Dimension-Wise Probing of Music Foundation Models for Chinese and Western Music: Timbre, Technique, Key, and Mode

Music foundation models are increasingly used as feature extractors in music information retrieval; yet, their treatment of Chinese tonal organization has received little controlled evaluation. We probe frozen MERT, MuQ, CultureMERT, and an 88-dimensional librosa baseline on instrument timbre, guzheng playing technique, Western key, and Chinese pentatonic mode. Probes are evaluated with task-specific splits that guard against leakage and with chance-normalized accuracy. Timbre is near ceiling (0.994–0.998), whereas source-aware guzheng-technique accuracy ranges from 0.382 to 0.787. In the unmatched external comparison, GiantSteps key scores 0.441–0.604 and CNPM mode pattern scores 0.294–0.355; this difference is descriptive because the corpora differ in genre, production, provenance, and label distribution. Same-audio CNPM controls identify an important boundary: the TongGong system reaches 0.649–0.734, and absolute tonic reaches 0.433–0.480, whereas their matched five-way mode-pattern labels reach 0.256–0.350 and 0.243–0.366, respectively. Chroma ablations show that pitch-class energy is central to the external key task but does not by itself account for CNPM mode-pattern performance. These results map which musical dimensions are readily decodable from the evaluated frozen representations while avoiding a cultural-capability interpretation of the unmatched cross-corpus comparison.