, yet its reliability depends heavily on the choice of embeddings and experimental settings. In this work, we comprehensively evaluate Fr\'echet Speech Distance (FSD) and its variant Speech Maximum Mean Discrepancy (SMMD) under varied embeddings and conditions. We further incorporate human listening evaluations alongside TTS intelligibility and synthetic-trained ASR WER to validate the perceptual relevance of these metrics. Our findings show that WavLM Base+ features yield the most stable alignment with human ratings. While FSD and SMMD cannot fully replace subjective evaluation, we show that they can serve as complementary, cost-efficient, and reproducible measures, particularly useful when large-scale or direct listening assessments are infeasible. Code is available at https://github.com/kaen2891/FrechetSpeechDistance.