Outdoor sound

This section is the path: what happens to a sound between a source of known power and a receiver hundreds of metres away. Its three pages go from the engineering method down to the physics it approximates, and then to the one thing both of them assume does not happen.

Outdoor Sound Propagation is the engineering method. Starting from a source’s sound power, ISO 9613-2 subtracts, octave band by octave band, every mechanism that attenuates sound on its way: geometrical divergence, atmospheric absorption (supplied by the pure-tone coefficient of ISO 9613-1), the ground effect and barrier screening, with a meteorological correction for long-term averages. The page keeps the per-term breakdown visible, so a prediction is never a black box: you can see exactly which mechanism buys how many decibels at which frequency. Start here — the other two pages exist to say when its terms can be trusted.

Spherical ground effect and advanced barriers goes underneath two of those terms to the wave acoustics they fit: the Weyl-Van der Pol spherical-wave reflection coefficient of a finite-impedance ground, and barrier diffraction by the Kurze-Anderson Fresnel number, the exact rigid half-plane, thick barriers and the coherent four-path barrier on the ground. What it resolves is the frequency-dependent interference the octave-band terms smooth away: a ground dip is a cancellation between a direct and a reflected path, at a frequency that depends on the geometry and the ground impedance, and a tabulated correction cannot know where it falls. Open this page when the answer is dominated by the ground or by a screen, or when the octave-band result has to be defended against a measurement.

Atmospheric refraction: rays and the GFPE removes the assumption both pages above are built on. Sound speed changes with height, so rays are curved rather than straight, and whether that matters is mostly a question of range: a representative surface-layer gradient bends rays on a radius of about 3.4 km, so over the first hundred metres the homogeneous models are accurate, and beyond a few hundred the geometry takes over. Downwind or under a nocturnal inversion the rays close over the ground and hold the level up; upwind the same profile opens an acoustic shadow into which the level collapses by 20 dB or more. That asymmetry — the same machine at the same distance, tens of decibels apart depending on which side you stand — is what ISO 9613-2 fixes by decree in its favourable-propagation convention and compresses into the scalar meteorological correction. This page computes it, with curved rays and closed-form shadow-zone distances, and with the Green’s function parabolic equation as the reference field.

Read them in that order. The rating that a predicted level ends in is not here: the period levels come from Integrated and Statistical Levels, and Lden, Ldn and the rating level from Environmental Levels (ISO 1996-1/-2) in the assessment subsection. The source strengths a prediction starts from are in Environmental sources for road, rail and wind turbines, in Sound power and intensity for a machine, and in Aircraft noise for aircraft.

Pages in this section

• Outdoor Sound Propagation: ISO 9613-1 atmospheric absorption and the ISO 9613-2 general method with a per-term octave-band attenuation breakdown.
• Spherical ground effect and advanced barriers: the Weyl-Van der Pol spherical-wave ground reflection and wave-theoretic barrier diffraction (Kurze-Anderson, exact rigid half-plane, thick barriers and the coherent four-path barrier on the ground).
• Atmospheric refraction: rays and the GFPE: the refracting atmosphere itself: effective sound-speed profiles, curved rays with their shadow zones, and the Green’s function parabolic equation.

See also

Pages elsewhere on the site that this section leans on:

• CNOSSOS-EU road traffic source emission and CNOSSOS-EU railway source emission, both in Environmental sources: the directional sound power per metre of source line that a prediction starts from.
• Impulsive-sound prominence (NT ACOU 112), in Assessment and regulation: the character adjustment applied to the level once it has arrived.

What this section does not cover

• These are point-to-point models, not a mapping engine. Each call takes one source, one receiver and the ground between them; there is no terrain elevation profile, no building geometry and no GIS layer, both refraction models assume flat ground at height zero and a profile that varies with height alone rather than along the path, and how a source line is broken into point sources is declared out of scope by CNOSSOS itself. The CNOSSOS-EU propagation method of section 2.5 is not implemented: it is a different model from ISO 9613-2, so a calculation that pairs CNOSSOS source powers with the path here is not a CNOSSOS calculation. The coherent barrier-on-ground model weights its four diffracted paths with a single reflection coefficient computed over the overall geometry, so it is coherent and reciprocal but is not a boundary-element solution, and no model here computes turbulent scattering: ISO 9613-2 absorbs it into the fixed caps on its screening term, and the wave-acoustic and refraction pages assume a non-turbulent atmosphere outright. Nothing on these pages produces a rating: no Lden, no limit value and no verdict — those are Assessment and regulation.