Real-time 2.5 dimension outdoor acoustic simulation
GeoNoise implements ISO 9613-based outdoor sound propagation with coherent phasor summation for multi-path interference modeling. All calculations are performed per octave band (63 Hz – 16 kHz).
Point source spherical spreading models how sound energy spreads over an expanding sphere.
where d = 3D distance (m), 11 dB = 10·log₁₀(4π) for hemispherical radiation.
Derivation: Power flux Φ = W/(4πr²), therefore Lp = LW - 10·log₁₀(4πr²)
Air absorbs sound energy through viscous, thermal, and molecular relaxation processes.
ISO 9613-1: Calculates α from temperature, humidity, and pressure.
The coefficient α combines classical losses with N₂ and O₂ molecular relaxation.
Simple: Uses the lookup table below at standard conditions (20°C, 70% RH).
| 63 Hz | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz | 8 kHz | 16 kHz |
|---|---|---|---|---|---|---|---|---|
| 0.1 | 0.4 | 1.0 | 1.9 | 3.7 | 9.7 | 33 | 117 | 392 |
Values in dB/km
Sound reaches the receiver via direct and ground-reflected paths, creating frequency-dependent interference.
S (source)
/|\
/ | \ r₁ = direct path
/ | \
/ |hs \ ─────────────────── R (receiver)
/ | \ /
/ | \ r₂ / hr
──────┴──────●─────────────/────── Ground (z=0)
reflection point
Path lengths:
Delany-Bazley normalized impedance:
where σ = flow resistivity (hard: 200,000, soft: 20,000 Pa·s/m²)
Coherent ground effect:
This captures constructive/destructive interference (ground dip phenomenon).
Sound diffracts over and around barriers. Attenuation depends on the path difference δ.
Over-Top Diffraction (vertical):
S R
\ /
\ δ = A+B-d /
A ──→ ● ←── B
│
═══════╧══════ Barrier
Insertion loss:
Side Diffraction (horizontal):
Sound can also bend around the ends of finite-length barriers:
════════════════ Barrier (top view)
● ●
Left Right
Edge Edge
↑ ↑
S ─┘ └─ R
(around left) (around right)
Path difference for side: δ = |S→Edge| + |Edge→R| - |S→R|
The path with minimum δ (over-top or around sides) determines the effective attenuation.
Side Diffraction Toggle:
| Mode | Behavior |
|---|---|
| Off | ISO 9613-2 infinite barrier assumption. Only over-top diffraction. |
| Auto | Enable side diffraction for barriers < 50m (recommended default). |
| On | Always compute side diffraction for all barriers. |
Low frequencies diffract easily → less attenuation. High frequencies are blocked effectively.
For thick obstacles, sound diffracts twice (entry + exit edges):
S R
\ /
A₁ ──→ ●────────● ←── A₂
│ ROOF │
═══════╧════════╧═══════ Building
Path: Total = A₁ + T (roof width) + A₂
Note: coefficient is 40 (not 20) for double-edge diffraction.
Horizontal diffraction: Also computed around building corners using single-edge Maekawa. The minimum-loss path is selected.
Reflections from building walls computed using virtual image sources:
Wall
│
S ──────●────── R
\ │ /
\ │ /
S' │ (S' = mirror of S across wall)
Reflections are validated for line-of-sight and checked against other buildings.
Multiple paths from the same source are summed with phase (capturing interference):
This captures:
Multiple sources are summed energetically (incoherent):
Known Limitations:
Numerical Guards: