What a brilliant observation! This is very similar to UHF RF propagation. A wonderful technical blog!
Deep rumbling road noise is a true companion on my nature recording trips around in Denmark. Maybe it is due to modern headphones or to bass sensitive microphones – the rumbling is just ever present when I get home and hear the recordings. Noise from cross-cutting motorways can be particularly audible over long distances, however the noise level do vary surprisingly from time to time at the same location.
I have started to realize that weather conditions play a major role for this ambient background noise. The typically far distant nature of these sounds gives a high sensitivity to weather conditions. The atmosphere plays an active role in reflecting and guiding long distance sounds. It is actually a well-founded but highly complex research field. Sound differences of 10-20dB are often stated.
To my experience, road noise (closely followed by aircraft noise) is the most common and persistent type of noise…
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