5. Aircraft noise characteristics

5.1 Noise propagation, brief reminder

Land transport noise sources (railways, roads) are linear, and noise decay as a function of distance is in principle "cylindrical", although shielding effects are possible. Airborne noise is 3D and, in free field conditions, its attenuation as a function of distance comprises two terms:

• geometric, spherical attenuation: L = L ₀ – 20log(d/d ₀ ), which corresponds to a decrease of 6 dB per doubling of distance;

• atmospheric absorption, which depends on frequency; negligible at low frequencies, it can be very significant at high frequencies, such as engine noise.

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