---
title: "Environment and transport"
description: "Sound outdoors and the transport sources that dominate it: the ISO 9613 propagation model, the NT ACOU 112 impulsive-sound adjustment, aircraft noise certification (EPNL, ECAC Doc 29), the rotorcraft hemisphere method and wind-turbine acoustic emission."
---

Environmental noise is a source-path-receiver problem stretched over hundreds
of metres of open air. This section covers both ends of it. The **outdoor
sound** pages handle the path and the assessment: ISO 9613 predicts, band by
band, how much level survives divergence, air absorption, the ground and any
barrier on the way to a receiver, and NT ACOU 112 quantifies when impulsive
character makes the received sound more annoying than its LAeq suggests.

The **aircraft and wind energy** pages handle transport sources that come with
their own internationally fixed metrics. Aircraft noise certification runs on
the Effective Perceived Noise Level of ICAO Annex 16, with the airport-contour
machinery of ECAC Doc 29 built on top; helicopters get the directional
noise-hemisphere source model of ECAC Doc 32; and wind turbines are rated by the
apparent sound power and tonal audibility of IEC 61400-11. What unites them is
the pattern: a carefully standardised source descriptor plus standardised
propagation adjustments.

This section leans on the core toolkit more than any other: the rating levels
and Lden that environmental assessment ends in live in
[Integrated and Statistical Levels](/phonometry/guides/levels/), and the
atmospheric absorption that every propagation model consumes is shared with
the room and materials pages. Start with
[Outdoor Sound Propagation](/phonometry/guides/outdoor-propagation/); it
introduces the source-path-receiver bookkeeping the transport pages reuse.

## [Outdoor sound](/phonometry/guides/sections/outdoor-sound/)

The path from an outdoor source to a receiver, and the character of what
arrives.

- [Outdoor Sound Propagation](/phonometry/guides/outdoor-propagation/):
  atmospheric absorption (ISO 9613-1) and the ISO 9613-2 general method with
  its per-term attenuation breakdown.
- [Impulsive-sound prominence (NT ACOU 112)](/phonometry/guides/impulse-prominence/):
  the predicted prominence of impulsive sounds and the graduated adjustment
  added to LAeq.

## [Aircraft and wind energy](/phonometry/guides/sections/aircraft-wind/)

Transport and energy sources with their own certification metrics.

- [Aircraft noise: Effective Perceived Noise Level](/phonometry/guides/aircraft-noise/):
  the ICAO Annex 16 EPNL, the IEC 61265 measurement-system verifier, SAE
  ARP 5534 atmospheric absorption and ECAC Doc 29 NPD interpolation.
- [Rotorcraft noise: the hemisphere method](/phonometry/guides/rotorcraft-noise/):
  the ECAC Doc 32 noise-hemisphere source model with its propagation
  adjustments.
- [Wind-turbine noise: sound power and tonal audibility](/phonometry/guides/wind-turbine-noise/):
  the IEC 61400-11 apparent sound power level and tonal-audibility chain.
