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I'm wondering if there are any common principles for how animals gauge the distance of a sound source. How to differentiate between a close, soft sound and a loud distant sound?

For example, hypothetically, if you alternately played back the same sound from two identical speakers, one 10m away from the focal animal and the other 50m away, with volumes such that the sounds arrive at the animal at equal amplitude - would the animal have any way to tell them apart?

My hunch is that attenuation of higher frequencies would have something to do with it, and maybe the relative level of echo ('wet') to 'dry' signal. Thoughts?

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Birds (and humans to a lesser extend) can "measure" signal degradation due to reverberation and frequency-dependent attenuation and estimate distance based on these cues.

Here is an interesting review on the topic:

Naguib, Marc, and R. Haven Wiley. "Estimating the distance to a source of sound: mechanisms and adaptations for long-range communication." Animal behaviour 62.5 (2001): 825-837.

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Depending on your frequency range and propagation environment, there are features other than amplitude that could be used by animals to estimate distance.

For example, low frequency baleen whale calls will undergo modal dispersion as they propagate in shallow water. It's been shown that you can use this to estimate range (given you have enough knowledge about the environment). At shorter ranges you can also use things like reflections from the surface/bottom.

Do animals use this? I have definitely wondered about it myself. Can we prove it? It seems tricky with marine mammals, but maybe it can be shown for certain terrestrial species.

Bonnel, J., Thode, A. M., Blackwell, S. B., Kim, K., & Macrander, A. M. (2014). Range estimation of bowhead whale (Balaena mysticetus) calls in the Arctic using a single hydrophone. The Journal of the Acoustical Society of America, 136(1), 145-155. doi:10.1121/1.4883358

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