Dolby CP750 – Loud bang and sirens Who knows this?

[Ryan Gallagher]

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.

Usually if it was THAT loud, the phone mic wouldn't be able to handle it either without distotions. Although maybe phone mics and recorders have gotten a lot smarter recently and do some fancy limiting and/or compression automatically.

Also above the pain thresholds, one's instinct should be to cover their ears... not get their phone out and use both hands to record it?

[Randy Stankey]

I think he was showing us another video that had a similar sound to the one he heard when the amp went bad, not the actual incident in the theater.

[Ryan Gallagher]

I think he was showing us another video that had a similar sound to the one he heard when the amp went bad, not the actual incident in the theater.

Perhaps, but the fact it is a dropbox link via Dennis's account will usually be a personal upload? If it was a youtube I would expect it to be "found footage" unless stated otherwise.

I think this is a customer and not a tech, looking for information about the technical aspects of this incident, perhaps related to legal. But to answer the original question, this type of failure mode with audio signal continuing is very uncommon, although things can fail in unintended ways sometimes.

[Randy Stankey]

I've seen this happen in theaters. I wouldn't say it's common but it does happen.

My local theater had Peavey amps in several of the houses. They were pieces of junk. I don't care what they said, Peavey amps are for rock and roll, not for theaters. They aren't even particularly good for that, either.

Those Peavey amps would die just like the OP explained. Right in the middle of a show, they would go into oscillation. Sometimes it was loud. Most of the time it was just in the background. Either way, the amp was toast. The solution would be to just turn the amp off until the end of the show and deal with it, then.

If the amp was on an important channel (L/C/R) we would swap the amp with another channel and keep the "swapped out" channel shut down until it could be sent out for repair. Sometimes, we would take an amp out of one of the smaller theaters and swap it in for the bad one.

While I am pretty certain that the problem at hand was because of a bad amp, I am disappointed that somebody didn't come along and, at least, shut that amp off until the end of the show when it could be dealt with properly.

[Marcel Birgelen]

Usually if it was THAT loud, the phone mic wouldn't be able to handle it either without distotions. Although maybe phone mics and recorders have gotten a lot smarter recently and do some fancy limiting and/or compression automatically.

I remember doing some recordings on my crappy 4 year old Android phone of a problem with a LED wall on a festival I was working on for a project (that used that very same LED wall). While recording the problem with some module going haywire in order to have it looked after, a pretty popular dance act was playing on stage... Yeah, I was pretty close to the stage, so the sound on that recording was just a piece of distorted junk. Now, there are plenty of "bootlegged" recordings of that same act, made by people's personal phones or camera gear without any of this distortion on-line on TikTok, YouTube, Instagram, etc..., some also from similar distances to the stage. While some might have done some post-production on the sound, I doubt all of them did.

Since recording at live concerts is something that happens quite often nowadays, there are probably a bunch of more evolved phones out there that do some kind of limiting or have microphones with far higher input tolerances for such scenarios. Those phones are constantly evolving... My current, 4 year old phone is better in filming and taking pictures in dark rooms, as my 8 year old, dedicated 4K Sony camcorder...

While I am pretty certain that the problem at hand was because of a bad amp, I am disappointed that somebody didn't come along and, at least, shut that amp off until the end of the show when it could be dealt with properly.

If it was the place from the recording, it looks like a standard multiplex. Who do you think, still works there, giving a rats ass of what actually happens in one of the auditoriums?

[John Thomas]

I experienced a very similar issue on a PM back in 2019 during my multiplex service days. I would say that it's definitely not common, since I haven't heard of it happening before or since. Here is the relevant portion of my service report, for what it's worth:

Began calibrating MCGD on house 4. While in the auditorium, suddenly heard loud 1khz square-wave through all audio channels. Found CP750 in an error state and unable to boot. Removed CP750 and installed on-site spare JSD-80.

[Tony Bandiera Jr]

Some CP-200's would, if you powered it down before the amps were shut off, make a loud BANG in most of the channels followed by a high pitch whistle that slowly faded out.

I have also had some amplifiers fail in the same way as in the video...notably Peavey (As Randy mentioned) and far too many Crowns of later models (macrotech, et al). The Crow D series would just suddenly put the full DC rails on the outputs, with a single loud pop as the woofer cone launched out of the basket and burned.

I would also suspect the CP750 of doing it, if the wrong caps on the motherboard let go....