Hearing Neighbors

My friend often hears her neighbors' conversations at night and it actually sounds like they are talking in her house. She lives in a rural area, but her neighbors are close. Her house sits in a sharp curve and the neighbors that she can hear are at a much higher elevation and are beside and behind her house. Prior to building her house here, there was another house that another lady lived in. This lady's son, who has always lived in that area, told my friend that she would probably be able to hear her neighbors talking at night like his mother and all her family did since they all grew up there. He said it's because her house is much lower than theirs and something about the way the sound travels. He actually called it something, but I can't remember what phrase he used. It's a little frightening for her, but sounds like there is a logical explanation. Her house is a log house with one open air bedroom upstairs. Most of the time it sounds like that's where the talking is. That is also the side of the house where the neighbors' houses are. Is it possible that the voice sounds are traveling down the hill and through the vents in her upstairs room?
- Debbie (age 60)
Wadesboro, NC, US

Sounds don't really travel more downhill, so that's not the direct explanation. It's got to be that the shape of the hollow is in effect acting like a lens and/or mirror for the sound.

If you've ever been in an elliptical stadiium, you may have noticed that standing at one focus of the ellipse, you can clearly hear a friend speaking at the other focus, because the stadium acts like a mirror refocussing the sound.

Why is the effect more noticable at night? Maybe that's just because it's quieter at night so it's easier to hear voices. It could also be that there's a tempperature diffeence between the low spots and the high spots, which makes a sort of lens for the sound. Maybe that helps focus sounds from the top on the house at the bottom.

If the main effect is just the mirroring from the groud shape, it won't vary much from night to night. If it's mainly lensing from temperature differences, then it will depend on the weather.

By the way, there's a reciprocity theorem you should know about. If your friend stands at a spot where she can easily hear the neighbors, they can also easily hear her!

Mike W.

(published on 07/23/2016)