Most nights around about 10pm we notice a faint mid-pitched "buzz"
sound emitting from our electrical appliances (similar sound to when
your ears ring). It's noticable in speakers, the back of my old CRT
monitor, on the phone - quite loud sometimes, even the fridge - it can
be sometimes hard to locate exactly.

The buzz seems to follows a consistant pattern when I notice it - like
buzzzzzzzzzzzzz, buz, buz, (gap 3 secs) ,
buzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz, buzzzzzzzzzzzzzzz,
buzzzzzzzzzzzzzz, buzzzzzz, buzzzz, buzz

It seems to end of like that starting with a long buzz, then repeated
but getting shorter and shorter.  Then it stops - and starts again.  
Always happens around 10pm at night - and notice it occaisionally
through to about 1am - repeats about every 10-20 mins.

Not always that exact pattern but often very similar, and you get to
know them after a while!

I've noticed the above specific pattern in 3 *different* houses during
the last 5 years - (all in the same area).  Does anybody know if this
is some kind of code or signal coming down the electricity lines? 
Maybe perhaps it's a signal that triggers off-peak hot water heaters
or something (although our last 2 houses were gas hot water).

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Request for Question Clarification by grimace-ga on 29 Jun 2002 00:50 PDT

Interesting.

Whereabouts do you live?

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Clarification of Question by webwrx-ga on 29 Jun 2002 02:08 PDT

Rooty Hill
Sydney, Australia

(general outer western Sydney area)

Hi, 

Electrical hum or the 60 cycle hum as it is more commonly known as, is
caused by the harmonics of the frequency of the power lines. This can
be exacerbated by transformers. The reason you notice it at night is
that there is less ambiant noise to disguise the sound. The cyclical
nature of the interaction of the sinewave gives you that pattern that
you speak of. The Harmonics that derive from several devices in the
home can interact and drive speakers as their magnetic fields move the
coils.

Here is a web site that goes into much detail about this phenomena:
http://www.epanorama.net/documents/groundloop/otherhumming.html

Search Term used

causes of electrical hum

There are many more sources with solutions on how to eliminate or
minimize the sources of hum just using that search term.

Good luck and I hope this helps you to not only understand your hum
but get rid of it!

Thanks you - concise, and informative.

How close is the nearest military base?

in reply to alienintelligence-ga question: Richmond RAAF base is about 15km north.

'probably' It's the natural noise from electrical components, the hum
from the transformers... and you notice it at night because that is
then sounds are amplified by the lack of environmental noise.
(my guess)

lot-ga: no - I'm absolutely certain this is night time only. The
buzzing pattern is too regular and distinctive to be a transformer hum
although similar in sound. Regular patterns repeated nightly that you
learn to recognize!

Richmond? Are you in Sydney, Australia?

In that case, the tones are control tones transmitted across the power
supply to switch on/off various off-peak power circuits.

Customers with off-peak hot water heating will have a unit in their
meter box (often branded "Zellweger") which detects these tones and
switches the off-peak circuit on and off at the appropriate times. The
control tones go to every house - not just those houses with off-peak
circuits.

Prior to the 1970's, mechanical timeclocks were used to switch
off-peak circuits. However, control tones are more versatile because
they can be varied to allow for daylight saving and exceptional power
system conditions.

The control signals are sent several times a day, because there is a
range of off-peak tariffs for domestic and industrial purposes.

I think the frequency is around 1100Hz and aplitude around 20V RMS,
but I can't find anything on the web to confirm this.