|
UP | |
GOOD STUFF | |
NEW STUFF | |
SEARCH |
Some info from John Dawes
about the "Bristol Hum" in the UK
Sounds like an idling diesel engine.
Most "hummers" are over the age of 50
At least one partially deaf person hears the hum without using a hearing
aid
"Hearing" of radar signals can be ruled out, since aluminum foil
enclosures do not attenuate the Hum.
If a signal generator and loudspeaker is used, a zero beat can be heard
around 100Hz
Steel enclosures slightly attenuate the perceived hum, but only if greater
than 1/8" wall thickness.
J. Hall of Bristol UK committed suicide in 10/96 after having been driven
crazy by the hum.
The Hum is perceived as loudest when the modulation frequency is 2.5 -
3.5Hz.
The Hum can be detected and recorded using coil detectors:
700 turns .010" diameter enameled wire wound on a 12" plastic bucket.
A 0.1uF capacitor in parallel tunes it to broad resonance at 200Hz.
Connect this to the MIC input of a battery powered cassette tape recorder
with 9ft of coax to separate the coil from e/m noise from the recorder.
70 turns of wire on a wooden frame 1 meter square, tuned with a
capacitor.
Ferrite rod 7" long, 1/2" diameter. Use .004" enameled wire and wind
a coil 1" diameter, 3/4" long in the center of the ferrite rod. Coil
resistance should be about 500 ohms. 0.1uF capacitor is connected in
parallel for tuning, vary the coils to tune either for 150Hz or 250Hz.
Coil output feeds an audio preamp with 100x gain, which feeds an op-amp
active filter tunable 30Hz to 300Hz. Filter output goes to an active
rectifier stage, which feeds a meter, chart recorder, etc.
Two ELF carrier waves are detected, 150Hz and 250Hz, amplitude
modulated with 0.5 - 1.5Hz.
The 150hz and 250hz signals do not rise and fall together, and the
modulation isn't the same.
The field strength is *not* proportional to the perceived loudness of
the Hum. But if the modulation consists of sharp rises and falls
combined with a high signal level, this appears to produce a loud hum.
The perceived loudness may be proportional to rate of change of
modulation. The movement of a signal level meter seems synchronized
to the modulation of the hum.
The loop antenna indicates the signal direction is on a N-S line.
On a 30mile E-W base line using the loop antenna for direction finding,
taking multiple readings along the line, no change in angle was
measurable, indicating a very distant source.
Taking a detector into a railway tunnel with 15ft - 20ft of soil and
rock overhead did not measurably attenuate the detected signal.
See John Dawes'
website
Created and maintained by Bill Beaty. Mail me at:
.