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SPECTRA: Subspace-Preserving Embedding Calibration, Transport, and Replay for Fully Few-Shot Class-Incremental Audio Classification

Giries Abu Ayoub, Loay Mualem, Simon Korman

Year: 2026Venue: arXiv preprintArea: cs.SDType: PreprintEmbeddings: 33

Intelligence

Status: succeeded | Model: Gemma-4-26B-A4B | Prompt: intel-v1 | Confidence: 94%

Last extracted: 8/27/2026, 4:33:39 AM

Summary

The paper introduces SPECTRA, a framework for Fully Few-Shot Class-Incremental Audio Classification (FFCAC) that addresses catastrophic forgetting without storing raw audio data. SPECTRA utilizes a frozen audio-language model encoder (PENGI) enhanced by three components: a lightweight trainable adapter for embedding calibration, subspace feature replay using low-rank SVD subspaces to synthesize old-class features, and transductive optimal transport for prototype refinement at test time. It outperforms the state-of-the-art TAPE method on benchmarks NSynth-100, FSC-89, and LS-100.

Entities (10)

SPECTRA · method · 100%FFCAC · task · 95%FSC-89 · dataset · 95%LS-100 · dataset · 95%NSynth-100 · dataset · 95%Subspace Feature Replay · technique · 95%Adapter · component · 90%Optimal Transport · technique · 90%Pengi · model · 90%TAPE · method · 90%

Relation Signals (8)

SPECTRA → evaluatedon → LS-100

confidence 95% · On three FFCAC benchmarks (NSynth-100, FSC-89, LS-100)

SPECTRA → evaluatedon → NSynth-100

confidence 95% · On three FFCAC benchmarks (NSynth-100, FSC-89, LS-100), SPECTRA improves average accuracy

SPECTRA → evaluatedon → FSC-89

confidence 95% · On three FFCAC benchmarks (NSynth-100, FSC-89, LS-100)

SPECTRA → outperforms → TAPE

confidence 95% · SPECTRA improves average accuracy and reduces forgetting over current state-of-the-art methods... TAPE... defines the current state of the art.

Adapter → partof → SPECTRA

confidence 95% · We propose SPECTRA... adds three components. (i) a lightweight trainable adapter

Subspace Feature Replay → partof → SPECTRA

confidence 95% · We propose SPECTRA... adds three components... (ii) subspace feature replay

SPECTRA → uses → Pengi

confidence 95% · The encoder is the frozen PENGI ALM

Subspace Feature Replay → solves → Catastrophic Forgetting

confidence 90% · subspace feature replay, an exemplar-free anti-forgetting scheme

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Abstract

Abstract:Fully few-shot class-incremental audio classification (FFCAC) requires recognizing new sound classes from only a handful of labeled examples per session, without forgetting previously learned classes and without any large base dataset. Existing methods typically freeze a pre-trained audio--language encoder and classify with point prototypes, but they suffer from significant performance degradation throughout the sessions due to generic feature representations. We propose SPECTRA, a framework built on a frozen encoder which adds three components. (i) a lightweight trainable adapter that calibrates the generic embeddings to the task; (ii) subspace feature replay, an exemplar-free anti-forgetting scheme that replays old classes by sampling from the low-rank subspace of their stored features; and (iii) a transductive optimal-transport refinement of prototypes at test time. Our central finding is that the subspace structure of the replay diminishes forgetting and outperforms naive Gaussian replay of equal variance. On three FFCAC benchmarks (NSynth-100, FSC-89, LS-100), SPECTRA improves average accuracy and reduces forgetting over current state-of-the-art methods, and our ablations statistically validate each component.

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SPECTRA: Subspace-Preserving Embedding Calibration, Transport, and Replay for Fully Few-Shot Class-Incremental Audio Classification Giries Abu Ayoub Loay Mualem Simon Korman Abstract Fully few-shot class-incremental audio classification (FFCAC) requires recognizing new sound classes from only a handful of labeled examples per session, without forgetting previously learned classes and without any large base dataset. Existing methods typically freeze a pre-trained audio–language encoder and classify with point prototypes, but they suffer from significant performance degradation throughout the sessions due to generic feature representations. We propose SPECTRA, a framework built on a frozen encoder which adds three components. (i) a lightweight trainable adapter that calibrates the generic embeddings to the task; (i) subspace feature replay, an exemplar-free anti-forgetting scheme that replays old classes by sampling from the low-rank subspace of their stored features; and (i) a transductive optimal-transport refinement of prototypes at test time. Our central finding is that the subspace structure of the replay diminishes forgetting and outperforms naive Gaussian replay of equal variance. On three FFCAC benchmarks (NSynth-100, FSC-89, LS-100), SPECTRA improves average accuracy and reduces forgetting over current state-of-the-art methods, and our ablations statistically validate each component. Index Terms: Few-shot class-incremental learning, audio classification, feature replay, catastrophic forgetting, audio–language models †address: 1Department of Computer Science, University of Haifa 2Institute for AI, University of Stuttgart 3IMPRS-IS 1 Introduction Audio classification underpins applications from assisted driving [12] and medical monitoring [11] to wildlife sensing [1]. In realistic deployments, the set of classes is not fixed, since new categories appear over time and must be learned from very few examples, without revisiting old data. Few-shot class-incremental audio classification (FCAC) formalizes this, and its hardest variant, fully FCAC (FFCAC), removes the assumption of an abundant base session: every session, including the first, provides only N classes with K examples each. FFCAC therefore combines the two classic difficulties of few-shot overfitting and incremental catastrophic forgetting. Pre-trained audio–language models (ALMs) such as CLAP [6] and PENGI [4] provide strong, transferable audio features. TAPE [8] is the first method to exploit an ALM for FFCAC and defines the current state of the art. It freezes the ALM encoder so representations stay stable across sessions, and adds two lightweight components: a Task-Transform that maps features and prototypes into a task-adaptive metric space through a closed-form orthogonalising transformation, and a Prototype Evolution step that refines each class prototype using low-entropy query samples. Related work. Classic few-shot class-incremental learning (FSCIL) methods decouple a frozen backbone from an evolving classifier [19, 23]; in audio, prototype-refinement [21] and capacity-growing methods address FCAC and FFCAC, while SpurAudio [2] shows that few-shot audio classifiers readily latch onto spurious correlations, underscoring the need for task-specific representation calibration. A complementary line fights forgetting by replaying past data: rather than storing exemplars, generative and feature-replay methods synthesise old-class samples [16], which is attractive when raw data cannot be retained. On the inference side, transductive methods such as PT-MAP [10] exploit the joint statistics of the query set through Sinkhorn optimal transport [3]. SPECTRA brings exemplar-free, subspace-structured feature replay and transductive transport to the frozen-ALM pipeline of TAPE [8], where the tiny support sets make both data-efficiency and forgetting acute. Motivation. Prototype-based representations provide an effective summary of each class for classification. However, in FFCAC the feature representation must continue to adapt as new classes arrive, while samples from previous sessions are no longer available. This raises a fundamental question: how can we continue adapting the representation without losing knowledge of earlier classes? Our key insight is that a compact representation of each class’s feature geometry can be used to synthesize representative feature embeddings for replay, providing continual supervision for previous classes without storing raw audio. Contributions. Motivated by the need to continually adapt the feature representation while preserving knowledge of previously learned classes, we propose SPECTRA, a prototype-based framework that augments each class with a compact subspace representation to enable exemplar-free feature replay. Our main contributions are: (i) A trainable residual-FFN adapter that calibrates frozen ALM embeddings to the target task (Sec. 2.3). (i) Subspace feature replay, an exemplar-free anti-forgetting mechanism that models each class using a compact low-rank subspace and synthesizes representative feature embeddings for continual rehearsal (Sec. 2.4). We show that preserving subspace geometry is crucial, significantly outperforming naive replay strategies. (i) A transductive optimal-transport refinement procedure that improves prototype estimation during inference (Sec. 2.5). (iv) Extensive experiments on three FFCAC benchmarks demonstrating consistent improvements over TAPE in both accuracy and forgetting, supported by per-session analyses and statistical significance tests (Sec. 3). Figure 1: (a) The FFCAC setting: sessions arrive over time, each introducing 55 new classes with K=5K=5 shots and no base session; the model is evaluated on all classes seen so far (cumulative test set growing →255\!→\!25). (b) SPECTRA. An audio clip is embedded by a frozen audio–language encoder (PENGI f), calibrated by a trainable residual-MLP adapter g, and mapped by TAPE’s closed-form orthogonalising transform tP_t before a prototypical classifier. Green (training only): each class’s stored support features ℳcM_c give a low-rank SVD subspace from which synthetic old-class features ~c x_c are sampled and passed through the same head, a rehearsal that counters forgetting without storing audio. Purple, dashed (inference only): a transductive Sinkhorn step refines the prototypes from the unlabelled query batch. 2 Methodology 2.1 Problem Setup We consider the fully few-shot class-incremental audio classification (FFCAC) setting. An audio-language model (ALM) encoder f maps an audio clip to a d-dimensional embedding ∈ℝdx ^d (d=1024d=1024). Classes arrive incrementally across sessions, where each new class is observed through only K support examples and samples from previous sessions are no longer available. The goal is therefore to continually incorporate new classes while preserving knowledge of previously learned ones under strict exemplar-free constraints. For each class c, we maintain the support feature set ℳc=ii=1KM_c=\x_i\_i=1^K together with its prototype c=1K∑i μ_c= 1K _ix_i. Following TAPE [8], we employ its closed-form Task-Transform, which projects prototypes into an orthogonal metric space prior to classification. This component is kept unchanged throughout our method, allowing us to focus on improving representation adaptation, continual rehearsal, and inference. 2.2 Overview Our framework consists of three complementary components. First, we introduce a lightweight embedding adapter that calibrates frozen ALM features to the target task without fine-tuning the backbone. Second, we preserve previous knowledge through subspace feature replay, which models each class by a compact low-dimensional subspace and synthesizes representative feature embeddings for continual rehearsal. Finally, we refine class prototypes during inference using transductive optimal transport over the unlabeled test batch. Figure 1 illustrates the complete pipeline. 2.3 Embedding Adapter Frozen ALM embeddings provide strong generic representations but are not optimized for the target taxonomy. Rather than fine-tuning the entire encoder, which is prone to overfitting in the fully few-shot regime, we introduce a lightweight residual adapter that calibrates the embedding space while preserving the stability of the frozen backbone. Specifically, we employ a transformer-style feed-forward residual block with LayerScale, g⁡()=+⊙2GELU(1LN())g(x)=x+ γ _2\,GELU\! (W_1\,LN(x) ), where 1∈ℝrd×dW_1\!∈\!R^rd× d, 2∈ℝd×rdW_2\!∈\!R^d× rd, and γ is initialized with small values so that the adapter starts close to the identity mapping while remaining expressive throughout training. The adapter is applied to both support and query embeddings before the Task-Transform, i.e., =tg()z=P_tg(x), ensuring that all features are represented in the same calibrated embedding space. It is the only substantial trainable component of our framework and is optimized end-to-end together with the classifier. 2.4 Subspace Feature Replay As the embedding adapter evolves to accommodate newly introduced classes, previously learned classes become susceptible to forgetting. Since FFCAC prohibits storing past audio, rehearsal must operate entirely in feature space. Our key idea is to represent each class not only by its prototype, but also by a compact low-dimensional subspace that captures its intrinsic feature variation. This representation enables the synthesis of representative feature embeddings, allowing previous classes to be rehearsed throughout continual learning without retaining any raw audio. Empirically, the support embeddings of a class occupy only a small region of the feature space and are well approximated by a low-rank affine subspace. Let ℳ~c=[1−c,…,K−c]⊤ M_c=[x_1- μ_c,…,x_K- μ_c] with SVD ℳ~c=ΣV⊤ M_c=U V . The top-k singular vectors c=:kU_c=U_:k define the principal directions of the class, while σj=Σjj/K−1 _j= _j/ K-1 captures the variation along each direction. At session t>0t>0, we generate n pseudo embeddings for every previously seen class by sampling within this subspace, ~c=c+∑j=1kzj(c)j,zj∼⁡(0,σj2), x_c= μ_c+ _j=1^kz_j\,(U_c)_j, z_j \! (0, _j^2 ), (1) The synthesized embeddings are processed by the current adapter and classifier exactly as real samples. Consequently, replay regularizes the evolving representation by encouraging previously learned classes to remain discriminative as new classes are introduced. The overall training objective combines the standard cross-entropy loss on the current session with a replay loss over the synthesized embeddings, ℒ=ℒnew+λℒreplay,ℒreplay=∑c< 5tCE⁡(head⁡(~c),c).L=L_new+λ\,L_replay, _replay=\!\! _c<\,5t\!CE (head( x_c),c ). (2) Unlike isotropic Gaussian replay, our approach preserves the principal directions of variation within each class, producing substantially more representative rehearsal samples. Section 3 compares subspace replay against Gaussian replay and replay-free training, and Figure 2 visualizes the difference between the two sampling strategies. 2.5 Transductive Prototype Refinement Few-shot prototypes are estimated from only K support examples and therefore provide imperfect estimates of the underlying class centers. Instead of classifying each query independently, we refine the prototypes by exploiting the collective structure of the unlabeled test batch through entropy-regularized optimal transport. Given the cosine-distance cost Mjc=1−j⊤cM_jc=1-q_j μ_c, Sinkhorn iterations [3] compute a soft transport plan =Sinkhornε(M)R= *Sinkhorn_ (M) under balanced row and column constraints. Each prototype is then updated using both its support embeddings and the softly assigned query embeddings, c←∑i+∑jRjcjK+∑jRjc μ_c← _ix_i+ _jR_jc\,q_jK+ _jR_jc, We repeat this refinement for T iterations before the final prediction. By jointly considering all unlabeled queries, the refined prototypes provide a more reliable estimate of the class centers than independent per-sample classification, leading to improved recognition performance in the few-shot setting. Figure 2: Replay geometry across sessions. As more classes are added, low-rank subspace replay remains class-aligned, whereas Gaussian replay produces overlapping clouds. 2.6 SPECTRA Training: We update only the adapter g and the learned prototypes and reference anchors; the encoder f is frozen and the transform tP_t is closed-form. The sole training objective is Eq. 2: the new-class support features and the synthetic replay features (1) are classified by cross-entropy. Inference (no gradients). Only at test time do we apply the transductive optimal transport step (2.5): we push the query-set through the trained adapter and project it using the transform, run T iterations to refine the prototypes from the unlabelled queries, and classify. This step updates no parameters. Algorithm 1 summarises one session. 3 Experiments Setup. We evaluate on NSynth-100 [7] (instrument notes), FSC-89 [20] (sound events), and LS-100 [14] (speaker identity) under the standard FFCAC protocol: 55 sessions, 55 classes per session, K=5K=5 shots. We report average accuracy (A, mean over sessions, ↑ ) and performance drop (PD == first −- last session accuracy, ↓ ); all numbers are means over 5050 seeds. The encoder is the frozen PENGI ALM and we keep TAPE’s closed-form transform and training otherwise untouched, reproducing its pipeline as our baseline. Unless stated otherwise, the adapter expansion ratio is r=3r=3, the replay subspace rank is k=3k=3, the replay weight is λ=1λ=1, and OT uses T=3T=3 prototype-refinement iterations. Main results. Tables 1–4 compare SPECTRA to various FFCAC baselines. SPECTRA improves average accuracy on all datasets while also reducing forgetting (PD), confirming that task calibration plus exemplar-free replay help on top of a strong frozen-ALM classifier. Table 1: Comparison of different methods on NSynth-100. Method S0 S1 S2 S3 S4 A ↑ PD ↓ iCaRLCVPR’20iCaRL_CVPR'20 [15] 71.70 53.51 53.66 49.07 49.48 55.48 40.08 PODNETECCV’20PODNET_ECCV'20 [5] 71.87 44.89 43.58 42.93 40.97 48.85 35.57 DERCVPR’21DER_CVPR'21 [22] 74.40 61.42 60.06 53.73 44.42 58.81 38.95 CECCVPR’21CEC_CVPR'21 [23] 76.13 58.15 53.80 48.26 44.34 56.14 37.49 FACTCVPR’22FACT_CVPR'22 [24] 74.97 51.94 51.43 46.54 43.45 53.27 38.65 PANTMM’23PAN_TMM'23 [13] 76.71 58.38 53.92 48.44 44.48 56.39 37.72 EDEInterspeech’24EDE_Interspeech'24 [17] 76.16 70.18 63.46 59.16 58.02 65.40 18.14 AISPTASLP’25AISP_TASLP'25 [18] 63.84 60.32 58.62 54.30 52.90 57.99 10.94 CLAPICASSP’23CLAP_ICASSP'23 [6] 43.80 22.14 16.15 12.69 12.31 21.42 31.49 COOPIJCV’22COOP_IJCV'22 [26] 78.35 44.45 30.99 23.23 17.91 38.98 60.45 COCOOPCVPR’22COCOOP_CVPR'22 [25] 69.60 39.79 27.24 21.53 16.67 34.96 52.94 PALMEMNLP’24PALM_EMNLP'24 [9] 94.09 57.34 41.43 34.08 31.32 51.65 62.77 TAPECVPR’26TAPE_CVPR'26 [8] 96.64 94.53 93.44 91.72 91.05 93.48 5.58 SPECTRA(ours)SPECTRA_(ours) 98.14 97.27 96.77 95.50 94.92 96.52 3.23 Table 2: Comparison of different methods on FSC-89. Method S0 S1 S2 S3 S4 A ↑ PD ↓ PANTMM’23PAN_TMM'23 [13] 41.48 23.72 18.08 15.27 12.25 22.16 37.72 EDEInterspeech’24EDE_Interspeech'24 [17] 53.25 37.65 35.59 32.69 27.60 37.36 30.05 AISPTASLP’25AISP_TASLP'25 [18] 62.66 48.92 43.81 37.73 33.43 45.31 29.23 CLAPICASSP’23CLAP_ICASSP'23 [6] 49.64 37.73 28.55 25.42 23.70 33.01 25.94 COOPIJCV’22COOP_IJCV'22 [26] 63.32 37.75 27.50 22.07 18.64 33.86 44.69 COCOOPCVPR’22COCOOP_CVPR'22 [25] 64.65 37.62 26.63 21.22 16.85 33.39 47.80 PALMEMNLP’24PALM_EMNLP'24 [9] 74.41 39.16 25.30 19.91 17.47 35.25 56.94 TAPECVPR’26TAPE_CVPR'26 [8] 81.24 72.65 67.07 63.93 61.29 69.24 19.96 SPECTRA(ours)SPECTRA_(ours) 82.24 74.41 69.25 66.21 63.48 71.12 18.77 Table 3: Per-session accuracy (%) under harder protocols: 1010-way × 10×\,10-session on NSynth-100 and LS-100, and 1010-way × 8×\,8-session on FSC-89 (FSC-89 has only 8989 classes, capping it at 88 sessions); 3030 seeds. Method S0 S1 S2 S3 S4 S5 S6 S7 S8 S9 A↑ PD↓ NSynth-100 (1010-way × 10×\,10-session) TAPE 95.6 92.2 90.8 89.6 88.4 86.5 85.1 84.0 82.9 81.5 87.66 14.11 SPECTRA 97.6 95.6 94.3 93.4 92.7 91.5 90.8 90.1 89.6 88.9 92.45 8.77 LS-100 (1010-way × 10×\,10-session) TAPE 90.1 85.4 80.7 77.9 75.2 73.4 70.7 69.0 67.2 65.7 75.53 24.43 SPECTRA 95.1 91.2 88.1 85.7 83.7 82.3 80.4 79.7 78.6 77.8 84.26 17.31 FSC-89 (1010-way × 8×\,8-session) TAPE 71.7 63.2 58.1 55.0 51.9 49.6 47.1 45.0 – – 55.22 26.66 SPECTRA 74.1 65.9 60.7 57.4 54.5 52.5 50.3 48.7 – – 58.02 25.44 Table 4: Comparison of different methods on LS-100. Method S0 S1 S2 S3 S4 A ↑ PD ↓ PANTMM’23PAN_TMM'23 [13] 85.70 52.20 39.17 32.95 29.88 47.98 37.72 EDEInterspeech’24EDE_Interspeech'24 [17] 91.90 70.23 54.21 46.97 45.94 61.85 45.96 AISPTASLP’25AISP_TASLP'25 [18] 91.20 70.26 53.71 47.08 45.15 61.48 46.05 CLAPICASSP’23CLAP_ICASSP'23 [6] 18.96 9.06 5.91 4.75 3.37 8.41 15.59 COOPIJCV’22COOP_IJCV'22 [26] 49.14 27.64 18.48 14.48 11.01 24.15 38.14 COCOOPCVPR’22COCOOP_CVPR'22 [25] 46.90 24.32 15.75 10.86 7.84 21.14 39.06 PALMEMNLP’24PALM_EMNLP'24 [9] 87.73 44.96 29.39 21.44 18.97 40.50 68.76 TAPECVPR’26TAPE_CVPR'26 [8] 92.34 87.41 84.64 82.24 80.84 85.49 11.50 SPECTRA(ours)SPECTRA_(ours) 97.22 94.81 92.32 89.99 88.55 92.58 8.67 Per-session analysis. The per-session columns of Tables 1–4 show where the gains arise: SPECTRA leads at every session, and on NSynth-100 the margin grows over sessions (S0 +1.5+1.5, S4 +3.9+3.9 over TAPE). The effect is largest on LS-100 (+7.1+7.1 A). Scaling to a harder protocol. To test generality beyond the standard 55-way protocol, we run a harder 1010-way × 10×\,10-session setting on all datasets (all dataset classes over 1010 incremental steps). Table 3 shows SPECTRA still beats TAPE by a wide margin. Algorithm 1 SPECTRA: training and inference at session t 1: frozen ALM f; adapter g; anchors A; memories ℳc\M_c\ 2: labels y y for queries of all classes seen so far 3: Training ⊳ only g and the head get gradients 4: ℳc←f⁡(ai)i=1KM_c←\f(a_i)\_i=1^K, c←1K∑i μ_c← 1K _ix_i ⊳ new classes 5: for each old class c do 6: c,Σ←SVD⁡(ℳc−c)U_c, (M_c- μ_c), σj←Σjj/K−1 _j← _j/ K\!-\!1 7: ~c←c+∑j≤kzjσj(c)j x_c← μ_c+ _j≤ kz_j _j(U_c)_j, zj∼⁡(0,1)z_j\! \!N(0,1) ⊳ Eq. 1 8: end for 9: ℒ←ℒnew()+λℒreplay(~)L _new(x)+ _replay( x) ⊳ Eq. 2 10: g,head←SGD⁡(ℒ)g,\,head (L) 11: t←pinv⁡(C)P_t (C)\,A ⊳ closed form 12: Inference ⊳ no gradients 13: j←ℓ2(tg(f⁡(aj)))q_j← _2(P_t\,g(f(a_j))), c←ℓ2(tg(c)) μ_c← _2(P_t\,g( μ_c)) 14: for TotT_ot iterations do 15: Mjc←1−j⊤cM_jc← 1-q_j μ_c, ←Sinkhornε(M)R _ (M) 16: c←ℓ2(c+∑jRjcj) μ_c← _2\! ( μ_c+ _jR_jcq_j ) ⊳ Eq. 2.5 17: end for 18: return y^j=arg⁡maxc⁡cos⁡(j,c) y_j= _c (q_j, μ_c) Table 5: Component ablation (A / PD, 50 seeds). Configuration NSynth-100 FSC-89 A↑ PD↓ A↑ PD↓ TAPE (baseline) 93.5 5.6 69.2 20.0 + Adapter 95.5 4.3 70.0 19.0 + Transport (OT) 95.8 4.7 70.5 19.9 w/ Gaussian replay 95.8 4.6 70.8 19.3 w/ subspace replay 96.5 3.2 71.1 18.8 Ablations. (i) Component contribution. Table 5 decomposes SPECTRA. Removing the adapter gives up the largest single drop in A and already lowers PD, so calibrating the frozen embedding is the main driver; optimal transport then adds accuracy at a small PD cost (discussed below); subspace replay yields the best A and the lowest PD. (i) Does the replay structure matter? We compare our low-rank replay (1) against (i) no replay and (i) an isotropic Gaussian rehearsal of matched total variance, ~c=c+ϵ x_c= μ_c+ ε, ϵ∼⁡(0,τ2d) ε\! \!N(0,τ^2I_d), τ2=1d∑jσj2τ^2= 1d _j _j^2. Subspace replay beats Gaussian replay on 2 datasets and both metrics. On NSynth-100 Gaussian replay barely improves over no replay, isolating the anisotropic, low-rank structure—not the act of replaying—as the source of the anti-forgetting effect. (i) Adapter capacity. Sweeping the expansion r∈2,3,4,8r\!∈\!\2,3,4,8\ moves A by less than the seed standard deviation on both datasets; we use r=3r=3, the smallest value on this plateau. (iv) Transport assignment. Optimal transport is the one component with a trade-off: across the chain it raises A (+0.3+0.3/+0.5+0.5) but increases PD (+0.4+0.4/+0.9+0.9). Subspace replay more than compensates, yielding the best PD overall; we therefore retain OT for its accuracy gain while replay supplies the anti-forgetting. 4 Conclusion We presented SPECTRA for fully few-shot class-incremental audio classification: a trainable adapter that calibrates frozen audio–language embeddings, exemplar-free subspace feature replay that fights forgetting by sampling old classes from their low-rank feature subspaces, and a transductive optimal-transport refinement. On three FFCAC benchmarks SPECTRA improves average accuracy and reduces forgetting over TAPE the current state of the art, and our ablations statistically establish that the subspace structure of the replay—not replay alone—drives the gain. Future work includes coupling the transport plan with the replay subspaces to further stabilise noisier datasets. References [1] P. Anuvind, C. Abhishek, C. Rahila, K. Neethu, et al. 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