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The other week me and my wife climbed into a Cwm/Corrie/Cirque in the lake district on the side of St Sunday Crag in the UK. I could distinctly hear someone talking. I could basically hear every word. I looked for this person only to realise they we're on the other side of the cwm. I checked my map, they we're about 0.5-1km away!

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I was at X they we're at Y. Each Square on this map is 1km.

Whats the process that allowed me to hear this person over such a long distance? I've experienced this phenomenon several times. Always in Cwms.

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    Slightly related and an interesting read: amphitheaters like in ancient Greece, e.g. sciencedaily.com/releases/2007/04/070404162237.htm – stijn Aug 1 '17 at 18:36
  • This is a great question! Were you at different altitudes or pretty much the same just a distance away? – Sue Saddest Farewell TGO GL Aug 5 '17 at 0:49
  • Hi @Sue they we're above me, I'd guess about 100m higher than I was – user2766 Aug 7 '17 at 7:46
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Sound reflected by concave surfaces will concentrate in a narrow area rather like light passing through a convex lens. You were probably at one of the focal points for the sound being reflected in the cwm behind the person you could hear.

  • I think you can also get reflections off the air at thermal boundary layers. – StrongBad Aug 3 '17 at 16:49
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    @StrongBad You can, but they're usually negligible. This answer is correct. Looking at Y, the point where the sound was being emitted from, you can clearly see that there is a positive height gradient to the southwest of it - and that it's actually concave (almost parabolic). It's almost as if X was placed at locus of the parabola-like rock formation the sound is being reflected from. As physics dictates: parabolas focus all incoming waves at the locus. – QuantumBrick Aug 9 '17 at 22:13
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    @QuantumBrick there are a lot of unknows. My recollection is that at speech frequencies air has an attenuation of 15 dB/km, conversational speech is 60 dB and whispering is 30 dB. A half open wave guide that has an attenuation of less than 15 dB/km seems unlikely which is why I suggested there might be a strong downwards reflection. – StrongBad Aug 9 '17 at 23:26
  • @StrongBad but how large must the temperature difference between layers be such that thermal reflection is relevant? I'd say pretty large. – QuantumBrick Aug 10 '17 at 10:47
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+50

Even barring special circumstances like focal points and such you can in general hear sounds from further away in the mountains. Reason one is all the air between two points. On flat ground there is often lots of sound absorbing stuff near the path of the sound, like soil and plants. Open air carries the sound better. The second reason are rocky walls serving as deflectors. If you talk in front of a large rocky wall all the sound that would usually go backwards is reflected, increasing the amount of sound going forward. If the listener is also standing in front of a rock, some of the sound that misses them on the way in gets reflected back and still makes it to their ears. In very short: you hear better for the same reason there are echo's. If a valley echoes well, it probably carries sounds very far.

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