Eliminating buzz from burglar alarm motion sensors

[Hywel]

Hi All,

I just figured out a bizarre audio problem recording in my house (which is where we do quite a lot of our shooting, as our studio is here).

I was getting buzz pickup on sound. I've narrowed it down to something from the motion sensors in our burglar alarm being picked up by the mic- although bizarrely it sounds an awful lot like 50 Hz hum to my ears. The burglar alarm sensors are stand alone battery powered units, not connected to the mains at all.

I tried changing mics, cables, transmitters, then going direct by wire into my H4n instead of wirelessly into my RED. I'm now fairly sure it isn't any sort of earth loop mains pickup, bad connection or equipment fault since I can replicate it with two completely independent chains of audio equipment simultaneously.

Pointing the mic close to the motion sensors if definitely seems to be coming from them. Bizarre that I've not noticed it before, but now I have noticed it, I am wondering what the hell I can do about it.

Does anyone have any suggestions?

The sensor units appear to be constantly active, even when the alarm is switched off. Each sensor is individually powered by battery and communicates with the base station by RF. So I'm guessing the buzz is actually a high frequency sound producing a lower frequency alias tone in the output (although it is really bugging me that it sounds like 50 Hz). I'm using K6 modules, and I hear it both with an ME66 short shotgun and the ME64 cardiod. I can find zones of lower buzz to place the mic, presumably these are nulls where the reflections from walls and ceilings cancel out, but it isn't well rejected by angling the mic, it seems to be primarily sensitive to the mic placement rather than pointing.

Anyone encountered a similar problem? What did you do?

Any ideas much appreciated. Can't really afford to rip out/replace the alarm system, having it is a requirement of insurance.

Cheers, Hywel.

 

[Hywel]

OK, I've answered my own post with more data. Turned the house electrics off at the consumer box. Buzz disappeared. It was indeed 50 Hz, should have trusted my ears, it is the house electrics.

The correlation with the placement of the burglar alarm sensors was very strong, but the buzz disappeared instantly when the electrics were switched off.

Time to call an electrician.

Cheers, Hywel.

 

[j1clark@ucsd.edu]

OK, I've answered my own post with more data. Turned the house electrics off at the consumer box. Buzz disappeared. It was indeed 50 Hz, should have trusted my ears, it is the house electrics.

The correlation with the placement of the burglar alarm sensors was very strong, but the buzz disappeared instantly when the electrics were switched off.

Time to call an electrician.

Cheers, Hywel.

While I don't know the particulars of your alarms, I presume they are using some sort of 'ultrasonic' (at least for humans) method, and may check ranges with 'bursts' of the ultrasonic signal. While you can't nominally hear the 'ultrasonic' sound... you can hear the 50 Hz (perhaps) bursting of the signal. (The one's I've used long ago in designs have had a 'click' when pulsed... but the burst rate is more like 10 a second.)

Most electronics has 'buzzing'. When I sit in a quite room, and am testing my 'serial ports' on some devices I here an audible high pitched 'whine/buzz' when the serial ports are busy with data traffic...

Then there's the old flyback on CRT displays... etc...

Probably the thing to do is to see if the motion sensors can be put on a switchable circuit, or some how the ones in your work area, switched off. But... an alarm turned off... isn't much of an alarm...