sound-propagation 0.1.0

ISO 9613-1/2 atmospheric sound absorption and ground attenuation

ISO 9613 Propagation Toolkit

A small, pure-Python library that implements the core calculations from the ISO 9613 series:

• ISO 9613-1:1993 -- Atmospheric absorption and geometric spreading of sound outdoors.
• ISO 9613-2:1996 -- Ground-attenuation model for flat terrain (three-region model).

The toolkit provides two high-level classes:

Class	Purpose
AtmosphericPropagation	Compute the frequency-dependent atmospheric absorption coefficient α (dB/m) and the resulting sound-pressure-level (SPL) change between a source and a receiver, optionally adding ground attenuation.
GroundAttenuation	Calculate the ground-attenuation term Agr (dB) for the standard octave-band centre frequencies (63 Hz -- 8 kHz) using the three-region model defined in ISO 9613-2.

Both classes are deliberately lightweight, have no external dependencies beyond NumPy, and are fully typed for IDE support. Requires Python 3.10+ (uses X | Y union syntax).

Validated against 102 data points from ISO 9613-1:1993 Table 1 spanning -20 °C to +50 °C, 10--100 % RH, and 50--8000 Hz.

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Table of Contents

1. Installation
2. Quick Start
3. Detailed API
   • AtmosphericPropagation
   • GroundAttenuation
4. Examples
5. Testing & Development
6. License

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Installation

pip install sound-propagation

Or install from source:

git clone https://github.com/commuted/sound-propagation.git
cd sound-propagation
pip install -e .

---

Quick Start

from sound_propagation import AtmosphericPropagation, GroundAttenuation

# 1. Define atmospheric conditions
prop = AtmosphericPropagation(
    temperature_c=20.0,           # °C
    relative_humidity_pct=50.0,   # %
    pressure_kpa=101.325,         # kPa (standard atmosphere)
    source=(0, 0, 0),             # source position (m)
    recording=(100, 0, 0),        # mic position (m)
    warn_frequency=True,          # optional warnings for out-of-range freqs
)

# 2. Compute atmospheric absorption for a single frequency
alpha = prop.absorption_coefficient(1000)   # dB/m at 1 kHz
print(f"Absorption coefficient at 1 kHz: {alpha:.4f} dB/m")

# 3. Get attenuation at an arbitrary 3-D point (shorthand via __call__)
result = prop(1000, (150, 0, 0))
print("Total SPL change (dB):", result["total_dB"])

# 4. Get attenuation at an on-axis offset from the recording position
result = prop.attenuation_at_offset(
    frequency=[500, 1000, 2000],
    distance_offset=20.0,         # 20 m beyond mic
)
print("Total SPL change (dB):", result["total_dB"])

# 5. Add ground attenuation (optional)
ground = GroundAttenuation(
    source_height=2.0,
    receiver_height=1.5,
    distance=120.0,
    G_source=0.8,
    G_receiver=0.5,
    G_middle=0.6,
)

# 6. Combine everything (atmosphere + geometry + ground)
full = prop.total_attenuation(
    frequency=AtmosphericPropagation.OCTAVE_BANDS,
    eval_pos=(30, 0, 0),
    ground=ground,
)
print("Total attenuation per octave band (dB):", full["total_dB"])

# 7. A-weighted convenience
a_weighted = prop.a_weighted_attenuation((30, 0, 0), ground=ground)
print("A-weighted attenuation (dB):", a_weighted)

---

Detailed API

AtmosphericPropagation

Constructor

AtmosphericPropagation(
    temperature_c: float,
    relative_humidity_pct: float,
    pressure_kpa: float | None = None,
    source: Position = (0.0, 0.0, 0.0),
    recording: Position = (100.0, 0.0, 0.0),
    copy: bool = False,
    warn_frequency: bool = False,
)

Parameter	Description
temperature_c	Ambient temperature (°C).
relative_humidity_pct	Relative humidity (0 % -- 100 %).
pressure_kpa	Atmospheric pressure (kPa). Defaults to standard 101.325 kPa.
source / recording	3-D position as a tuple, list, or np.ndarray of shape (3,).
copy	If True, NumPy position arrays are copied; otherwise stored by reference.
warn_frequency	Emit a warning when a frequency falls outside the ISO-validated 50 Hz -- 10 kHz range.

Core Methods

Method	Return	Description
absorption_coefficient(frequency)	float or np.ndarray	Pure-tone atmospheric absorption α (dB/m) per ISO 9613-1 §4. Accepts f = 0 (returns 0).
absorption_coefficient_octave(center_frequencies=None)	np.ndarray	Same as above but for the eight standard octave-band centre frequencies.
attenuation_at_offset(frequency, distance_offset)	dict	SPL change at a point displaced distance_offset m from the recording position along the source-to-receiver line.
attenuation_at_position(frequency, eval_pos)	dict	SPL change at any arbitrary 3-D point.
total_attenuation(frequency, eval_pos, ground=None)	dict	Combines atmospheric, geometric, and optional ground attenuation.
a_weighted_attenuation(eval_pos, ground=None)	np.ndarray	A-weighted total attenuation for the eight standard octave bands.
__call__(frequency, eval_pos)	dict	Shortcut for attenuation_at_position.

Return Dict Keys

Methods returning dict include these keys:

Key	Type	Description
total_dB	float or np.ndarray	Net SPL change (positive = louder, negative = quieter).
atmospheric_dB	float or np.ndarray	Contribution from atmospheric absorption only.
geometric_dB	float	Contribution from geometric (1/r²) spreading only.
ground_dB	float or np.ndarray	Ground attenuation (only present when ground is provided).
distance_to_source	float	Distance from source to evaluation point (m).
distance_to_receiver	float	Distance from recording to evaluation point (m).

Class Constants

Constant	Value
OCTAVE_BANDS	np.array([63, 125, 250, 500, 1000, 2000, 4000, 8000])
A_WEIGHT	np.array([-26.2, -16.1, -8.6, -3.2, 0.0, 1.2, 0.9, -1.1])

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GroundAttenuation

Constructor

GroundAttenuation(
    source_height: float,
    receiver_height: float,
    distance: float,
    G_source: float = 1.0,
    G_receiver: float = 1.0,
    G_middle: float = 1.0,
)

Parameter	Meaning
source_height (hs)	Height of the source above ground (m).
receiver_height (hr)	Height of the receiver above ground (m).
distance (dp)	Horizontal source-to-receiver distance (m).
G_source, G_receiver, G_middle	Ground factors (0 = hard surface, 1 = porous).

Core Method

Method	Return	Description
ground_attenuation(frequency)	float or np.ndarray	Calculates Agr (dB) for the supplied octave-band centre frequencies using the three-region model (ISO 9613-2 §7.3.1).

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Examples

1. Plotting atmospheric absorption vs. frequency

import matplotlib.pyplot as plt
import numpy as np
from sound_propagation import AtmosphericPropagation

prop = AtmosphericPropagation(
    temperature_c=25,
    relative_humidity_pct=60,
    source=(0, 0, 0),
    recording=(150, 0, 0),
)

freqs = np.logspace(np.log10(50), np.log10(10000), 200)
alpha = prop.absorption_coefficient(freqs)

plt.semilogx(freqs, alpha)
plt.title("Atmospheric Absorption Coefficient (ISO 9613-1)")
plt.xlabel("Frequency (Hz)")
plt.ylabel("α (dB/m)")
plt.grid(True, which="both")
plt.show()

2. Comparing ground attenuation for different surfaces

from sound_propagation import GroundAttenuation

freqs = GroundAttenuation.OCTAVE_BANDS

hard   = GroundAttenuation(1.5, 1.5, 300, G_source=0.0, G_receiver=0.0, G_middle=0.0)
porous = GroundAttenuation(1.5, 1.5, 300, G_source=1.0, G_receiver=1.0, G_middle=1.0)

print("Hard ground:  ", hard.ground_attenuation(freqs))
print("Porous ground:", porous.ground_attenuation(freqs))

3. Full ISO 9613 prediction (atmosphere + ground)

from sound_propagation import AtmosphericPropagation, GroundAttenuation

prop = AtmosphericPropagation(20, 55, source=(0, 0, 0), recording=(80, 0, 0))
ground = GroundAttenuation(2.0, 1.0, 80, G_source=0.8, G_receiver=0.5, G_middle=0.6)

total = prop.total_attenuation(
    frequency=AtmosphericPropagation.OCTAVE_BANDS,
    eval_pos=(120, 0, 0),
    ground=ground,
)

print("Total attenuation per octave (dB):", total["total_dB"])

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Testing & Development

The test suite contains 256 tests including validation against all 102 data points from ISO 9613-1:1993 Table 1.

# Run all tests
./env-sound_prop/bin/pytest tests/ -v

# Run only Table 1 validation (102 parametrized cases)
./env-sound_prop/bin/pytest tests/test_atmospheric_absorption.py -v -k "test_absorption_coefficient_against_table1"

# Run ground attenuation tests
./env-sound_prop/bin/pytest tests/test_ground_attenuation.py -v

Tolerances: 1 % relative for values >= 1 dB/km, 0.01 dB/km absolute for smaller values.

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License

This code is released under the MIT License. See the accompanying LICENSE file for the full text.

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Contributing

Open an issue or pull request on the repository.