How do elliptical walls focus sound between the foci?

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loreberto911
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I was in Urbino a walled city in the Marche region of Italy. At the foot of Duke's palace we found an elliptical wall.

It has been mentioned that the guards were once placed at the ends of the wall. In fact the city is located on top of a mountain, the guards had to watch over the whole valley from potential invaders. In order to talk to each other, they used a nice acoustic reflection effect. Speaking directly to the wall it is possible to converse with extreme ease. Even in a low voice. My friend and I have tried it and it works very well.

I wondered how the reflection mechanism worked. I used this simple tool to simulate the effect.
https://amcoustics.com/tools/amray
In the photo we see that when the sound source is placed at the edge of the curve, the waves are concentrated at the other end.

I wanted to ask if the simple simulation is conceptually correct. And what other factors are involved in this phenomenon.
Thank you
riflessione 2.png

wall.png
 
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You should get best communications when each speaker is at a focal point of the ellipse.
So they should step a bit away from the wall before speaking and listening.
 
loreberto911 said:
I wanted to ask if the simple simulation is conceptually correct.
Using an isotropic source is not a good assumption for a human voice.