The “mysterious hums that keep us awake” (Letters, 27 November) may have a longer history and extend further than Paul Birchall speculates. In the early 1990s, HTV broadcast a one-hour documentary searching for the source of the Bristol hum. This programme went some way towards identifying the source of the extreme problems I was experiencing in Coventry, and found similar issues with people living near RNAS Culdrose on the Lizard peninsula in Cornwall. The answer appeared to be connected with overlapping transmissions from microwave towers, especially when mixed with radar, producing an ultra-low-frequency signal. In the extreme, it has considerably greater impact than merely lost sleep.
Ken Rees
Manaccan, Cornwall
• The Derby hum that Paul Birchall refers to is not a mystery. When I lived there, it was well known that it comes from the large aero engine test facility, which runs day and night. There were few complaints, because the success of Rolls-Royce is essential to the city. I doubt you can hear it in Omagh, though.
Eric Sharp
Newcastle upon Tyne
