I've been to a warehouse with so much noise from machinery that I was given ear plugs, yet the door at each storey exit had a dorgard on it. All doors were held open. How does that work then?
The noise from the machinery wasn't at the frequency(ies) needed to trigger the dorgard?
Is that a more than likeley answer or another question?
SidM, it is a guess at the answer put in the format of a question based on the fact that I can't possibly definitely know the answer because I don't have all the facts and knowledge!
The guess I made was on the assumption that the dorgards are in proper working condition and that my understanding is that they are 'tuned' to react to the frequencies of fire alarm sounder warning devices. Since they didn't close (and assuming they are not faulty) it must be assumed that the machinery is not giving out the frequencies required to operate them.
However, I agree it does seem strange. I would have thought that loud machinery operating must create a wide spectrum of harmonics across many frequencies, and based on the many stories of dorgard units operating to the noise created by a simple vacuum cleaner then it is stranger still.
The effect of general noise on Dorgard units is supported by the fact that I believe they also make a version that has a RF transmitter that should be sited close to sounders and which sends a radio signal to operate nearby 'non-acoustic' Dorgards. In fact, I wonder if maybe they use these wireless signal versions at the site you are talking about precisely because of the noise of the machinery!
Finally, one would assume these devices are tested on a regular basis and that they function correctly whichever version they use. Maybe the warehouse owners can provide the answer.