Science Questions

Why do sound recorders pick up electrical interference and radio stations?

Sun, 13th Apr 2008

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Stephen Bralls asked:

I send my students out into the field to do biology and ornithology recordings with parabolic reflectors near microphones so they can record the sound they’re picking up. Every now and then a student returns with a recording on tape that’s picked up some music or a distant radio station. Where’s that coming from?


We put this question to Chris Davis, from the Rutherford Appleton Laboratory:

That’s very interesting. It all depends at what frequency their equipment’s working at but the radio station’s reflected off the Earth’s ionosphere which is an electrified layer on the edge of space which is where the aurora interferes with. The ionosphere by night can reflect radio stations over large distances and during the day they’re much more absorbed in the lower atmospheres. It sounds like, if they’re going out around dawn it might just be around the time these radio stations are changing and they’re picking up signals from a distance when they wouldn’t normally expect to.


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