spherical-array-processing 0.5.0

Pythonic spherical array processing toolkit — Ambisonics, spherical arrays, beamforming, DOA

spherical-array-processing

A self-contained Python toolkit for spherical microphone array signal processing.

This repository is organised as an open-source Python package plus a repository-only reproduction workbench for MATLAB parity checks. The stable installable package lives under spherical_array_processing/. The scripts/, examples/, and src/ directories are development and reproduction assets used by the repository test suite; they are intentionally not part of the runtime wheel.

Features

• Spherical harmonics — complex and real SH basis matrices, ACN/orthonormal normalisation, ACN↔degree/order helpers, forward and inverse SHTs, coefficient conversions, and Wigner-D rotation of SH-domain signals with both the classical Sakurai summation and a numerically-stable J_y-eigendecomposition backend (method="jy") for high-order rotations.
• Spatial sampling — Fibonacci, Gauss-Legendre (exact quadrature), and t-design grids with integration weights.
• Array simulation — free-field plane-wave transfer functions and closed-form modal-coefficient simulators for open / rigid / cardioid / arbitrary-α directional spherical arrays (simulate_sh_array_response).
• Array presets — ready-to-use geometries for the Eigenmike-32 rigid-sphere array, tetrahedral A-format 4-mic, cubic 8-mic, and evenly-spaced circular arrays.
• Fixed beamforming — cardioid, hypercardioid (max DI), supercardioid (max front-back ratio), MaxEV, Butterworth, and spherical Dolph-Chebyshev weight vectors.
• Adaptive beamforming — MVDR and LCMV in SH domain.
• DOA estimation — narrow-band PWD and MUSIC spatial spectra, broadband SRP / SRP-PHAT (mic-domain and covariance-based), closed-form spherical ESPRIT (esprit_doa), and AIC / MDL source-count estimators (estimate_n_sources).
• Ambisonic encoding — typed AmbisonicSpec / AmbisonicFrame containers, plane-wave source encoding (ambi.encode_plane_wave, ambi.encode_plane_wave_frame), order/channel helpers and signal health reports (ambi.per_order_energy, ambi.ambisonic_signal_report), Tikhonov and WNG-limited inverse filters (encoding.radial_equalizer) for turning raw microphone SHT into plane-wave-steering signals, plus measured-steering-matrix encoders (encoding.measured_array_equalizer, regLS / regLSHD) with encoding.measured_array_diagnostics for condition, WNG, gain, and reconstruction-error reporting.
• Ambisonic decoding — sad, mmd / mad, epad, and allrad (VBAP-based) loudspeaker decoders plus the unified decoder_matrix entry point, dual-band (D_lf, D_hf) decoders with max-rE tapering (dual_band_decoder_matrix / apply_dual_band_decoder), general frequency-dependent decoder banks (frequency_dependent_decoder_matrix) with max_re / in_phase / no-taper policies, layout helpers (layout_itu_5_1, layout_itu_7_1_4, layout_t_design), decoder diagnostics (decoder_diagnostics), and imaginary-loudspeaker hull closure (suggest_imaginary_loudspeakers) for hemispherical / partial layouts.
• Binaural rendering — Magnitude Least Squares (MagLS, Schörkhuber-Zotter 2018) and its bilateral-Ambisonics extension (BiMagLS, Engel et al. 2021) that time-aligns HRTFs around each ear for better low-order reproduction (binaural.magls_binaural_filters, binaural.bimagls_binaural_filters), plus an end-to-end one-call renderer binaural.ambi_to_binaural_time_domain with optional head-tracking. HRTFs can be loaded from SOFA AES69 SimpleFreeFieldHRIR files via hrtf.load_sofa (requires the optional [hrtf] extra) or synthesised analytically with hrtf.rigid_sphere_hrtf for testbed scenarios.
• Ambisonic format converters — ambi.convert_ambi_normalization (N3D ↔ SN3D ↔ orthonormal) and ambi.acn_to_fuma / ambi.fuma_to_acn for the AmbiX ↔ legacy B-format round trip through third order, plus raw-array and typed-frame AmbiX WAV interop (ambi.read_ambix_wav, ambi.write_ambix_wav, ambi.read_ambix_frame, ambi.write_ambix_frame) and ambi.nfc_hoa_distance_filter for near-field distance compensation.
• Room convolution — room.convolve_mono_to_ambi (dry-mono → ambi-reverb via an SH-RIR, e.g. from shoebox_sh_rir) and room.convolve_sh_to_sh (channel-diagonal SH-RIR application).
• Acoustic metrics — ISO 3382-family descriptors (RT60 via T20/T30/T60 regression, EDT, C50, C80, D50, EDC) via room.rir_metrics or the individual room.reverberation_time / room.clarity / room.definition / room.early_decay_time / room.energy_decay_curve.
• Statistical room acoustics — Sabine, Eyring, Millington-Sette, and Arau-Puchades RT60 prediction, equivalent absorption area, room constant, critical distance, Schroeder frequency, rectangular-room modes, and ISO 9613-1 atmospheric absorption coefficient utilities via room.* / acoustics.*.
• Frequency-dependent shoebox — room.shoebox_rir_banded with per-wall per-octave reflection magnitudes for realistic spectral decay modelling.
• Plane-wave encoding — ambi.encode_plane_wave to turn one or more monaural source signals into an ambisonic mixdown at the chosen directions.
• DirAC parametric audio — per-bin DOA / diffuseness analysis and loudspeaker resynthesis of SH-domain STFTs (dirac.dirac_analysis, dirac.dirac_synthesize).
• Room simulation — shoebox image-source monaural and Ambisonic RIR generator (room.shoebox_rir, room.shoebox_sh_rir) for reverberant DOA / beamforming test vectors.
• Head-tracking — time-varying Wigner-D rotation of an ambisonic signal with linear crossfade between keyframes (sh.rotate_ambi_over_time).
• Covariance regularisers — Ledoit-Wolf and OAS shrinkage, forward-backward averaging, and trace-relative diagonal loading for robust DOA / adaptive beamforming on short SH records (covariance.*).
• STFT utilities — multichannel (F, M, T) layout wrappers compatible with the SRP and encoding APIs (sap.stft).
• Diffuseness — IE, TV, SV, and CMD estimators from FOA / SH covariance.
• Coherence — sinc-based diffuse-field coherence models.
• Acoustics — spherical Bessel, Neumann, and Hankel functions, wavenumber helpers, modal coefficients for open/rigid/cardioid/directional arrays.
• Plotting — 3-D array geometry, 2-D spatial maps, MATLAB-like figure style.

Installation

pip install spherical-array-processing        # from PyPI (when published)
pip install -e ".[dev]"                        # editable install for development

Python >= 3.9. Core dependencies: numpy, scipy. Install the plotting extra if you want sap.plotting entry points.

Extra	Adds
audio	soundfile for WAV I/O
hrtf	h5py for SOFA HRTF I/O
plotting	matplotlib for visualisation (sap.plotting)
image	scikit-image for image utils
dev	build, pytest, pytest-cov

Quick start

import numpy as np
import spherical_array_processing as sap

# -- Spatial sampling --
grid = sap.array.gauss_legendre_sampling(3)
print(f"weights sum: {grid.weights.sum():.4f}")  # 4*pi

# -- SH basis and transforms --
spec = sap.SHBasisSpec(max_order=3)  # defaults: complex basis, az_colat convention
Y = sap.sh.matrix(spec, grid)            # (32, 16) complex SH matrix
nm = np.random.randn(spec.n_coeffs) + 1j * np.random.randn(spec.n_coeffs)
f = sap.sh.inverse_sht(nm, Y)            # synthesize a band-limited field
nm_rec = sap.sh.direct_sht(f, Y, grid)   # weighted least-squares recovery
print(np.max(np.abs(nm - nm_rec)))       # ~ machine precision on exact quadrature grids

# -- Fixed beamforming --
b = sap.beamforming.beam_weights_hypercardioid(3)
theta = np.linspace(0, np.pi, 181)
pattern = sap.beamforming.axisymmetric_pattern(theta, b)

# -- Array simulation (open-sphere free-field) --
sensor_grid = sap.array.fibonacci_grid(32)
geometry = sap.ArrayGeometry(radius_m=0.042, sensor_grid=sensor_grid)
freqs, H = sap.array.simulate_plane_wave_array_response(
    fft_len=256, fs=16000.0, geometry=geometry,
    source_grid=sap.array.fibonacci_grid(4),
)
# H.shape == (129, 32, 4) — (n_bins, n_mics, n_sources)

# -- Rigid-sphere array simulation via modal coefficients --
em32 = sap.array.em32_eigenmike()
freqs, H_rigid = sap.array.simulate_sh_array_response(
    fft_len=256, fs=16000.0, geometry=em32,
    source_grid=sap.array.fibonacci_grid(1),
    max_order=6, array_type="rigid",
)

# -- SH-domain signal rotation (head-tracking, scene rotation) --
alpha, beta, gamma = 0.3, 0.2, 0.1                    # ZYZ Euler angles
nm_rot = sap.sh.rotate_sh_coeffs(nm, spec.max_order, alpha, beta, gamma)

# -- Radial encoding filters (Tikhonov / WNG-limited) --
from spherical_array_processing.encoding import radial_equalizer
kr_vec = sap.acoustics.kr(freqs, radius_m=em32.radius_m)
eq = radial_equalizer(6, kr_vec, array_type="rigid",
                      regularization="tikhonov", tikhonov_lambda=0.01)

# -- DOA estimation (broadband SRP-PHAT) --
scan = sap.array.fibonacci_grid(2000)
result = sap.doa.srp_map(H[:, :, 0], freqs, geometry, scan,
                          weighting="phat", freq_range_hz=(500, 6000))
print("SRP-PHAT peak direction (rad):", result.peak_dirs_rad[0])

Module reference

Module	Key symbols
sap.sh	matrix, complex_matrix, real_matrix, direct_sht, inverse_sht, complex_to_real_coeffs, real_to_complex_coeffs, acn_index, acn_to_nm, degree_order_pairs, replicate_per_order, wigner_small_d (method={"sakurai","jy"}), wigner_D, sh_rotation_matrix_complex, sh_rotation_matrix_real, rotate_sh_coeffs, rotate_ambi_over_time
sap.array	fibonacci_grid, gauss_legendre_sampling, equiangle_sampling, get_tdesign_fallback, simulate_plane_wave_array_response, simulate_sh_array_response, em32_eigenmike, tetrahedral_array, cubic_array, circular_array
sap.acoustics	besseljs, besseljsd, besselys, besselysd, besselhs, besselhsd, besselhs2, besselhs2d, wavenumber, kr, plane_wave_radial_bn (open/rigid/cardioid/directional with dir_coeff), bn_matrix, sph_modal_coeffs, sabine_rt60, eyring_rt60, millington_sette_rt60, arau_puchades_rt60, schroeder_frequency, critical_distance, rectangular_room_modes, air_absorption_coefficient_iso9613
sap.beamforming	beam_weights_cardioid, beam_weights_hypercardioid, beam_weights_supercardioid, beam_weights_maxev, beam_weights_butterworth, beam_weights_dolph_chebyshev, normalize_axisymmetric_weights, axisymmetric_pattern, mvdr_weights, lcmv_weights
sap.doa	pwd_spectrum, music_spectrum, esprit_doa, srp_map, srp_map_from_covariance, estimate_n_sources, peak_pick_spectrum, peak_pick_spectrum_nms, spatial_spectrum_from_map
sap.encoding	radial_equalizer, radial_equalizer_tikhonov, radial_equalizer_wng_limited, apply_radial_equalizer, measured_array_equalizer, apply_measured_equalizer, measured_array_diagnostics
sap.room	ShoeboxRoom, shoebox_rir, shoebox_sh_rir, shoebox_rir_banded, convolve_mono_to_ambi, convolve_sh_to_sh, rir_metrics, RIRMetrics, energy_decay_curve, reverberation_time, early_decay_time, clarity, definition, shoebox_acoustic_stats, sabine_rt60, eyring_rt60, millington_sette_rt60, arau_puchades_rt60, schroeder_frequency, critical_distance, rectangular_room_modes, air_absorption_coefficient_iso9613, fdn_reverb, fdn_sh_tail
sap.decoding	DecoderConfig, save_decoder_config, load_decoder_config, decoder_matrix, apply_decoder, sad_decoder, mmd_decoder, epad_decoder, allrad_decoder, vbap_gains, layout_from_directions, layout_from_directions_deg, layout_t_design, layout_itu_5_1, layout_itu_7_1_4, suggest_imaginary_loudspeakers, check_layout_coverage, decoder_diagnostics, dual_band_decoder_matrix, apply_dual_band_decoder, frequency_dependent_decoder_matrix, decoder_taper_weights, apply_decoder_taper, max_re_sh_weights, in_phase_sh_weights
sap.binaural	magls_binaural_filters, bimagls_binaural_filters, ambi_to_binaural_time_domain
sap.hrtf	HRTFDataset, load_sofa, save_sofa, rigid_sphere_hrtf
sap.ambi	AmbisonicSpec, AmbisonicFrame, AmbisonicSignalReport, channel_count, infer_order, order_channel_slices, order_channel_mask, per_order_energy, ambisonic_signal_report, convert_ambi_normalization, acn_to_fuma, fuma_to_acn, read_ambix_wav, write_ambix_wav, read_ambix_frame, write_ambix_frame, nfc_hoa_distance_filter, encode_plane_wave, encode_plane_wave_frame, uhj_encode, uhj_decode, intensity_vector, doa_from_intensity, translate_foa
sap.covariance	ledoit_wolf_shrinkage, oas_shrinkage, forward_backward_average, diagonal_loading
sap.dirac	dirac_analysis, dirac_synthesize, dirac_render_time_domain, DirACParameters
sap.stft	stft, istft
sap.diffuseness	diffuseness_ie, diffuseness_tv, diffuseness_sv, diffuseness_cmd, intensity_vectors_from_foa
sap.coherence	diffuse_coherence_matrix_omni, diffuse_coherence_from_weights
sap.coords	sph_to_cart, cart_to_sph, azel_to_az_colat, az_colat_to_azel, unit_sph_to_cart
sap.plotting	plot_mic_array, plot_directional_map_from_grid, apply_matlab_like_style, figure_repro_context

SH conventions

Channel ordering follows ACN (Ambisonic Channel Numbering):

index q = n(n + 1) + m,   n = 0..N,   m = -n..n

Complex SH use orthonormal normalisation. Real SH use the tesseral form. Round-trip complex -> real -> complex is lossless for conjugate-symmetric inputs.

direct_sht returns the weighted least-squares projection onto the chosen SH basis. On exact quadrature grids such as gauss_legendre_sampling, this coincides with the usual weighted projection formula. On approximate grids such as Fibonacci sampling, it is more accurate than treating Y^H W as an exact inverse.

Running tests

For a full repository checkout, run the complete suite:

pytest tests/ -q

The source distribution published to PyPI contains a smaller test subset that does not depend on repository-only MATLAB reference sources or example scripts. Use the full repository checkout when validating MATLAB parity, image regression, or repository-only developer tooling.

Running the bundled examples (after pip install)

Since 0.4.0, a small set of self-contained, stable-API-only demos ships inside the wheel as a regular sub-package at spherical_array_processing.examples. Wheel users can therefore run them directly without cloning the repository:

python -m spherical_array_processing.examples.plane_wave_doa
python -m spherical_array_processing.examples.binaural_em32_to_ears

Programmatic use:

from spherical_array_processing.examples.plane_wave_doa import run_example
out = run_example(az_deg=73.0, col_deg=62.0)
print(out["pwd_error_deg"], out["music_error_deg"])

The repo-side examples/ tree is still the right place for longer, research-flavoured scripts (Rafaely chapter figures, Politis comparisons) that depend on repository-only reference assets and the spherical_array_processing.repro developer-only layer. Those are intentionally not part of the runtime wheel.

Development and release checks

python -m pip install -e ".[dev]"
python -m pytest -q --tb=short
python -m build

Before publishing, verify both the wheel and source distribution from a clean environment. The CI workflow runs the full repository suite on supported Python versions and separately checks that the source distribution can run its packaged test subset.

An optional external numeric cross-check is available for validating core mathematical contracts against independent open-source packages:

python scripts/external_numeric_audit.py --repo-root /tmp/sap-cross-repos

MATLAB source provenance

Some repository-only reproduction tests compare Python behavior against Rafaely and Politis MATLAB reference materials. Those files are kept outside the runtime package and are documented in THIRD_PARTY_NOTICES.md. They are reference and parity assets, not required for normal pip install usage.

License

MIT. See LICENSE.

References

• Rafaely, B. (2015). Fundamentals of Spherical Array Processing. Springer.
• Zotter, F. & Frank, M. (2019). Ambisonics. Springer.
• Schmidt, R. O. (1986). Multiple emitter location and signal parameter estimation. IEEE Trans. Antennas Propag., 34(3), 276-280.