Siren sounder wire length

[fourbie]

This is something I have never come across before even though I have installed a good few alarm systems in my time as a DIYer.
I know it is a cheapo alarm but it shouldn't be any different. It came with a little siren and about two feet of flex, black and red two core. I intended to put in to the existing bell box after taking off the non functioning alarm system that came with the house.
Great - I thought I'd use the wire that ran from the old alarm to the outside bell box using two wires (the cable had three cores black, red and brown).
I got an awful distorting buzz so bad I could not make out the verbal prompts like "please enter password" etc. I ran a few tests and any (extra) wire used to connect the siren to the host box in any orientation created humming buzz even with the siren not attached.
My first reaction though, was that the fridge or some other electronic item was interfering with the unit but connecting the siren to the host using its pathetic little length of wire showed no interference at all. It is so bad that the spoken prompts are almost impossible to make out.
I have thought of using a screened twin core cable so the wire is shielded from aerial interference. Other than that I have no idea how to deal with this, it is a pity because it spoils what is an excellent alarm and I really do want an outside sounder.

 

[ian.]

It sound like its picking interference from somewhere, can you turn the mains power completely off to prove this, if the problem stops, start turning the breakers on one at a time until it returns.

It may well be that one piece of equipment is radiating noise that you can track down, failing that the original cable has been ran parallel with a mains carrying cable and has an induced voltage issue, a simple test for this is to use a multimeter to measure for an AC voltage with the sounder cable disconnected.

 

[fourbie]

It sound like its picking interference from somewhere, can you turn the mains power completely off to prove this, if the problem stops, start turning the breakers on one at a time until it returns.

It may well be that one piece of equipment is radiating noise that you can track down, failing that the original cable has been ran parallel with a mains carrying cable and has an induced voltage issue, a simple test for this is to use a multimeter to measure for an AC voltage with the sounder cable disconnected.

Yes, thanks, that is what I suspected but why does it do it with any extended wire. I ran about 6 feet of wire and laid it (with the siren on the end) along the floor as an experiment and it buzzed just as badly.
It is an idea to try shutting of the mains. Might be a bit awkward though.
Also I had a feeling the host was receiving an untuned radio station. Could it be acting as a radio antenna? Could the alarm box be creating the distortion. I know it sounds weird but I assume the previous alarm system did not exhibit this problem.

"a simple test for this is to use a multimeter to measure for an AC voltage with the sounder cable disconnected"
I ran a short wire extension and the noise was there. I can check it but I am fairly sure the test extension had no induced voltage.

If I used screened twin core for the bell wire would it help and how would I earth the screening?

 

[SparkyChick]

Because the longer the wire, the more of an aerial it will be. You could try screened cable with the screen connected to earth, but you should keep in mind that very often, audio circuits are designed to work with a specific output device impedance and lengthening the cables could be adversely affecting this.

 

[fourbie]

audio circuits are designed to work with a specific output device impedance

Surely this is not classed as an audio circuit!!

 

[SparkyChick]

Why wouldn't it be? You're talking about a sounder which is an audio output device. There will usually be a small analogue amplifier to drive it.

 

[AlmondSauce]

Surely this is not classed as an audio circuit!!

Depends.

I got an awful distorting buzz so bad I could not make out the verbal prompts like "please enter password" etc.

Is the buzz constantly there in the background, or only there when the sounder is trying to say something?

If it's constantly there, then it's almost certainly picking up interference from somewhere, and the longer your wire the worse it will be. (The old cable could also be running right alongside other power cables, which really won't help things.)

If it's absolutely fine when off, but distorted when it's saying something, then it's much more likely to be some form of impedence matching issue.

 

[fourbie]

Why wouldn't it be?

But is it an audio output device!? It is just sending 12v to a siren like you would get on a car horn. The signal noise you hear at 115db is generated by the siren itself when it gets current.

 

[fourbie]

Depends.


Is the buzz constantly there in the background, or only there when the sounder is trying to say something?

If it's constantly there, then it's almost certainly picking up interference from somewhere, and the longer your wire the worse it will be. (The old cable could also be running right alongside other power cables, which really won't help things.)

If it's absolutely fine when off, but distorted when it's saying something, then it's much more likely to be some form of impedence matching issue.

The host buzzes all the time while longer wire is connected; you would think a kitchen blender is running in the distance. When the alarm is being set and a voice says please enter user password it's worse than a railway station tannoy thanks to the distortion.

Even if I shut down the mains supply area by area till I find the cause of the transmitted interference I would still need to deal with it and bear in mind that all the facilities in the house have to remain on. lol This is why I am thinking I need to use screened flex. I am not too concerned about the old cable because this is happening with fresh cable in a very different orientation and it is only about 6 feet of length as well. Six to ten feet is about all I need to get some height from the host's placement and through a wall to the bell box's situation.
Ii have run bell box wires alongside mains runs for 50+ feet in the past with no problem.

 

[SparkyChick]

But is it an audio output device!? It is just sending 12v to a siren like you would get on a car horn. The signal noise you hear at 115db is generated by the siren itself when it gets current.

I interpreted this statement:-

"I got an awful distorting buzz so bad I could not make out the verbal prompts like "please enter password" etc."

To mean that the device on the end was playing the prompts. Clearly I was wrong.

 

[fourbie]

Clearly I was wrong.

My bad for being rather ambiguous.

 

[ian.]

Have you tried the extension wire laid out outside to see if the buzz still exists to eliminate if the noise is from inside the house.

It my well be that the sounder rather than being a simple speaker has some active components as others have said.

Screened cable may work, you only earth one end, or try cat5 cable as that might Work

 

[fourbie]

Have you tried the extension wire laid out outside to see if the buzz still exists to eliminate if the noise is from inside the house.

It my well be that the sounder rather than being a simple speaker has some active components as others have said.

Screened cable may work, you only earth one end, or try cat5 cable as that might Work

I didn't try the sounder on extension outside as it was not long enough to escape the house. The main test was simply connecting a length of wire with no sounder attached as I did with the already installed sounder wire and still I got the buzz.
Is cat5 the broadband cable. I have reels of telephone cable but is that cat5?
What is the best way to earth the screen - should I earth at the host unit? Should I use single and screen or twin?

 

[ian.]

So the buzz is coming from the main control unit, this sounds like a dodgy power supply, how is the unit powered, maybe try it from a battery.

 

[Jim_e_Jib]

The supplied red and black flex suggests the sounder has a polarity? If so, have you followed this?

Do the instructions make any reference to maximum cable length or a requirement for screened cable? The supplied cable length could be very telling or just suggest the manufacturer is saving a few pence...

If it is a cheap alarm, perhaps it is just a design feature. If this is the case, it could probably be engineered out but maybe you're best off purchasing a system you've used before?