REVIEW 5 major objections 6 minor 30 references
The Computation of Generalized Embeddings for Underwater Acoustic Target Recognition using Contrastive Learning
T0 review · 5 major / 6 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read A self-supervised contrastive encoder trained on unlabeled single-hydrophone audio produces embeddings that transfer to ship-type and marine-mammal classification as well as supervised baselines.
desk verdict A solid proof of concept for self-supervised pretraining on unlabeled hydrophone data, but the 'generalized embeddings' claim would be stronger with a random-initialization baseline and repeated runs. 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 central object is the unsupervised contrastive-learning pipeline built on the VICReg loss, Variance-Invariance-Covariance Regularization. VICReg needs no negative samples: it trains the encoder by pulling augmented views of the same clip together in embedding space, pushing the variance of each embedding dimension toward a target, and minimizing off-diagonal covariances to prevent dimensional collapse. Four Conformer blocks serve as the encoder, and a two-layer MLP projects the embeddings for optimization. The augmentation family is the domain-knowledge injection: low-pass filtering at 1 kHz targets the band where ship-type discriminative information concentrates, and a mixup augmentation combines low- and high-frequency content from nearby time windows. This machinery lets the model learn from raw unlabeled audio and still produce features that separate ship types and whale calls in other recordings.
What would settle it
Train the identical VICReg pipeline on unlabeled recordings from a hydrophone far from Vancouver, ideally near the ShipsEar recording region, then attach the same linear classifiers. If accuracy on ShipsEar and Watkins drops substantially compared with the Vancouver-trained model, the claim that a single representative hydrophone yields generalized embeddings is falsified.
Extended reading notes
Core claim
The central claim is that an unsupervised contrastive-learning pipeline, trained without labels on raw audio from one hydrophone, yields generalized embeddings for underwater acoustic target recognition. The authors define positive pairs by augmenting two-second audio windows with domain-specific functions, then optimize a four-block Conformer encoder with the VICReg loss, which enforces invariance between augmented views, keeps feature variance alive, and decorrelates embedding dimensions. Linear classifiers built on these embeddings reach 54.87% accuracy on Deepship, 57.42% on ShipsEar, and 86.10% on Watkins, close to the supervised Conformer baseline, and they hold up better than the supervised baselines when the labeled fraction of Deepship falls to 25% and below. The paper offers this as a proof-of-concept that unlabeled ocean audio can replace labeled corpora as the training signal for UATR backbones.
Load-bearing premise
The load-bearing premise is that recordings from one hydrophone near Vancouver are representative enough of the acoustic environments in all three benchmark datasets, including ShipsEar off Spain and the decades-spanning Watkins collection, and this premise is never tested.
Editorial extensions
If this is right
- Passive acoustic monitoring pipelines could be initialized from unlabeled ocean recordings, removing the dependence on large labeled corpora for underwater acoustic target recognition.
- The same backbone may transfer to other underwater acoustic analysis tasks, such as climate-change monitoring or detection of human-made sounds, because the embeddings are not tied to one dataset's label set.
- Time-wise evaluation of Deepship suggests that models trained on a random split overstate real-world performance; time-wise generalization is the harder and more deployment-relevant test.
- Unsupervised models are more robust than supervised contrastive baselines to reduced label availability on Deepship, so label collection effort could be shifted to downstream evaluation.
- Expanding the augmentation family with speech-based augmentations helps supervised models but can hurt unsupervised transfer, making augmentation choice a key design lever for self-supervised UATR.
Reading between the lines
- A stronger version of the claim would hold if training on a hydrophone far from all three benchmarks still transferred; that test would distinguish general acoustic-structure learning from site-specific similarity to Deepship.
- The ShipsEar result, where the unsupervised Conformer exceeds the supervised Conformer baseline, is consistent with supervised training on Deepship overfitting site-specific class boundaries, but the paper does not identify the exact spectral or temporal features driving the transfer.
- A direct extension would measure whether linear probes trained on Deepship embeddings transfer to ShipsEar and Watkins labels in a zero-shot or few-shot protocol, and compare against spectrogram-statistics baselines.
- A systematic sweep over hydrophone location, depth, season, and recording hardware would define the practical envelope of the single-hydrophone claim.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes an unsupervised contrastive learning pipeline for underwater acoustic target recognition (UATR). A Conformer encoder is trained with the VICReg loss on unlabeled recordings from a single ONC hydrophone near Vancouver, and the learned embeddings are evaluated by training a logistic regression classifier on three labeled benchmarks: Deepship (ship type), ShipsEar (ship type), and The Best of Watkin's (marine mammal vocalizations). The method is compared against supervised contrastive learning baselines (ResNet18 and Conformer) and is additionally analyzed with respect to VICReg loss weights, augmentation functions, reduced labeled data, embedding size, and computational cost. The central claim is that the unsupervised contrastive approach produces robust and generalized embeddings that transfer across geographically and temporally diverse datasets, with accuracy comparable to supervised baselines and greater robustness when labeled data are scarce.
Significance. If the claims hold, the paper offers a useful proof-of-concept that publicly available unlabeled hydrophone data can serve as pretraining data for multiple underwater acoustic classification tasks, reducing reliance on scarce labeled data. The paper is honest about using held-out test sets and a time-wise split for Deepship, and it makes the code available. The study addresses a relevant problem for passive acoustic monitoring and is likely to interest the underwater acoustics and applied self-supervised learning communities. However, the strength of the central claim depends on the missing controls and evaluation issues discussed in the major comments. The manuscript is a solid empirical contribution in need of revision rather than a breakthrough result.
major comments (5)
- [Section 3, Table 1] The central claim that VICReg training on unlabeled ONC data produces generalized embeddings is not supported by the current baseline set. Tables 1, 3, and 4 compare only VICReg-trained encoders with supervised-CL-trained encoders; there is no control using the same encoder with random weights or with VICReg trained on a non-representative unlabeled corpus. This control is essential because Table 3 shows that the LowPass augmentation alone matches the full augmentation family (87.00 vs 87.00 on Watkin's, 55.93 vs 55.61 on Deepship), suggesting that a large part of the class signal may be present in the input spectrogram itself. Please add (a) linear-probe accuracy on randomly initialized Conformer and ResNet18 encoders and (b) a VICReg training run on a mismatched unlabeled corpus (e.g., white noise or a different hydrophone) to attribute the observed accuracy to the contrastive objective and the specific ONC data.
- [Sections 3.2-3.3, Tables 2-4] The VICReg loss weights (Table 2), the augmentation family choice (Table 3), and the decision to expand the augmentation family (Table 4) are selected using accuracy on the Deepship test set, which is also used in the final evaluation in Table 1. This is test-set selection and inflates the reported generalization numbers, undermining the claim that the embeddings generalize without dataset-specific tuning. Use a held-out validation split (separate from the test sets) or nested cross-validation to select hyperparameters, and report the corresponding final accuracies. This is load-bearing because the selected configuration is then presented as the proposed method's performance.
- [Equation (2)] The variance term in the VICReg loss is written as abs(γ − S(z^j, ε)), but the original VICReg formulation (Bardes et al., 2021) uses max(0, γ − S(z^j, ε)), a hinge function that only penalizes standard deviations below the target γ. The absolute value also penalizes standard deviations above γ, which is not part of VICReg and can change the learned representation. If the implementation matches the equation, this is a substantive deviation; if not, the equation misleads. Please correct the formula and, if the implementation used the absolute value, report whether the results change under the correct VICReg variance term.
- [All result tables (Tables 1, 3, 4, Figures 5, 7, 8)] All reported accuracy and F1 scores are point estimates from a single training run. Several key comparisons involve small differences, e.g., Deepship 54.87 vs 55.16 and Watkin's 86.10 vs 87.10 between unsupervised and supervised Conformer in Table 1. Without multiple random seeds and error bars or a significance test, the claims of 'comparable performance' and 'more robust' are not statistically supported. Please report mean ± standard deviation over at least three independent runs, or provide a paired significance test for the main comparisons.
- [Section 2.1.1] The load-bearing premise that unlabeled recordings from a single ONC hydrophone near Vancouver are sufficiently representative of the acoustic conditions in all three labeled benchmarks—including ShipsEar off Spain and the decades-spanning Watkins database—is never tested empirically. The paper provides no spectral or distributional analysis comparing the unlabeled data to the evaluation datasets, and no experiment with an alternative unlabeled corpus. This makes it difficult to rule out the possibility that the cross-dataset accuracy reflects generic spectrogram statistics rather than genuinely transferable embeddings learned from the ONC data. Please add an analysis of the acoustic feature distributions or a sensitivity experiment using a different unlabeled corpus.
minor comments (6)
- [Equation (8)] The symbol v is used both for the variance component function v(Z) and as the scalar weight for the covariance term in Eq. (8). Rename the scalar weight ν (or similar) to avoid confusing the variance function with the covariance weight.
- [Table 3 caption] The caption says 'The influence of the augmentation functions on the classification of Ship types,' but the table also reports results on the Watkin's marine mammal dataset. Adjust the caption to reflect that both ship type and marine mammal classification are evaluated.
- [Section 2.2, item 2] The Gaussian noise augmentation constrains the output SNR between 0.3 dB and 0.5 dB, which would make the noise overwhelmingly stronger than the signal. Please confirm that this range is intentional or whether the SNR values are defined differently (e.g., SNR in dB of the noise added relative to the signal).
- [Abstract and Section 1] The sentence 'This shows to potential of unsupervised methods' contains a typo; it should be 'This shows the potential of unsupervised methods.'
- [Figure 2b caption] The caption reads 'Visualization fo the proposed augmentation functions'; 'fo' should be 'of.'
- [Section 1, contribution 1] The claim 'The first implementation of unsupervised CL on a separate unlabeled underwater acoustic dataset' is strong given that the reference list includes self-supervised work on underwater acoustics (e.g., Xu et al., 2024). Suggest softening to 'to the best of our knowledge' and clarifying the precise distinction from prior self-supervised underwater acoustic studies.
Circularity Check
No significant circularity: embeddings are trained without labels on separate ONC data and evaluated on held-out labeled benchmarks; the only self-citation (augmentation design from authors' prior survey) is empirically validated, not load-bearing.
full rationale
The central derivation chain is self-contained against external benchmarks. Unlabeled audio from a single ONC hydrophone is used to train a Conformer encoder with the VICReg loss (no labels, Sections 2.1.1, 2.5); the frozen embeddings are then used to fit a logistic regression on labeled training splits of Deepship, ShipsEar, and Watkins, and accuracy is reported on genuinely held-out test splits (Deepship time-wise future data; the other two 80/20 splits, Section 2.1.2). No fitted quantity is renamed as a prediction: the test accuracies in Table 1 are out-of-sample measurements, and the label information from the benchmarks never enters the unsupervised training stage. The only self-citation is the authors' own prior survey (Hummel et al., 2024) used to justify the 50 Hz-1 kHz low-pass augmentation (Section 2.2). This is not load-bearing: the claim is externally falsifiable and is in fact tested against the external benchmarks in the Table 3 ablation, where LowPass alone roughly matches the full family. The choice of the ONC hydrophone to match the Deepship site (Section 2.1.1) shapes representativeness, but it does not force any result by construction: the ShipsEar and Watkins results are cross-site, and the Deepship test set covers ships not seen in the pretraining period under a time-wise split. Concerns about missing controls (random-initialized Conformer or a non-representative unlabeled corpus) bear on causal attribution of the accuracy numbers to the contrastive objective, which is an experimental-design risk rather than a reduction of the claimed result to its own inputs. The paper therefore does not exhibit the specific reductions required to establish circularity.
Assumptions & free parameters
free parameters (7)
- VICReg loss weights (lambda, mu, v) =
lambda=5, mu=5, v=1
- Low-pass filter cutoff frequency =
1 kHz
- Gaussian noise SNR range =
0.3-0.5 dB
- Mixup temporal sampling sigma =
50 seconds
- Number of Mel filters =
128
- Embedding dimension =
2048
- Batch size and LARS learning rate =
batch=2048, LR=0.01
assumptions (5)
- domain assumption Unlabeled recordings from the single ONC hydrophone near Vancouver are representative for the acoustic conditions in all three labeled benchmarks (Deepship, ShipsEar, Watkins).
- domain assumption Two-second non-overlapping windows preserve the class-discriminative information for ship type and marine mammal recognition.
- domain assumption The four augmentations (identity, Gaussian noise, low-pass filter, temporal mixup) produce valid positive pairs that do not change the latent class identity.
- domain assumption Logistic regression on frozen embeddings is a sufficient probe for measuring embedding quality and generalization.
- domain assumption VICReg's variance and covariance regularization prevent representation collapse for spectrogram inputs as they do for images.
Cite this review
Pith. "Pith review of The Computation of Generalized Embeddings for Underwater Acoustic Target Recognition using Contrastive Learning." pith.science (2026). https://pith.science/paper/W2MAEP63
@misc{pith2026250512904,
author = {Pith},
title = {Pith review of: The Computation of Generalized Embeddings for Underwater Acoustic Target Recognition using Contrastive Learning},
year = {2026},
howpublished = {\url{https://pith.science/paper/W2MAEP63}},
note = {Machine review of arXiv:2505.12904}
}
read the original abstract
The increasing level of sound pollution in marine environments poses an increased threat to ocean health, making it crucial to monitor underwater noise. By monitoring this noise, the sources responsible for this pollution can be mapped. Monitoring is performed by passively listening to these sounds. This generates a large amount of data records, capturing a mix of sound sources such as ship activities and marine mammal vocalizations. Although machine learning offers a promising solution for automatic sound classification, current state-of-the-art methods implement supervised learning. This requires a large amount of high-quality labeled data that is not publicly available. In contrast, a massive amount of lower-quality unlabeled data is publicly available, offering the opportunity to explore unsupervised learning techniques. This research explores this possibility by implementing an unsupervised Contrastive Learning approach. Here, a Conformer-based encoder is optimized by the so-called Variance-Invariance-Covariance Regularization loss function on these lower-quality unlabeled data and the translation to the labeled data is made. Through classification tasks involving recognizing ship types and marine mammal vocalizations, our method demonstrates to produce robust and generalized embeddings. This shows to potential of unsupervised methods for various automatic underwater acoustic analysis tasks.
Figures
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Reference graph
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Reviewed August 15, 2026 · model on record in the stance chip above.
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