Learnable Spectral Activations

Tamir Shor; Or Litany; Alex Bronstein

cs.LGOct 5, 2026

Learnable Spectral Activations

Authors: Tamir Shor, Or Litany, Alex Bronstein

Organizations: Technion – Israel Institute of Technology, Haifa, Israel · NVIDIA · Institute of Science and Technology Austria (ISTA), Klosterneuburg, Austria

Abstract

Implicit neural representations (INRs) are shaped by the spectral structure induced by their input encodings and activation functions. Existing methods improve fitting primarily by modifying which frequencies are available to the network, through coordinate encodings or periodic nonlinearities. However, frequency access is not the only bottleneck: signals with localized or spatially varying structure require the network to efficiently compose frequencies into multi-harmonic internal responses. We introduce learnable spectral activations (LSA), which replace fixed neuron-level nonlinearities with a residual truncated Fourier series whose harmonic amplitudes are learned during training. LSA does not expand the asymptotic function class. Instead, it changes the factorization of the representation: linear weights select features while activation coefficients control spectral shaping, and the two are updated by separate gradients. Because the activation output is affine in the coefficients given fixed pre-activations, spectral tuning becomes a more direct subproblem compared to architectures where it is entangled with feature selection. Empirically, this factorization concentrates more target-signal energy in the leading eigenmodes of the neural tangent kernel, consistent with improved optimization behavior. Across audio, image, neural radiance field, and neural acoustic field tasks, LSA also improves reconstruction quality.

Figures & tables

Panel of Figure 1 — Figure 1 : (a) Operator families and parameter efficiency. Dashed curves show individual atoms and solid curves show the fitted signal. SIREN and FINER use prescribed single-unit responses, whereas LSA synthesizes multi-harmonic waveforms and reaches accurate fits with fewer atoms. (b) Target energy concentration in leading empirical NTK modes. LSA places more target energy in the top modes than SIREN or FINER, indicating better alignment between the induced dictionary and the target.

Figure 2 : SIREN/FINER/LSA comparison of on-grid audio fitting across Fourier-feature bandwidths

\sigma

. LSA achieves the strongest performance at moderate and high bandwidths on both speech and musical-note signals, indicating that learned spectral activations improve the internal processing of Fourier-feature harmonics.

Figure 3 : On-grid vs. off-grid reconstruction under increasing Fourier-feature bandwidth. SIREN exhibits a large on-grid/off-grid gap already at

\sigma=0

and severe interpolation artifacts for

\sigma=100

. Our method remains stable under inference of interpolated, unseen grid points.

Figure 4 : Per-image Kodak fitting comparison under fixed configurations. Per-image PSNR is shown for LSA, FINER, SIREN, and STAF under 5,000-update fits.

Figure 5 : Poisson reconstruction from gradient-only supervision. Our method produces more consistent gradient fields and integrates them into a coherent image, while FINER and SIREN exhibit structured residuals and global reconstruction errors.

Figure 6 : Qualitative Kodak reconstructions for illustrative examples kodim21 (left) and kodim13 (right), selected for qualitative clarity. LSA, FINER, and SIREN use the fixed configurations in Figure 4 . We show reconstructions, spatial residuals, and spectral residuals, with a common display scale for each residual type across both images and all methods. Full-dataset comparisons are in Figure 4 and Section C.1 .

Metric	Method	Chair	Drums	Ficus	Hotdog	Lego	Materials	Mic	Ship
PSNR $\uparrow$	SIREN [ 2 ]	33.48	24.89	28.22	32.85	29.60	27.13	33.28	22.48
	FINER [ 24 ]	33.90	24.90	28.70	33.05	30.04	27.05	33.96	22.47
	Ours	34.18	24.79	28.85	33.36	29.39	27.22	33.91	22.60
SSIM $\uparrow$	SIREN [ 2 ]	0.973	0.912	0.959	0.960	0.948	0.932	0.979	0.797
	FINER [ 24 ]	0.973	0.911	0.958	0.959	0.951	0.928	0.981	0.792
	Ours	0.977	0.922	0.967	0.967	0.950	0.936	0.983	0.803

Table 1 : Controlled activation comparison on novel view synthesis, over Blender [ 3 ] scenes.

Figure 7 : Spatial improvement of LSA over the NAF baseline. Each point corresponds to a spatial query in the scene, and color indicates relative improvement. LSA yields broad spatial gains, especially for log-spectral and multi-resolution STFT metrics, across the queried locations.

Appendix figures & tables31 assets

Supplementary material from the paper’s appendix.

Appendix

Quantity	Setting
Datasets	LibriSpeech fixed 40-clip manifest. NSynth fixed 10-clip manifest
Sampling rate	48 kHz
Samples per clip	$N=48{,}000$ samples, one-second window
Channels	Mono waveform fitting
Coordinate range	$t\in[-1,1]$
Output dimension	1 waveform amplitude

Appendix

Table 2 : Audio fitting hyperparameters. FF16 denotes

M=16

Gaussian Fourier-feature bands. The selected

\sigma

values are those used for the main audio fitting results. Full

\sigma

-sweep curves are reported separately.

$\alpha/\alpha_{0}$	0.10	0.03	0.01
Mean $R(\alpha)$	0.806	0.942	0.981

Appendix

Table 3 : One-step validation on six Kodak images and three initializations per image. The normalization in Equation 26 tests the local descent prediction.

Figure 8 : Qualitative waveform fits on LibriSpeech and NSynth. Each column compares the target with a reconstruction and its residual on the original audio scale. Learned spectral activations closely match transient and harmonic waveform structure, leaving substantially smaller residuals than FINER and SIREN.

Figure 9 : Full NSynth on-grid vs. off-grid diagnostic across bandwidth settings.

Figure 10 : Convergence curves for all 24 Kodak images under fixed LSA, FINER, and SIREN configurations and matched 5,000-update budgets. LSA separates from SIREN early, while its advantage over FINER emerges progressively. Early-stage behavior varies across images.

Figure 11 : Qualitative novel view synthesis results. We show target views, reconstructions, and absolute RGB residuals for representative scenes. Learned spectral activations reduce structured residuals around fine object boundaries and high-detail regions compared to SIREN and FINER.

	Macro-scene aggregation			Micro-position aggregation
Metric	Baseline	LSA	Improvement	Baseline	LSA	Improvement
PSNR from MSE $\uparrow$	11.417	11.846	+0.429 / +3.76%	11.550	11.993	+0.443 / +3.84%
MSE $\downarrow$	0.07238	0.06572	+0.0067 / +9.20%	0.07	0.063	+0.0068 / +9.71%
Log Spectral L1 $\downarrow$	4.053	2.233	+1.820 / +44.90%	3.981	1.957	+2.024 / +50.84%
MRSTFT Log-L1 $\downarrow$	4.008	2.165	+1.842 / +45.97%	3.925	1.902	+2.023 / +51.54%
TDOA $\downarrow$	122.406	39.727	+82.679 / +67.55%	114.675	31.634	+83.041 / +72.41%

Appendix

Table 4 : Neural acoustic field results. We replace the activation in the NAF model with LSA (

K=8

) while keeping the architecture and training protocol fixed. Improvements are signed so that positive values indicate better performance.

Figure 12 : Spatial improvement of LSA over the NAF baseline. frl apartment scene 4

Figure 13 : Spatial improvement of LSA over the NAF baseline. Apartment Scene 1

Figure 14 : Spatial improvement of LSA over the NAF baseline. Apartment Scene 2.

Figure 15 : Spatial improvement of LSA over the NAF baseline.Room Scene.

Figure 16 : Spatial improvement of LSA over the NAF baseline. Office Scene

Figure 17 : Sparse context $\rightarrow$ dense reconstruction under different positional encodings. Top: reconstructions. Bottom: absolute error maps.

Method	Candidates/image	PSNR (dB)	SSIM	PSNR wins
LSA	3	38.308 $\pm$ 2.849	0.9568 $\pm$ 0.0111	24/24
STAF	56	35.425 $\pm$ 2.190	0.9209 $\pm$ 0.0119	0/24

Appendix

Table 5 : Per-image Kodak tuning. Values are mean and standard deviation across 24 images. Both methods select their configurations separately for each image.

Method	Parameters	Holdout PSNR	All-24 PSNR	LSA wins
LSA	199,043	38.045	38.110	-
SL 2 A, full	330,243	37.857	37.298	10/18
SL 2 A, parameter matched	199,171	35.910	35.995	18/18
SL 2 A, author protocol	330,243	37.463	37.541	15/18

Appendix

Table 6 : SL 2 A comparison on Kodak. The 18-image holdout is the primary comparison after calibration. The 24-image means also include the six calibration images. Wins refer to LSA relative to each row on the holdout.

Method	Holdout PSNR	LSA gain (dB)	95% interval	LSA wins
LSA	38.045	-	-	-
fJNB	22.985	15.060	[14.595, 15.559]	18/18
WIRE	36.477	1.568	[1.143, 1.970]	17/18
MFN GaborNet	35.086	2.959	[2.764, 3.157]	18/18

Appendix

Table 7 : Additional Kodak holdout results after six-image calibration. Confidence intervals are paired bootstrap intervals for LSA’s mean PSNR gain. Means and paired gains are rounded independently.

Method	Mean PSNR (dB)
SIREN	34.990
FINER	37.317
STAF	35.146
WIRE	36.596
MFN GaborNet	35.151
fJNB	22.948

Appendix

Table 8 : Descriptive Kodak results over all 24 images with globally selected configurations. Calibration images are included. STAF, WIRE, and MFN receive up to 10,000 updates. LSA, SIREN, and FINER use 5,000.

Image	LSA PSNR	STAF PSNR	LSA SSIM	STAF SSIM
kodim01	35.671	32.973	0.9562	0.9240
kodim02	39.563	35.636	0.9529	0.8902
kodim03	42.714	38.166	0.9737	0.9321
kodim04	39.194	35.999	0.9563	0.9086
kodim05	35.157	31.669	0.9605	0.9157
kodim06	38.192	34.668	0.9614	0.9256

Appendix

Table 9 : Per-image STAF comparison after selecting from 56 STAF and three LSA configurations per image.

Input	Terms $\tau$	NSynth	LibriSpeech
Raw coordinates	5	$68.79\pm 3.04$	$68.40\pm 6.29$
FF16, $\sigma=20$	5	$55.11\pm 11.63$	$50.24\pm 1.38$
FF16, $\sigma=100$	5	$51.83\pm 4.47$	$51.85\pm 1.28$
FF16, $\sigma=100$	16	$50.60\pm 2.44$	$51.21\pm 1.43$

Appendix

Table 10 : STAF audio configurations. Entries are mean PSNR and standard deviation across clips. The raw-coordinate setting uses first-layer

\omega_{0}=3000

Degree	Rank	Parameters	NSynth	LibriSpeech
512	128	329,473	$68.03\pm 10.20$	$47.42\pm 6.22$
512	64	231,169	$67.16\pm 10.57$	$46.35\pm 6.62$
256	128	263,937	$59.16\pm 16.05$	$38.84\pm 7.18$
256	64	165,633	$59.47\pm 15.35$	$38.37\pm 7.28$

Appendix

Table 11 : SL 2 A audio results for the tested degree and rank settings. Values are mean PSNR and standard deviation across clips. All rows use learning rate

10^{-3}

Dataset	Baseline	Input	Baseline PSNR	LSA PSNR	Gain	Wins
NSynth	fJNB	Raw	16.45	47.41	30.96	9/10
LibriSpeech	fJNB	Raw	25.23	31.96	6.73	31/40
NSynth	fJNB	FF16, $\sigma=100$	55.09	86.72	31.63	10/10
LibriSpeech	fJNB	FF16, $\sigma=100$	43.97	90.78	46.81	40/40
NSynth	WIRE	Selected	47.11	86.72	39.61	10/10
LibriSpeech	WIRE	Selected	51.79	90.78	38.99	40/40

Appendix

Table 12 : Additional audio fitting comparisons. Gains and wins refer to LSA relative to the specified baseline on the same clips. fJNB uses the fractional Jacobi construction.

Scene	WIRE (published)	fJNB (adapted)	LSA
Chair	29.31	30.15	34.18
Drums	22.22	18.08	24.79
Ficus	25.91	22.24	28.85
Hotdog	30.11	31.81	33.36
Lego	25.76	22.67	29.39
Materials	25.05	24.73	27.22

Appendix

Table 13 : Additional Blender PSNR results. WIRE is reproduced from Liu et al. [24] . fJNB and LSA use the shared-backbone evaluation described above. Means are across eight scenes.

$K$	Mean PSNR	Std. PSNR	Median PSNR	Mean SSIM
4	30.44899	5.28322	29.17142	0.81556
8	35.35890	4.63919	34.52907	0.92290
16	38.67377	4.34150	40.33058	0.96070
32	39.50432	4.32379	41.38003	0.96614
64	39.34945	4.14439	40.49803	0.96494

Appendix

Table 14 : Harmonic-count ablation on six Kodak images. Standard deviations and medians are computed across images.

Evaluation	Coefficients	PSNR	SSIM	LSA wins	Gain 95% interval
18 holdout	Learned	$38.05\pm 2.63$	0.9545	18/18	$[17.45,18.87]$
18 holdout	Frozen	$19.91\pm 1.78$	0.4861
24 pooled	Learned	$38.11\pm 2.90$	0.9557	24/24	$[17.56,19.17]$
24 pooled	Frozen	$19.76\pm 1.95$	0.4834

Appendix

Table 15 : Learning versus freezing the near-identity coefficients. PSNR is mean and standard deviation across images. Confidence intervals refer to the paired mean PSNR gain.

Figure 18 : Hidden-representation diagnostics on image fitting. For each method, we visualize layer-wise RMS hidden activity, the two highest-variance signed hidden channels, and the radial spectra of those selected channels. LSA produces more spatially localized and less purely periodic hidden features than SIREN and FINER, supporting the view that learned spectral activations modify the internal basis rather than only the final reconstruction.

Figure 19 : Learned LSA coefficient spectra across Kodak images. Each panel shows $\log_{10}(|a_{k}^{(\ell)}|+10^{-8})$ for harmonic index $k$ and layer $\ell$ . The learned operators exhibit layer-dependent harmonic profiles and image-dependent variation, indicating that LSA adapts its internal spectral basis rather than using a fixed spectrum.

Figure 20 : Learned activation coefficient spectra across Kodak images. Each panel shows the magnitude

|a_{k}^{(\ell)}|

of the learned harmonic coefficients for one image and layer. The coefficients exhibit layer-dependent profiles and image-dependent variation, indicating that LSA learns nontrivial spectral operators rather than using a fixed harmonic profile.

Figure 21 : Learned activation waveforms across Kodak images. Each panel plots the resulting activation

\phi^{(\ell)}(u)

for one image and layer. Although the learned functions remain close to the residual identity shape, they contain localized harmonic deviations whose form varies across layers and fitted images.

Depth	Method	Params	Extra act. params	Train time (ms)	Train mem. (GB)	Eval time (ms)	Eval mem. (GB)
4	SIREN FF16	206,337	0	8.26	0.48	3.56	0.16
	FINER FF16	206,337	0	14.20	0.85	5.55	0.21
	LSA FF16, $K=16$	206,401	64	97.93	7.53	35.08	1.58
6	SIREN FF16	337,921	0	12.77	0.67	5.46	0.16
	FINER FF16	337,921	0	21.65	1.22	8.45	0.21
	LSA FF16, $K=16$	338,017	96	147.84	10.56	52.87	1.58

Appendix

Table 16 : Resource usage under the matched FF16 input regime on full-grid 48k audio fitting. All models use width 256. Training time includes forward, backward, and optimizer update. Evaluation time is forward-only. Memory is peak CUDA allocation.

Depth	$K$	Train time (ms)	Train mem. (GB)	Eval time (ms)	Eval mem. (GB)
4	8	52.78	3.88	19.24	0.85
	16	98.57	7.53	35.33	1.58
	32	189.87	14.87	67.26	3.05
	64	341.09	29.55	115.23	5.98
6	8	79.13	5.44	28.77	0.85
	16	147.84	10.56	52.87	1.58

Appendix

Table 17 : LSA scaling with the number of harmonic terms

K

under the matched FF16 full-grid 48k setting. All models use width 256. The implementation is the batched PyTorch harmonic evaluation used in Table 16 .

$K$	Implementation	Train time (ms)	Train mem. (GB)	Eval time (ms)	Eval mem. (GB)
8	Batched	6.63	0.35	2.26	0.09
	Accumulate	6.98	0.30	2.35	0.04
16	Batched	11.43	0.66	4.08	0.15
	Accumulate	13.13	0.55	4.40	0.04

Appendix

Table 18 : Implementation diagnostic for LSA in a no-FF, small-batch setting (

B=4096

, width 256, depth 4, fp32). The accumulate implementation avoids explicitly materializing the full harmonic tensor, reducing memory at the cost of additional runtime.

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