Post #3585275

Observed a bizarre physics effect in our hotel: My wife noticed an annoying humming noise. Turned out to be 50 Hz AC hum, which was very localised. It wasn’t audible any more half-way along the bed. So we slept with our heads towards the foot end. I did use my spectrum analyser app and confirmed that the amplitude dropped by 18 dB over 1.5m! (see figures). How can the loudness drop so much within less than a quarter of a wavelength?? Standing waves? Interference? Near-field effects?