Outdoor sound

Outdoor sound assessment has two halves: predicting the level a source delivers to a distant receiver, and judging the character of the sound that actually arrives. The two pages of this section take one half each.

Outdoor Sound Propagation is the prediction half. Starting from a source’s sound power, the ISO 9613-2 general method 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.

Spherical ground effect and advanced barriers goes underneath the ISO 9613-2 fits to the wave acoustics they approximate: 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, all resolving the frequency-dependent interference the octave-band terms smooth away.

Impulsive-sound prominence (NT ACOU 112) is the assessment half. Noise containing distinct impulses (hammering, riveting, pile driving) annoys more than a steady sound of the same LAeq, and the Nordtest method quantifies that: from the onset rate and level difference of each impulse it computes a predicted prominence, and converts it into the graduated adjustment KI that is added to the measured LAeq in a rating level.

The surrounding machinery lives nearby: the rating levels and Lden that assessments end in are covered in Integrated and Statistical Levels, the tonal counterpart of the impulsive adjustment in Objective audibility of tones in noise, and the sources that feed a propagation calculation in Sound Power and the Aircraft and wind energy section.

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).
• Impulsive-sound prominence (NT ACOU 112): the predicted prominence of impulsive sounds and the graduated LAeq adjustment KI.