What's this noise?

[@rt]

I hear it mostly from audio amplifiers that are turned up and playing nothing,
but sometimes this ceiling fan makes a very similar or same noise:
https://youtu.be/l_Rd13q6QZU?si=u_BFHc3Wq9F2IfOe

[NiHaoMike]

Any solar inverters in the area? Might be the anti islanding test pulses.

[@rt]

Any solar inverters in the area? Might be the anti islanding test pulses.

Certainly nothing I can see, and I've never heard any obvious culprit on radio...
just the typical s9 modern urban noise floor, but no lone noise maker.

I remember this from my Dad's hifi in the mid 90's though. The exact same thing with different timings.
It's almost like two near frequencies heterodyning, but there's no beat frequency.

I really thought it would be something well known, and I'd look a fool with everyone jumping in with apparent obvious answer

It's happening fairly frequently with amps now since I took up guitar.

[SteveThackery]

We're talking about that ringing sound that comes and goes?

[@rt]

We're talking about that ringing sound that comes and goes?

Yes. The rest is me messing with the phone once I heard it.

[SteveThackery]

If you'd said your fan was making a funny noise, I would have been quite happy to theorise that something was rubbing, and maybe inducing some kind of resonance in the blades.

But the thought that you can also hear it coming from your amplifier when it is turned up is so weird it's actually pretty creepy. Obviously the fan and the amplifier have completely different mechanisms for making sound, so how the same noise can come from each.....  Well, like I say, "creepy".

Sorry I can't be of any help but I'd love to hear some theories from our experts.

[David Hess]

I remember this from my Dad's hifi in the mid 90's though. The exact same thing with different timings.
It's almost like two near frequencies heterodyning, but there's no beat frequency.

It could still be that.  Check for any cell phones in close proximity.  Audio amplifiers have a tendency to rectify the AM envelope of RF signals at their inputs and especially outputs.  RF suppression may need to be used at one or both.

What I did not hear is the typical white and pink noise hiss of an audio amplifier.

[@rt]

If you'd said your fan was making a funny noise, I would have been quite happy to theorise that something was rubbing, and maybe inducing some kind of resonance in the blades.

But the thought that you can also hear it coming from your amplifier when it is turned up is so weird it's actually pretty creepy. Obviously the fan and the amplifier have completely different mechanisms for making sound, so how the same noise can come from each.....  Well, like I say, "creepy".

Sorry I can't be of any help but I'd love to hear some theories from our experts.

It would be easier and cleaner to capture from the guitar amps. I'll upload that next time.

@David, it may still be that, but in the mid 90's was also an entirely different house and suburb.

A commonality is an amp with high impedance inputs pre-amplified presumably also by some separate op-amps.
In the mid 90's that would have been a stylus for playing vinyl, but it was a complete Marantz system, so no rubbish.

To be honest, not knowing it's origin is more annoying than the actual interference. It doesn't overpower anything.

[SteveThackery]

To be honest, not knowing it's origin is more annoying than the actual interference. It doesn't overpower anything.

And to me, the same noise coming from a fan and an amplifier is the real mystery.  Does anyone have any thoughts on that aspect?

[tom66]

Do you live near any electrically driven train system?  Others have had some problem with electrical noise in their properties in London, it turned out that the underground trains were producing vast amounts of EMI due to a fault at the power conversion station and this was being injected into power lines nearby.  These trains work on -210V/+420V split DC power.  The issue was eventually fixed, but it took over a year.

[elektryk]

Maybe something is drawing distorted current and fan/transformer windings are generating hum?

[SeanB]

Here is the explanation, from the little country to the right of australia with all the sheep, and this Belgian import,









those are the tones you are hearing.

[tom66]

Interesting on Wiki:

https://en.wikipedia.org/wiki/Zellweger_off-peak

"The noise is often picked up by other equipment, especially audio amplifiers and stereos and the noise can cause problems with other electrical devices. It is especially audible from ceiling fans running at low speed."

These types of systems are going obsolete in the UK (or may already be obsolete), all switching to be done via smart meters or older radio timeswitches.  Used to be common for streetlighting too, though I guess at different frequencies.

[SteveThackery]

That is fascinating!  I never knew such a thing existed. So it's just a signalling system - wow!

[@rt]

Wow! Thanks everyone for the replies, and of course for the answer.

It occurred to me the fan could make some mechanical noise that disturbed our mains circuit,
and next check was to run guitar amps with ceiling fans on and off. I didn't quite get to it.

Next, I considered it might be to do with correction of our 50Hz mains frequency where it might still be used for timing.

Now that I know the answer, to maintain intellectual honesty, I can't complain about the noise too much!!
"To be honest, not knowing it's origin is more annoying than the actual interference. It doesn't overpower anything."