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Author Topic: Cinema Sub Limiter?  (Read 590 times)

Peter Kowalczyk

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Cinema Sub Limiter?
« on: April 12, 2018, 05:11:34 pm »

A cinema client burned up the driver in his subwoofer.  I'm exploring options to address the issue.  One is a bigger box that can handle more power and generate more output (enough rig!).  Another is to repair the driver and add a limiter to the system to protect it from future damage. 

So, I'm looking for a simple average-power limiter to place before the subwoofer amp.  Preference for something with phoenix or screw-terminal connections (this will live permanently in their equipment rack) and a small form factor (1RU or even smaller 'black box').  dbx 160A seems to be an option, but there's no control of time constant and I'd love to not have to solder XLRs to connect it;  RDL's ST-CL2 looks like a handy and cost-effective option, but I'm wary of its 'fully automatic operation' and absence of typical control parameters.

What would you use?
Thanks!
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John L Nobile

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Re: Cinema Sub Limiter?
« Reply #1 on: April 12, 2018, 06:13:54 pm »

I have to ask...was there a HiPass filter on the sub?
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Mike Caldwell

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Re: Cinema Sub Limiter?
« Reply #2 on: April 12, 2018, 06:20:53 pm »

Is this in a professional movie theater or a home theater/cinema.

Tim McCulloch

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Re: Cinema Sub Limiter?
« Reply #3 on: April 12, 2018, 06:21:46 pm »

A cinema client burned up the driver in his subwoofer.  I'm exploring options to address the issue.  One is a bigger box that can handle more power and generate more output (enough rig!).  Another is to repair the driver and add a limiter to the system to protect it from future damage. 

So, I'm looking for a simple average-power limiter to place before the subwoofer amp.  Preference for something with phoenix or screw-terminal connections (this will live permanently in their equipment rack) and a small form factor (1RU or even smaller 'black box').  dbx 160A seems to be an option, but there's no control of time constant and I'd love to not have to solder XLRs to connect it;  RDL's ST-CL2 looks like a handy and cost-effective option, but I'm wary of its 'fully automatic operation' and absence of typical control parameters.

What would you use?
Thanks!

Is there an applicable Dolby® or other standard that is being met?

Is this in a professional movie theater or a home theater/cinema.

That's where I was headed with this...
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Caleb Dueck

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Re: Cinema Sub Limiter?
« Reply #4 on: April 12, 2018, 06:29:40 pm »

A cinema client burned up the driver in his subwoofer.  I'm exploring options to address the issue.  One is a bigger box that can handle more power and generate more output (enough rig!).  Another is to repair the driver and add a limiter to the system to protect it from future damage. 

So, I'm looking for a simple average-power limiter to place before the subwoofer amp.  Preference for something with phoenix or screw-terminal connections (this will live permanently in their equipment rack) and a small form factor (1RU or even smaller 'black box').  dbx 160A seems to be an option, but there's no control of time constant and I'd love to not have to solder XLRs to connect it;  RDL's ST-CL2 looks like a handy and cost-effective option, but I'm wary of its 'fully automatic operation' and absence of typical control parameters.

What would you use?
Thanks!
If the issue was improper HPF or not enough Rig for the Gig - a limiter isn't the answer.  Like everyone so far - I'd explore more details first before talking about which limiter.

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Stephen Kirby

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Re: Cinema Sub Limiter?
« Reply #5 on: April 12, 2018, 07:04:30 pm »

Not sure about modern 4K projection systems but back in the days when things had to be able to revert to optical, the mixes were pretty brick walled.  The calibrated levels were with the CP500 or whatever at "fader 7" but that would chase most audiences out so most theaters would turn it down 6-10dB just to keep Michael Bay's explosions reasonable.

I recently went to a movie though and was stunned at how loud it was.  Seemed like they were turning things back up.  Might be related to the new LuFS thing which keeps average levels down and things sound more dynamic than the smashed old days.  Should be easier on the sound system compared to what we had running a smashed action flick at cal'd levels.
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Scott Holtzman

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Re: Cinema Sub Limiter?
« Reply #6 on: April 12, 2018, 07:17:32 pm »

Not sure about modern 4K projection systems but back in the days when things had to be able to revert to optical, the mixes were pretty brick walled.  The calibrated levels were with the CP500 or whatever at "fader 7" but that would chase most audiences out so most theaters would turn it down 6-10dB just to keep Michael Bay's explosions reasonable.

I recently went to a movie though and was stunned at how loud it was.  Seemed like they were turning things back up.  Might be related to the new LuFS thing which keeps average levels down and things sound more dynamic than the smashed old days.  Should be easier on the sound system compared to what we had running a smashed action flick at cal'd levels.

We saw a movie in Imax last year (the watered down theater iMax).  They had all the amps and processors racked up in IT style cabinets behind a glass window so you could see the blinky lights.  It was all QSC amps.

It was nuts loud.  Average LF extension. Everything in the room was rattling too.   

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Peter Kowalczyk

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Re: Cinema Sub Limiter?
« Reply #7 on: April 12, 2018, 08:00:20 pm »

This is a professional theater that sells tickets to paying customers.  The audio stream is managed by a Dolby branded processor, but given the age of the speakers themselves and the relative hodgepodge of other components, I'd be surprised if this was calibrated to meet any particular standard.  Will have to ask the owner.

I'll have to double check the HPF setting.  Good call, and my oversight for not verifying this parameter.  If engaged, it'd be through the QSC Amp's DIP switch settings, or possibly in the Dolby Audio Processor that drives the system.

As usual, more digging required.  Thanks all for asking the right questions....





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Corey Scogin

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Re: Cinema Sub Limiter?
« Reply #8 on: April 13, 2018, 12:02:53 am »

We saw a movie in Imax last year (the watered down theater iMax).  They had all the amps and processors racked up in IT style cabinets behind a glass window so you could see the blinky lights.  It was all QSC amps.

It was nuts loud.  Average LF extension. Everything in the room was rattling too.

The one IMAX room near me is consistently excellent. It's the only theater I'll pay to see a movie in because the others suck and/or are often broken.
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Stephen Kirby

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Re: Cinema Sub Limiter?
« Reply #9 on: April 13, 2018, 08:39:38 pm »

Peter, I think you should reach out to Dolby.  They may have gone public since I worked there, but I'll bet the culture of helping get things right is still there.
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David Buckley

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Re: Cinema Sub Limiter?
« Reply #10 on: April 15, 2018, 10:47:09 pm »

You say "burned up the driver in his subwoofer".  "The" driver, not "a" driver.  It would have to be a small cinema to have a single sub with "a" driver.  So I'm in the "not enough sub for the gig" camp.

For digital cinema, the subs are supposed to be able to deliver 113dB to a seat two thirds of the way back from the screen.  This is achieved with the fader on 7, and a movie that goes to movie full loudness, which is -20dBFS. 

Here is a handy calculator.  Enter the distance as whatever is 2/3rds back from the screen, enter the SPL required as 113dB, enter the appropriate sensitivity of the speaker you've chosen, and that's how much power needs to be delivered. 

Fortunately, most cinemas play movies quieter than 7, and most movies don't hit "full". 


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Craig Hauber

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Re: Cinema Sub Limiter?
« Reply #11 on: April 16, 2018, 12:50:15 am »

You say "burned up the driver in his subwoofer".  "The" driver, not "a" driver.  It would have to be a small cinema to have a single sub with "a" driver.  So I'm in the "not enough sub for the gig" camp.


Yes, this.
So check the other three sub boxes to see if the employees stole the woofers out of them to bump-up their car stereos ;)
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John L Nobile

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Re: Cinema Sub Limiter?
« Reply #12 on: April 16, 2018, 11:34:31 am »

Yes, this.
So check the other three sub boxes to see if the employees stole the woofers out of them to bump-up their car stereos ;)

Employee theft is real. I inherited 2 JBL 2x 15" "subs" from a former AV company install. When I finally got around to using them, there was no bass. I opened them up and found that someone had taken just 1 woofer out of each box. I wound up putting both speakers in one box but that was a waste of time. I imagine that the speakers were damaged flopping around with that big hole in the box.
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Peter Kowalczyk

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Re: Cinema Sub Limiter?
« Reply #13 on: April 16, 2018, 05:46:54 pm »

Thanks all,

The theater is a small one, <200 seats, and yes this is (was) the only sub in the room (he got a bose 2x10 or something that's been handling the LFE channel since this EV box died, quite a while ago.)  The owner tells me it was calibrated to dolby standard.  If so, it's not surprising that single 18" driver gave up trying to push 113dB two-thirds of the way back. 

Regarding calibration, I'm curious why 'Full Movie Loudness' would be -20dBFS - that must be a long-term average, not a peak figure, right?  That'd be a lot of digital headroom to waste...
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Jean-Pierre Coetzee

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Re: Cinema Sub Limiter?
« Reply #14 on: April 17, 2018, 04:50:10 am »

Thanks all,

The theater is a small one, <200 seats, and yes this is (was) the only sub in the room (he got a bose 2x10 or something that's been handling the LFE channel since this EV box died, quite a while ago.)  The owner tells me it was calibrated to dolby standard.  If so, it's not surprising that single 18" driver gave up trying to push 113dB two-thirds of the way back. 

Regarding calibration, I'm curious why 'Full Movie Loudness' would be -20dBFS - that must be a long-term average, not a peak figure, right?  That'd be a lot of digital headroom to waste...

Cinema crest factor is recommended at 20dB so it makes sense for the meter(which is usually averaged) to be -20dBFS.

Re: the LUFS statement, LUFS is only applicable at 85dBSPL. LUFS is pretty much a K-Weighting scale with an average constant to hit, it's good science but not difficult to implement, if the movie was consistently above 85dB(probably something close to A-weighting) then the cinema was cranking it to give you a "larger than life" experience.

The explosions and such can be much louder(up to 20dB as I said) but the average should be tolerable.

The measurement is averaged over the entire length of the film, has to hit -24LKFS(america) or -23LUFS(everywhere else) which is supposed to be calibrated to 85dBsplA
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Joe Pieternella

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Re: Cinema Sub Limiter?
« Reply #15 on: April 17, 2018, 05:07:45 am »



Thanks all,

The theater is a small one, <200 seats, and yes this is (was) the only sub in the room (he got a bose 2x10 or something that's been handling the LFE channel since this EV box died, quite a while ago.)  The owner tells me it was calibrated to dolby standard.  If so, it's not surprising that single 18" driver gave up trying to push 113dB two-thirds of the way back. 

Regarding calibration, I'm curious why 'Full Movie Loudness' would be -20dBFS - that must be a long-term average, not a peak figure, right?  That'd be a lot of digital headroom to waste...

I'm pretty sure the reference level is specified differently.
As far as I know it says that when applying pink noise at -20dBFS to a single channel that channel should produce 85dB. Movie dialogues seem to be around these levels mostly. This gives 20dB of headroom for explosions and car crashes etc.
The LFE channel should produce 95dB at - 20dBFS.

A single 18inch driver pushing 120+ dB at (and sometimes below) 20Hz... Can't really think of an EV box suited to this application especially without a (steep) HPF. the double ten is probably close to failing too.

Ps. I assume the spl levels in the specification are a-weighted RMS values.

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