It is not only in Omagh that a hum is to be heard (‘It’s a bit of a mystery’: what’s causing Omagh’s hum?, 21 November). In Derby, I have been aware of a low-frequency hum for some years. It is much more noticeable at night and in winter, but only disturbing when lying in bed – probably because it could be a ground-transmitted vibration, which at its worst is on the boundary between hearing and feeling on the eardrum.
It used to be a distinctive two-tone sound, like a mechanism changing between in gear and neutral, but this has changed over the last couple of years to a single deep humming tone. Its wave form seems in some locations to hit a resonant frequency with the room, which may be why the hum is more evident in some homes than others. I find it to be just below the threshold of preventing sleep, but it may be worse for anyone with a tendency to insomnia.
Paul Birchall
Derby
• Read Madame Bovary to solve the hum mystery – it’s a retired customs official endlessly turning napkin rings on his lathe.
Keith Barnes
Frome, Somerset
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