APRIL: Ambiguity-Aware Expert Routing for Respiratory Sound Domain-Incremental Learning

Xinhong Li; Weiping Tu

cs.SDOct 20, 2026

APRIL: Ambiguity-Aware Expert Routing for Respiratory Sound Domain-Incremental Learning

Authors: Xinhong Li, Weiping Tu

Organizations: School of Computer Science, Wuhan University, Wuhan, Hubei, China

Abstract

Automated respiratory sound classification enables continuous, non-invasive respiratory assessment, yet long-term deployment remains challenging due to domain shifts from evolving recording devices and acquisition periods. When privacy and governance regulations prohibit replaying historical data, sequential learning easily overwrites previously acquired knowledge. Preserving dedicated domain experts prevents parameter interference, but selecting the correct expert at inference is difficult under overlapping acoustic distributions and absent domain labels. To resolve this dilemma, we propose APRIL, an exemplar-free domain-incremental framework for RSC. Built on a shared frozen BEATs backbone, APRIL sequentially inherits and isolates low-rank experts to enable new-domain adaptation without altering earlier models. A multi-level prototype router retrieves the top-two candidate experts and dynamically selects the final model by integrating domain-routing posteriors with classification confidence, adaptively weighted by matching ambiguity. We establish standardized device- and temporal-incremental benchmarks on the ICBHI and SPRSound datasets across forward and reversed domain orders. Evaluated over five random seeds, APRIL consistently achieves the highest final-session average Score among compared continual learning approaches, reaching 64.4% and 65.4% on ICBHI, and 83.1% and 82.2% on SPRSound under forward and reversed sequences, respectively. Crucially, average forgetting remains under 1.0% across all protocols, demonstrating robust domain adaptation and long-term knowledge retention without data replay. Overall, APRIL offers a reliable and privacy-preserving paradigm for deploying continuous respiratory sound classification across real-world healthcare settings.

Figures & tables

Panel of Figure 1 — Figure 1: Catastrophic forgetting under sequential fine-tuning. Each curve reports the Score on a previously learned domain after successive domain updates. Lines and shaded regions denote the mean and standard deviation over five seeds, respectively. $\Delta$ denotes the change between the stage at which a domain is first learned and the final stage.

Figure 2: Overview of APRIL. Each new domain expert inherits the preceding expert’s LoRA adapter and classification head, while the shared backbone and earlier experts remain frozen. During inference, multi-level prototype matching retrieves two candidate experts, and routing posterior and classifier confidence jointly determine which candidate’s prediction is returned.

Table 1: Summary of the domain-incremental protocols. Each entry is reported as training/test count. Patient totals count unique subjects, and the global training and test patient sets are disjoint.

Method	Year	Forward order				Reversed order
Method	Year	FSA ( $\uparrow$ )	AF ( $\downarrow$ )	BWT ( $\uparrow$ )	OS ( $\uparrow$ )	FSA ( $\uparrow$ )	AF ( $\downarrow$ )	BWT ( $\uparrow$ )	OS ( $\uparrow$ )
LwF	2016	62.58 $\pm$ 2.13	6.97 $\pm$ 3.69	-6.36 $\pm$ 3.97	59.98 $\pm$ 1.60	65.19 $\pm$ 0.91	2.60 $\pm$ 1.43	-0.30 $\pm$ 2.50	60.21 $\pm$ 0.54
EWC	2017	55.63 $\pm$ 1.54	14.20 $\pm$ 2.25	-14.20 $\pm$ 2.25	57.04 $\pm$ 1.11	64.79 $\pm$ 1.56	4.53 $\pm$ 2.26	-3.69 $\pm$ 3.47	59.43 $\pm$ 0.41
ER	2019	58.96 $\pm$ 2.31	11.16 $\pm$ 2.60	-11.09 $\pm$ 2.49	60.18 $\pm$ 1.06	64.41 $\pm$ 1.18	3.25 $\pm$ 1.53	-2.71 $\pm$ 1.49	61.02 $\pm$ 0.90
DER++	2020	56.68 $\pm$ 2.12	12.60 $\pm$ 2.20	-12.22 $\pm$ 2.68	58.69 $\pm$ 0.61	64.19 $\pm$ 1.89	2.81 $\pm$ 1.69	-2.21 $\pm$ 1.98	60.72 $\pm$ 1.91
InfLoRA	2024	59.39 $\pm$ 2.41	6.12 $\pm$ 4.77	-5.55 $\pm$ 4.46	57.23 $\pm$ 1.70	62.20 $\pm$ 1.49	0.89 $\pm$ 0.58	1.74 $\pm$ 2.43	58.36 $\pm$ 1.71

Table 2: Performance comparison on ICBHI under forward and reversed device orders. Results are mean

\pm

standard deviation over five seeds. Bold and underlined means indicate the best and second-best results among continual-learning methods, respectively. Seq. FT and Joint Train. denote sequential fine-tuning and joint training, respectively.

Figure 3: Forgetting dynamics on ICBHI and SPRSound. Columns (a)–(d) show ICBHI forward/reverse and SPRSound forward/reverse, respectively. Rows report stage-wise AF, BWT, and final domain-wise changes

\Delta\mathrm{Score}_{j}=R_{T-1,j}-R_{j,j}

. Values are five-seed means; shading and error bars show one standard deviation for APRIL.

Method	Forward order				Reversed order
Method	FSA ( $\uparrow$ )	AF ( $\downarrow$ )	BWT ( $\uparrow$ )	OS ( $\uparrow$ )	FSA ( $\uparrow$ )	AF ( $\downarrow$ )	BWT ( $\uparrow$ )	OS ( $\uparrow$ )
ICBHI
w/o isolation	53.74 $\pm$ 2.09	16.50 $\pm$ 3.27	-16.35 $\pm$ 3.17	59.15 $\pm$ 1.15	62.62 $\pm$ 0.80	4.56 $\pm$ 1.71	-3.98 $\pm$ 1.52	56.38 $\pm$ 0.84
Fre. / Fre.	63.03 $\pm$ 0.46	0.97 $\pm$ 0.32	-0.97 $\pm$ 0.32	60.27 $\pm$ 0.71	61.64 $\pm$ 0.63	0.41 $\pm$ 0.19	-0.34 $\pm$ 0.26	59.14 $\pm$ 1.22
Inh. / Fre.	63.08 $\pm$ 0.80	0.96 $\pm$ 0.33	-0.96 $\pm$ 0.33	59.86 $\pm$ 0.81	63.04 $\pm$ 0.61	0.29 $\pm$ 0.09	0.21 $\pm$ 0.27	59.77 $\pm$ 1.39
Inh. / Inh.	64.43 $\pm$ 0.85	0.37 $\pm$ 0.14	-0.32 $\pm$ 0.25	61.39 $\pm$ 0.78	65.41 $\pm$ 0.56	0.33 $\pm$ 0.32	0.02 $\pm$ 0.32	61.27 $\pm$ 1.16

Table 4: Expert isolation and initialization ablation. w/o isolation sequentially updates a shared head and LoRA adapter; other rows use isolated experts. Method names specify head/LoRA initialization: Fre. denotes fresh initialization (random head or standard LoRA initialization), and Inh. denotes inheritance from the preceding expert. Best means are bold.

Dataset	Order	Routing features	Expert selection	FSA ( $\uparrow$ )	AF ( $\downarrow$ )	BWT ( $\uparrow$ )	OS ( $\uparrow$ )
ICBHI	Forward	Final layer	Hard	64.09 $\pm$ 0.92	0.39 $\pm$ 0.20	-0.32 $\pm$ 0.29	61.02 $\pm$ 0.91
		Multi-level	Hard	64.48 $\pm$ 0.83	0.29 $\pm$ 0.11	-0.23 $\pm$ 0.21	61.41 $\pm$ 0.79
		Final layer	Top-2 confidence	64.11 $\pm$ 0.92	0.39 $\pm$ 0.20	-0.32 $\pm$ 0.29	61.09 $\pm$ 0.94
		Multi-level	Top-2 confidence	64.43 $\pm$ 0.85	0.37 $\pm$ 0.14	-0.32 $\pm$ 0.25	61.39 $\pm$ 0.78
	Reverse	Final layer	Hard	65.17 $\pm$ 0.38	0.18 $\pm$ 0.10	0.43 $\pm$ 0.24	61.16 $\pm$ 1.02
		Multi-level	Hard	65.43 $\pm$ 0.55	0.29 $\pm$ 0.31	0.09 $\pm$ 0.36	61.28 $\pm$ 1.16

Table 5: Factorial ablation of routing features and expert selection on ICBHI and SPRSound under forward and reversed domain orders. Hard selects the top-ranked expert. Top-2 confidence selects between the two top-ranked experts using routing and classifier confidence.

Dataset	Order	Top-1 Acc. (%)	Top-2 Recall (%)	Domain-oracle FSA $\uparrow$	Routed FSA $\uparrow$	Gap $\downarrow$
ICBHI	Forward	83.60	93.53	65.16 $\pm$ 0.74	64.43 $\pm$ 0.85	0.72 $\pm$ 0.45
ICBHI	Reverse	83.49	93.27	65.81 $\pm$ 0.50	65.41 $\pm$ 0.56	0.41 $\pm$ 0.23
SPRSound	Forward	55.98	83.66	82.92 $\pm$ 0.39	83.12 $\pm$ 0.32	-0.20 $\pm$ 0.18
SPRSound	Reverse	55.90	83.63	82.13 $\pm$ 0.43	82.23 $\pm$ 0.91	-0.11 $\pm$ 0.79

Table 6: Routing diagnostics. Top-1 accuracy and Top-2 recall indicate whether the true domain is retrieved first or among two candidates. Domain-oracle uses the corresponding domain expert. Gap is domain-oracle FSA minus routed FSA, computed per seed. FSA and Gap are mean

\pm

standard deviation over five seeds. Domain labels are used only for diagnostics.

Figure 4: t-SNE visualization of multi-level domain-matching scores under the forward order. The same recordings are colored by their true domains (left) and selected experts (right). Paired insets magnify the same region; black outlines mark recordings assigned to an expert from a different domain.

Dataset	Bank params. (M)	Storage (MiB)	Latency (ms/sample)
ICBHI	2.372	9.12	4.81 $\pm$ 0.01
SPRSound	2.365	9.09	4.97 $\pm$ 0.04

Table 7: Deployment cost of APRIL using Top-2 confidence selection. Latency is measured with a batch size of 32 and reported over three repetitions. Storage excludes the shared frozen backbone.

Figure 5: Sensitivity to prototype count

K

. Curves and shading show five-seed means and standard deviations.

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Automated respiratory sound classification (RSC) aims to identify normal and adventitious respiratory sounds, such as crackles and wheezes. Most existing methods formulate RSC as a flat four-class classification problem over norma, crackle, wheeze and both. However, unlike conventional classification tasks with semantically independent and mutually exclusive categories, the commonly used four-class RSC labels have a compositional structure and can be represented by two clinically meaningful sound attributes: crackle presence and wheeze presence. Existing flat classifiers do not explicitly exploit this structure, which may increase the learning burden by treating the four labels as independent targets. Meanwhile, crackles and wheezes exhibit distinct temporal characteristics, with sparse transient crackle events and sustained continuous wheeze patterns, making a single temporal aggregation strategy less suitable for modeling different adventitious sound evidence. To address these limitations, we propose an Acoustic-Aware Asymmetric Framework (AAAF) for RSC. AAAF reformulates four-class recognition as two sound-attribute detection tasks and recovers the final category through logical composition during inference. Built upon a pretrained BEATs encoder, AAAF employs branch-specific multi-layer fusion and asymmetric temporal aggregation to capture localized crackle evidence and sustained wheeze patterns. Auxiliary normal--abnormal supervision and a batch-adaptive dynamic loss are further introduced to improve global discrimination and robustness under imbalanced labels. Experiments on ICBHI and SPRSound demonstrate that AAAF improves overall RSC performance and generalizes across datasets, achieving a state-of-the-art four-class Score of 65.30% on ICBHI.

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May 28, 2026

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Abstract

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