REVIEW 2 major objections 4 minor 36 references
Foundation Model Hidden Representations for Heart Rate Estimation from Auscultation
T0 review · 2 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Heart rate can be estimated from heart-sound recordings using representations from pre-trained audio foundation models, and one contrastively trained encoder beats the acoustic-feature baseline at 1.88 vs 1.91 bpm mean absolute error.
desk verdict A useful layer-wise benchmark of audio FMs for PCG heart rate estimation, but the headline CLAP advantage is a post-hoc 0.03 bpm gap with no significance testing; the 'comparable performance' finding is solid, the 'outperforms' claim is not. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is the layer-wise extraction of hidden representations: for each foundation model, the output of the $n$-th embedding layer of its audio encoder is taken from 5-second PCG snippets and fed into a downstream 2D CNN classifier that predicts heart rate as one of 141 classes ($HR \in [40,180]$ bpm) with cross-entropy loss. The in-house CLAP model, which carries the main positive result, is a 12-layer ViT audio encoder with 768 hidden dimensions, pre-trained in two stages: first by masked spectrogram reconstruction in the style of AudioMAE on roughly three million audio samples, then by contrastive alignment with a text encoder on about five million captions. This two-stage, general-audio-event pretraining is what the paper credits for the encoder's ability to preserve cardiac cues in mid-level layers.
What would settle it
Rerun the same layer-wise comparison on the same snippets but with heart-rate labels obtained from an independent reference, such as a synchronized ECG recording or expert re-annotation of S1/S2 boundaries; the central claim fails if the in-house CLAP's 1.88 bpm result no longer beats the baseline's 1.91 bpm under independently verified labels.
Extended reading notes
Core claim
The central claim is that pre-trained acoustic foundation-model representations are a viable feature source for heart-rate estimation from auscultation, and that a contrastively trained audio encoder can beat a purpose-built acoustic-feature method. In the paper's layer-wise evaluation, each FM's embedding-layer output feeds a small 2D CNN that classifies heart rate into 141 bins from 40 to 180 bpm; the best single result is the sixth embedding layer of the in-house CLAP model, which reaches an average MAE of 1.88 bpm across six train/validation/test splits, versus 1.91 bpm for the baseline. The paper further reports that the shallow layers of ASR-oriented models (HuBERT, wav2vec2, WavLM, Whisper) generally perform better than deep layers, that larger model size does not guarantee better heart-rate estimates, and that averaging the baseline's predictions with FM-based predictions improves accuracy on every split. The authors attribute the in-house CLAP advantage to its training on a wide range of audio events rather than speech-dominated corpora.
Load-bearing premise
The ground-truth heart rate of each snippet is derived from the S1 onset annotations in the public PCG dataset using the labeling method of the baseline paper, so if those onset annotations carry systematic bias or noise, every reported MAE is measured against an unreliable target.
Editorial extensions
If this is right
- General audio foundation models can be used as frozen feature extractors for heart-rate estimation, avoiding task-specific pretraining and achieving accuracy comparable to handcrafted acoustic features.
- The in-house CLAP's advantage over the baseline (1.88 vs 1.91 bpm MAE) identifies contrastive audio-event pretraining, rather than speech-oriented pretraining, as the more transferable recipe for cardiac acoustics.
- The layer-wise curves provide a direct fine-tuning target: for ASR-based models, shallow and middle layers retain cardiorespiratory information while deep linguistic layers lose it, so adaptation should focus on the earlier layers.
- Averaging FM-based and acoustic-feature predictions improves accuracy on every split, suggesting ensemble schemes that combine both feature families are a practical route to more robust HR estimation.
- Evaluation across multiple data splits is necessary; the paper finds that data-split variation can affect model comparisons more than model size, and larger FMs do not systematically help.
Reading between the lines
- A testable extension: probe the same six encoders on other cardiorespiratory sounds (breathing, cough, murmurs); the layer-wise pattern predicts that mid-level layers of contrastive audio encoders will again carry the most signal.
- A concrete next experiment: concatenate the baseline acoustic features with the in-house CLAP layer-6 vector before the downstream CNN; the paper's averaging result suggests fusion would lower the MAE below 1.88 bpm.
- Because the reported difference is only 0.03 bpm, an independent-label rerun (ECG-synchronized) is needed to know whether the true effect is bigger, smaller, or absent; the paper does not provide that check.
- The paper's attribution of the advantage to audio-event pretraining breadth could be isolated by ablating the two pretraining stages of the in-house CLAP encoder, but that ablation is not reported here.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper benchmarks hidden representations from six pre-trained acoustic foundation models (HuBERT, wav2vec2, wavLM, Whisper, CLAP, and an in-house CLAP) for heart rate (HR) estimation from phonocardiogram snippets. Using the CirCor dataset, heart sound snippets are generated with 5-second windows, HR targets are derived from S1-onset annotations via the method of Nie et al. [1], and a downstream 2D CNN is trained on embeddings from each layer of each model. The reported result is that FM representations generally achieve MAE comparable to the acoustic-feature baseline (average MAE 1.91 bpm), with the 6th layer of in-house CLAP achieving an average MAE of 1.88 bpm, which the abstract and conclusions describe as outperforming the baseline.
Significance. If the findings are properly supported, the paper would provide a useful benchmark showing that frozen self-supervised audio representations are competitive with handcrafted acoustic features for auscultation-based HR estimation. The layer-wise analysis across six models and six held-out splits is a useful contribution, and the reporting of model sizes and per-split variance is commendable. The strongest parts are the breadth of the layer sweep and the use of a public dataset with multiple splits. However, the headline superiority claim for in-house CLAP rests on a 0.03 bpm difference that is not statistically tested, and the label-generation procedure is not independently validated; these issues are load-bearing for the paper's central claim.
major comments (2)
- [Section 5, Table 1, Abstract] The claim that in-house CLAP 'outperforms' the baseline is not statistically supported. The evidence is min(MAE)=1.88 for the 6th layer versus baseline MAE=1.91, a difference of 0.03 bpm, while the reported split-level spread is σ=0.37 for in-house CLAP at that layer and STD=0.32 for the baseline. The 6th layer was selected as the best among 12 embedding layers after examining test performance on the same six splits, so the minimum-of-12 comparison is subject to selection bias. No paired test, confidence interval, or multiple-comparison correction is reported for this key comparison. The authors should report per-split paired differences and a paired significance test, or use a held-out layer-selection procedure; otherwise the conclusion should be limited to 'comparable performance.'
- [Section 2 and Section 5] The ground-truth HR for each snippet is generated from S1-onset annotations in the CirCor dataset using the method of the authors' prior paper [1], and the manuscript itself acknowledges 'biases and errors in segmentation annotations' (Section 2) and 'annotation bias' (Section 5). Because both the baseline method and the FM-based models are evaluated against these derived labels, the reliability of the label-generation procedure is load-bearing for every reported MAE. No independent validation of the derived HR targets is provided (for example, against ECG-derived HR or manual reference measurements). The authors should either provide such validation or explicitly and consistently frame the results as relative performance under this particular label-generation protocol.
minor comments (4)
- [Section 1] The phrase 'train/validation/data splits' should read 'train/validation/test splits.'
- [Section 4, Eq. (1)] The displayed cross-entropy formula has mismatched subscripts (x_{a,c} versus x_{n,i}) and is not written in standard form; please correct the notation so the sum over classes and the softmax argument are unambiguous.
- [Table 1] Please clarify that min(MAE) is the minimum over embedding layers of the mean MAE across splits, and that σ_{MAE,j*} is the standard deviation across splits at the layer achieving that minimum; the current header is dense and could be misinterpreted as a minimum over individual split results.
- [Section 5, Figure 5] The shaded standard-deviation bands in Figure 5 are useful, but the figure legends do not state the number of splits underlying each mean; adding that information would improve interpretability.
Circularity Check
No significant circularity: the FM representations are evaluated independently against an implemented acoustic baseline on labels derived from external S1-onset annotations.
full rationale
The paper's central comparison is not circular. The heart-rate labels are computed from adjacent S1-onset intervals in the CirCor dataset using the method of [1], but those onsets are external annotations, not outputs of the baseline or of the foundation models being evaluated. The baseline itself is implemented from [1] as an acoustic-feature 2D CNN, and the FM embeddings are passed through a separately trained downstream classifier on the same six splits. No parameter is fitted on the test set in a way that defines the reported outcome; the 'outperforms' claim is based on selecting the minimum mean MAE across embedding layers, which is a model-selection and statistical-inference concern rather than a circularity. The paper's own caveat about annotation bias in the CirCor dataset (Section 5) is a data-quality limitation, not a step that reduces the derivation to its inputs. The self-citations to [1] are used as an external baseline and label-generation recipe, but the baseline is implemented and the labels are objectively derived, so the cited prior work does not carry the argument by assumption. Overall, the derivation chain is self-contained for the purposes of circularity analysis.
Assumptions & free parameters
assumptions (2)
- domain assumption The S1 onset annotations in the CirCor dataset, combined with the averaging method from [1], produce a valid ground-truth heart rate for each 5-second snippet.
- domain assumption Representations from pre-trained foundation models, despite domain mismatch with cardiac acoustics, can be fed into a 2D CNN for heart rate classification.
Cite this review
Pith. "Pith review of Foundation Model Hidden Representations for Heart Rate Estimation from Auscultation." pith.science (2026). https://pith.science/paper/QVA4C26U
@misc{pith2026250520745,
author = {Pith},
title = {Pith review of: Foundation Model Hidden Representations for Heart Rate Estimation from Auscultation},
year = {2026},
howpublished = {\url{https://pith.science/paper/QVA4C26U}},
note = {Machine review of arXiv:2505.20745}
}
read the original abstract
Auscultation, particularly heart sound, is a non-invasive technique that provides essential vital sign information. Recently, self-supervised acoustic representation foundation models (FMs) have been proposed to offer insights into acoustics-based vital signs. However, there has been little exploration of the extent to which auscultation is encoded in these pre-trained FM representations. In this work, using a publicly available phonocardiogram (PCG) dataset and a heart rate (HR) estimation model, we conduct a layer-wise investigation of six acoustic representation FMs: HuBERT, wav2vec2, wavLM, Whisper, Contrastive Language-Audio Pretraining (CLAP), and an in-house CLAP model. Additionally, we implement the baseline method from Nie et al., 2024 (which relies on acoustic features) and show that overall, representation vectors from pre-trained foundation models (FMs) offer comparable performance to the baseline. Notably, HR estimation using the representations from the audio encoder of the in-house CLAP model outperforms the results obtained from the baseline, achieving a lower mean absolute error (MAE) across various train/validation/test splits despite the domain mismatch.
Figures
Figures from the paper (2 more)
Reference graph
Works this paper leans on
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[1]
Introduction Health-related acoustic sounds have significant potential for health, fitness, and wellbeing. Heart sound auscul- tation (from phonocardiograms, a.k.a PCGs) is a non- invasive method that offers crucial vital sign data (such as heart rate (HR) information), and machine learning can be used to explore features and enable high-value use cases. ...
work page Pith review arXiv 2025
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[2]
Each PCG recording spans from 5.1 to 64.5 seconds, totaling about 20 hours
Data The PCG dataset used in our study is theCirCor DigiScope Phonocardiogram dataset, containing 3,163 heart sound recordings from 942 subjects collected across four main auscultation sites in hospitals. Each PCG recording spans from 5.1 to 64.5 seconds, totaling about 20 hours. All recordings in this dataset are low-pass filtered with a cut- off frequen...
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16 kHz raw audio using a 25 ms Hann window and 10 ms hops
In-House CLAP Model Audio Encoder Architecture:The audio encoder takes a 128 LogMel spectrogram as input, computed from mono Figure 3:The heart rate distribution for 6 training, valida- tion, and test splits and the number of unique subjects. 16 kHz raw audio using a 25 ms Hann window and 10 ms hops. The model is a 12-layer ViT-B with around 86 mil- lion ...
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Downstream Model and Training Since HR is typically represented as an integer in beats per minute, we followed a similar approach to [1], treating HR estimation as a 141-class classification problem, where HR∈[40,180](bpm). The cross-entropy (CE) loss func- tion was used for HR estimation: CE= PA a=1(− PB b=1 log exp (xa,c)PB i=1 exp (xn,i) ya,b),(1) wher...
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Results Figure 5 shows theM AEi,j for splitiusing the represen- tations from embedding layerjof the different FMs. The thick purple line represents the average MAE ( M AEj) across the 6 splits, with the light purple shaded area indi- cating the standard deviation (σ M AEj ). Table 1 further il- lustrates the performance of using the representations from d...
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Conclusion and Future Work In this study, we evaluated and benchmarked the ability of pre-trained acoustic foundation models (FMs) to en- code heart sound auscultation for heart rate (HR) estima- tion. Through a layer-wise investigation of six FMs, such as HuBERT, wav2vec2, wavLM, Whisper, CLAP, and an in-house CLAP model, we demonstrated that these FMs o...
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2024 arXiv
Reviewed August 7, 2026 · model on record in the stance chip above.
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