Dolby CP750 – Loud bang and sirens Who knows this?

[Dennis_Dirksen]

I was at the cinema a few days ago and the following happened.
There was a short loud bang in the film.
Then the sound went out and 10 seconds later an ear-splitting siren sounded. Is this normal procedure in the event of a failure?
Or who is familiar with this process with other Dolby systems?

The cinema uses the Dolby CP750 system.

[Marcel Birgelen]

That's not typical CP750 behavior. What is normal behavior though is that a fire alarm will kill the running show, raise house lights, etc. To me it feels like you just experienced a fire or burglar alarm going off during the movie.

[Dennis_Dirksen]

No fire alarm.
It was a failure of the power supply for the audio processor.

[Marco Giustini]

The PSU will kill the sound and I'm not aware of an emergency system which would sound sirens if the sound stops ?

What MIGHT have happened is that the PSU shorted, it tripped some breaker which triggered some emergency procedure. It still feels excessive though, as long as emergency lights can be turned on, there is no need to sound sirens to evacuate a room.

As Marcel pointed out, in all the cinemas I've worked for, emergency evacuation messages would only be required in case of a fire or some major emergency - that is, manually triggered by the staff. A sound processor biting the dust shouldn't trigger that.

[Frank Cox]

I think you need to provide more information about exactly what happened in order to get good answers.

You have two threads going now about this same issue.

Can you describe exactly the sequence of what you saw and heard then? The movie was playing as normal and then....

I'm not sure if it's relevant, but was it a blast-em movie or a quiet drama?

Did you hear the bang through the speakers or from behind you, if you were sitting in the auditorium. Did you hear the siren through the speakers or from some other source or direction?

Did the siren stop? If so, did it stop within a few seconds, within a minute, ten minutes later?

And, again, did you ask the people running the movie what had happened? What did they tell you?

[Marcel Birgelen]

If the PSU failed on the audio processor, it's unlikely the sound of the sirens came through the speakers, unless there is a rather elaborate emergency system in-place that switches over the B-Chain input in such a case.

Marco's reasoning sounds plausible: A short in a PSU might have triggered one or more breakers and/or RCDs, the resulting power loss might have activated the fire/evacuation alarm.

Emergency lights should be automatically activated in the event of a power-loss on their input, the alarm you heard might have been "wired" to do the same, although I agree, that this feels somewhat excessive.

In some rare cases, local fire departments can make some special requests. I know about one location, where the biggest room has a mandatory evacuation procedure for every fire alarm that occurs, even if it's just a pre-alert, which cannot be manually overridden by the staff, while the other rooms operate on a short delay. The reason being, the fear not being able to clear the biggest room in-time, if a real emergency happens.

[Dennis_Dirksen]

here is the sound
That's how it sounded in the movie theater.

https://www.dropbox.com/scl/fi/d8d6a5pnvgffp3wjurwht/Video-13-11-2025-22-09-21.mov?rlkey=labl94o04p6mwpqiz5ve1v7v6&st=m8u5ud19&dl=0

[Sascha Roll]

Mind if I ask:
What exactly is your goal here?

[Dennis_Dirksen]

As described,
find out if this happens often.

[Marco Giustini]

you mentioned permanent tinnitus in the other thread so I wonder if you're looking for facts to back up a claim against the cinema?

Not sure what that noise is - when a device fails, it can output noise. The 750 has some protection on that: output relays should open the output lines if the system is compromised but nothing is fail-proof and those protections can fail to activate too.

Hint: next time instead of filming you could cover your ears ?

[Dennis_Dirksen]

Nice tip. But that's the best proof, and I always carry earplugs with me, which I put in after the bang.

[Randy Stankey]

That is the sound of an amplifier biting the dust and going into oscillation.

The loud noise in the soundtrack of the movie caused an amp to blow.

[Dennis_Dirksen]

That is the sound of an amplifier biting the dust and going into oscillation.

The loud noise in the soundtrack of the movie caused an amp to blow.

Hi Randy, what does “swing” mean?

[Randy Stankey]

Hi Randy, what does “swing” mean?

Oscillate: To alternate position or to transition between different states.

In this case, I'm referring to something going wrong with the amplifier's circuitry that caused an oscillating voltage that was passed to the amplifier's output.

It could be a diode or a capacitor or something like that. I don't know for certain because it's been too long and I forget too many things.

Regardless of the cause, when I was working in theaters, to hear that sound or one like it was a sure sign that an amplifier has failed.

[Marcel Birgelen]

Like Randy correctly identified, this isn't a siren, but a clear sign of some electronic component going into oscillation after failure. I've had similar sound coming out of amplifiers after a component failure. Most modern theaters don't rely on a single amp, but a stack of amplifiers for the different channels. Some speakers even might have multiple amplifiers, depending on size of the sound system. What's interesting is that there is no other sound besides this whining, slowly oscillating noise. Either this noise is so loud, it drowns out all the other sound in the movie or something in the A-chain failed.