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I recently purchased a twin pack of Kidde smoke detectors. One thing I noticed is that one sounds off at a slightly higher pitch than the other. A friend purchased two detectors of the same mode as mine, but purchased them separately, one at a time. Each of his alarms also sounds off at a different pitch. I could not find anything online or in the manual. Is this normal? Or would it be a defect in their current production batch? It's after hours so I could not reach Kidde, just something that's keeping me interested in my caffeine-induced insomnia!

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    I'm a little surprised that there would be such variation within a specific model of smoke detector. However, a bit of quick searching found 3kHz, 3.1kHz and 3.2kHz as frequencies for residential smoke detectors, so at least from the point of view of "normal", there is quite a range. – manassehkatz-Moving 2 Codidact Jul 29 '19 at 4:44
  • ...and this is a matter of concern why, exactly? If anything, the dissonance should help it be more annoying and get you out of the house faster than a soothing symphony of perfectly tuned alarms... – Ecnerwal Jul 29 '19 at 13:58
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The device that produces the sound is a piezo-electric transducer: smoke alarm

It works like a speaker, but as you can see, it has three wires, not two.

The detector sends the alarm signal to the transducer through two of the wires to produce the basic sound. The third wire sends feedback to the controller, which allows it to slightly adjust the frequency of the signal it is sending so that it matches the natural resonant frequency of the transducer.

This positive feedback mechanism means that the transducer will be sounding at its loudest possible volume. Think of a wine glass ringing when someone sings the perfect note that matches its resonant frequency.

It also means that even though the controller initially sends the same frequency signal, any two units will not necessarily resonate at exactly the same frequency.

See What's the third wire on a piezo buzzer? - Electrical Engineering Stack Exchange for a more technical explanation.


A similar thing happens in two-stroke gasoline engines. If you've ever ridden a dirt-bike, you might have noticed that there is an RPM sweet-spot that provides maximum power. Again, this is produced by resonant feedback from the exhaust system that causes, at that one frequency, unburnt fuel in the exhaust gas to be forced back into the cylinders. Every engine will have a slightly different sweet-spot and sound slightly different from every other engine.

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A slightly different pitch would be normal manufacturing tolerance.

What is required (at least in the UK, and probably throughout Europe as there's an EN standard for fire alarm systems) is that all interconnected fire alarm sounders should be synchronised in cadence so:

1. !!!!!-----!!!!!-----!!!!!
2. ---!!!!!-----!!!!!-----!!!!!

would not be allowed.

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The alarms sound different as the sounder units are likely set to slightly different settings.

They may be adjustable or, depending how they are driven, the supply circuit may be adjustable.

If there is some sort of adjustment then you might consider trying to adjust them. You could find software or an app to show the frequency & pitch etc to try to get them to be similar, but at a guess identical won’t happen.

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