What’s this ‘noise’?

londonmidland

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I’ve heard this noise, which kinda sounds like a faint alarm, across many areas on third rail land. It’s quite a faint sound, but if you listen carefully, you’ll be able to hear it. Strangely I’ve only ever heard it in areas where there’s third rail, so I assume it’s something to do with that, giving out a high frequency sound?
 
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Surreytraveller

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I’ve heard this noise, which kinda sounds like a faint alarm, across many areas on third rail land. It’s quite a faint sound, but if you listen carefully, you’ll be able to hear it. Strangely I’ve only ever heard it in areas where there’s third rail, so I assume it’s something to do with that, giving out a high frequency sound?
I've never noticed it before!
 

Need2

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I’ve heard this noise, which kinda sounds like a faint alarm, across many areas on third rail land. It’s quite a faint sound, but if you listen carefully, you’ll be able to hear it. Strangely I’ve only ever heard it in areas where there’s third rail, so I assume it’s something to do with that, giving out a high frequency sound?
I’ve got my medical next week, I can’t hear a thing! o_O
(Can you pinpoint a time on the video so that I know the sound is definitely there?)
 

alxndr

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The high pitched slightly warbling sound at the linked timestamp? An audio frequency track circuit, so named because they're in the audible frequency range! Specifically, these would be EBItrack 200 (or TI21 by their original name).

They're definitely not unique to third rail/electrified areas.
 

Surreytraveller

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Maybe the recording or playback device changes the frequency to make it more audible.
Bit like how mobile phone cameras enable you to see Infrared LEDs
 

londonmidland

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I’ve got my medical next week, I can’t hear a thing! o_O
(Can you pinpoint a time on the video so that I know the sound is definitely there?)
The video has been time stamped, so it’ll automatically skip to the bit I mean.

The high pitched slightly warbling sound at the linked timestamp? An audio frequency track circuit, so named because they're in the audible frequency range! Specifically, these would be EBItrack 200 (or TI21 by their original name).
Yes, that sounds like what I’m talking about. Thanks.
 

RichardKing

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I’ve heard this noise, which kinda sounds like a faint alarm, across many areas on third rail land. It’s quite a faint sound, but if you listen carefully, you’ll be able to hear it. Strangely I’ve only ever heard it in areas where there’s third rail, so I assume it’s something to do with that, giving out a high frequency sound?
I know exactly what sound you're talking about. I first noticed it at Plumpton. You can hear it in this video
 

alxndr

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Maybe the recording or playback device changes the frequency to make it more audible.
Bit like how mobile phone cameras enable you to see Infrared LEDs
It's always audible, although phones do seem to make it particularly bad. It's a nightmare if you're on a phonecall and the person you're talking to has decided to put their phone on loudspeaker and sit it on the equipment.
 

richieb1971

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I hear a noise through my camera at sharnbrook junction. A warble electrical noise that sounds like interference from somthing nearby. I would imagine where 3rd rail is used this warble is more evident due to the amount of transformers and heavy duty cabling being used.
 

John Webb

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I'm not clear about this - is the AF track circuit (TC) being picked up by the video equipment through the TC's electromagnetic radiation or is it actually audible and is being picked up via the video equipment's microphone? If the latter what is acting as a speaker?
 

Annetts key

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Audio frequency track circuits are used so that there is no need for insulated rail joints (IRJ). The noise comes from the track side units (older units were a cream colour, later units are a light grey colour).
The noise is present whenever there is no train on the track circuit concerned.

The cause is the inductors in the track side units vibrating due to the pulsing electrical current flowing through them. As the current changes, so does the magnetic field, and hence this force causes just enough movement to cause a vibration.

Because the sound is in the range of human ears, yes, you can hear it directly. And obviously microphones can pick it up as well.

With the older type, each track circuit uses a single constant frequency. But the more modern later type use two frequencies that alternate, just like some sirens.

I hear a noise through my camera at sharnbrook junction. A warble electrical noise that sounds like interference from somthing nearby. I would imagine where 3rd rail is used this warble is more evident due to the amount of transformers and heavy duty cabling being used.
The third rail supply equipment will vibrate at the supply frequency. For rectified DC, the frequency is doubled compared to the AC input to the system.

Large mains transformers can often be heard vibrating at 50Hz. Substations can often be heard for example. As can some older style fluorescent light fittings that use a magnetic choke (inductor).

But none of these will warble like a siren.

In this screen grab from the video in post 7, you can see some of the track side units.

994ADB1D-48F5-4B6E-8FA9-F3F6614F2431.jpeg
 
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MadMac

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I'm not clear about this - is the AF track circuit (TC) being picked up by the video equipment through the TC's electromagnetic radiation or is it actually audible and is being picked up via the video equipment's microphone? If the latter what is acting as a speaker?
Oh, it’s definitely audible! There used to be an “acoustic jacket” available for the trackside units where they were in proximity to residences.
 

richieb1971

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At Sharnbrook, the noise is not audible through the human ear. My video camera picks it up though.

Here is a video I made a few years ago -

Skip to 09:10. You can hear it distinctly as the 66 comes in from the distance.
 

Annetts key

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At Sharnbrook, the noise is not audible through the human ear. My video camera picks it up though.

Here is a video I made a few years ago -

Skip to 09:10. You can hear it distinctly as the 66 comes in from the distance.
I’m not sure that the noise in this video at 9:10 is from a ‘joint less’ audio’ frequency track circuit. It also doesn’t sound right. This type of track circuit is normally not used through point work. Instead conventional (normally DC type) track circuits are used. I also can’t see any of the track side units in the picture (although there could some out of shot).

So this noise could be something different. Some equipment cubicles use data monitoring systems that use the mobile telephone network. If you are near one, it can interfere with audio equipment in the same way a mobile telephone can. I’m not saying this is the cause here, because again it does not sound right.

-

Incidentally, here is a list of frequencies used by ‘joint less’ audio frequency track circuits:

Older type (continuous constant ‘tone’):
1700Hz
2000Hz
2300Hz
2600Hz

Later type (normally constantly switching between the two frequencies):
1682Hz / 1716Hz
2279Hz / 2313Hz
1979Hz / 2013Hz
2576Hz / 2610Hz
1532Hz / 1566Hz
2129Hz / 2163Hz
1831Hz / 1865Hz
2428Hz / 2462Hz
 
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