what keeps a dc bell from vibrating on supervisory voltage.

Theory question: what keeps an alarm bell from vibrating based on
The supervisory voltage?

I’m looking at a troublesome system with two bells, one of which makes a dull rumble at all times. That bell is marked 24dc .03 amp polarized supressed. The other bell is marked .06 amp 1.8k type. The polarity matches up, as does the of line resistor. The bell voltage at idle is 24 vdc and at alarm 25 vdc. Both alarms ring at full volume in alarm. The .03 amp bell test normal on the bench with a power supply.

Thanks for your insight.

Diode polarization. There’s a diode inside the bell that only allows it to ring when current is flowing a certain way.

When the panel goes into alarm the polarity on the NAC flips and the bell rings.

Bell/supervisory polarity would a good topic to mention at <URL url="How to wire a pull station to a horn/strobe text=“viewtopic.php?f=60&t=6312”>How to wire a pull station to a horn/strobe


Here the install sheet for the bell shows + from the panel going to a red wire, and - to a black wire.
These have to be reversed to get the expected behavior. Panel plus must go to black to prevent the supervisory current from jiggling the bell softly.

Is this normal? A miunderstandnig of the instructions? A defect in the bell (seems unlikely)? Or just the way these things are meant to be wired up?

(As expected the bell rings on the bench when 24 volts is hooked up with + on red, and it does nothing when polarity is reveresed. The voltage drop is consistent with a single diode drop (the unit is not easily dissassembled in order to inspect the coil or diode))