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Topic: Paranormal Activity blowing out Center Speakers

Adam Sherwood
Film Handler

Posts: 20
From: Wicklow, Eire ,Eire, Ireland
Registered: Jul 2013

posted 01-05-2014 11:56 AM

How many times have you had Customers come out and Demand surround sound for the whole film ?. Esp on film were their is just 2 people talking.

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Harold Hallikainen
Jedi Master Film Handler

Posts: 906
From: Denver, CO, USA
Registered: Aug 2009

posted 01-05-2014 02:19 PM

"I guess very few systems are - on paper - capable of coping with a 0dB FS soundtrack when the sound processor is set to 7.0, and you always need to prove that your system is properly calibrated."

"I have the feeling that it would be quite tough to prove that the speaker has failed because of something wrong on the soundtrack. Studios can easily say "the speaker wouldn't have failed if your speaker had been properly spec'ed for the size of the room". "

If one speaker can do 85dB with -20dBFS (-23rms), and we gain 3dB every time we double the power put into the room by doubling the number of speakers at this level, we get 85 (C) +3 (doubling by adding L), +3 (doubling from 2 to 4 by adding R, Ls+Rs at 82 each) for a total of 91 for a 5.1 system with all speakers except LFE driven at -20dBFS. Add 20dB for FS and we have 111dB in the room. That's loud!

Headroom is for transient signals above nominal, not for continuous signals above nominal. It seems that mechanical failures in speakers could be due to instantaneous amplitude, while electrical failure would be due to longer term signals, depending on power dissipated, heat capacity, heat dissipation (convection, conduction, etc.), and temperature limits of materials. Recent amplifier designs have speaker protection circuits. Are these adequate for the job?

Harold

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Marco Giustini
Film God

Posts: 2713
From: Reading, UK
Registered: Nov 2007

posted 01-05-2014 03:34 PM

Harold,
111dB is correct, but what if the sound designer/mixer just pushed a FS sound through the centre channel for a short period of time? It would 'only' be 105dB. I know tons of cinema with speakers unable to cope with such demand if running at 7.0. The amp may or may not be able to protect - depending on the amp, the limiter and whether the limiter has been engaged or not.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 01-05-2014 03:51 PM

I can say that "we" design sound systems that are capable of playing 0dBFS from any/all speakers with the fader set to 7.0 (or 0dB for those people)...I think it is incompetent to not design a system that is capable of playing the program material.

Part of the confusion is what is perpetrated by the speaker companies in the quest for sales and for exhibitors that have the "Ghostbusters" mentality. That is..."We are ready to believe you."

Speaker companies rate their drivers in either continuos ("AES"), program or peak values. Where program is double pink noise, and peak is double program. While it is true that typical signals are not well represented by pink noise or a continuous signal, subwoofer IS best represented by continuous pink noise (band limited to the pass band of the system).

Another thing that will zap the inexperienced are passive crossovers. First, most suck nowadays...they do the minimum, it would seem, to have the various sections play the frequencies that they are supposed to play but very little matching seems to be going on. Also, with passive crossovers, any distortion is going to likely take its vengeance out on the higher frequency drivers because that is where the distortion is going to end up and the passive crossover is all to happy to send it that way.

When one looks at the response of a speaker...if you have a speaker that needs a lot of inherent EQ, particularly boost...that boost is on TOP of the normal power going to the driver. If you boost 1.3KHz by 3dB to make things look flat on the analyzer...well you just doubled the power going to that driver at that frequency. If the crossover isn't well matched you are going to be doing a lot more EQing...that all comes off of power handling.

In short, don't cheap out on speakers and if you are going to buy a 3-way or 4-way system, tri or quad amp it...you WILL get better results. In cinema, a speaker that requires little EQ to be flat in response is going to be a better choice than a cheap speaker that needs a lot of electrical help to play all of the frequencies equally. A 3 (or 4) way speaker is NOT necessarily a better speaker than a 2-way. A good 2-way beats a mediocre 3-way EVERY TIME. If you can't afford a good 3-way or 4-way, buy a good 2-way for less. You'll get more speaker for your money and have fewer problems. Get the crossover right too. Make sure that the crossover point is correct, the slopes are correct and any time delay actually matches the actual speaker. Balance each section properly. "Out of the box" crossover settings rarely work as well as you would hope.

105dBc, the required level from all channels except Subwoofer...which has to be able to hit 115dBc, takes more power than you think. If you were running optical sound (SR) before DCinema, you need about 10dB more power than you used to. If you were running Dolby digital before, you need 2dB more...and with each 3dB meaning TWICE as much POWER...this all adds up fast.

When looking at speaker sensitivity...watch out...the proper rating is the decibel level measured with 1-watt at 1-meter. Some will cheat this and use a voltage rating like 2.83V...do the math and get it into watts.

If one downloads the Dolby Technical Manual here on Film-Tech...you can skip the math, for the most part...since it was for Dolby Digital, it is 2dB light for today's needs and it is 5dB light on the subwoofer spec. Look up the pages with the nomographs where one can see sensitivity, compare it to room length and then see what the required power is.

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

posted 01-05-2014 05:27 PM

I'm with Steve on the passive x/overs not being much. It is quite easy to overload and saturate the inductors and produce all sorts of nasty harmonics that end up in the HF.

Also agree a good two way beats a so/so 3 way speaker. Meyer's cinema speakers are 2 way, albeit at great cost, but don't cut any corners.

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Mike Blakesley
Film God

Posts: 12767
From: Forsyth, Montana
Registered: Jun 99

posted 01-05-2014 11:05 PM

quote: Frank Cox
You should consider playing them. They always do reasonably well here. They've never been really huge, but still worth the week on the screen.

We don't really do well with horror movies. We played "The Conjuring" last summer and it tanked, despite being a huge hit and well-reviewed and everyone who DID see it loved it. I have all sorts of theories about why scare flicks don't work here but it's too boring and off-topic to go into at this point, I guess.

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Kurt Werbrouck
Film Handler

Posts: 7
From: 9000 gent Belgium
Registered: Aug 2004

posted 01-06-2014 01:11 AM

Hello,

In Granada Spain, they have a lot of damage speakers already since the screening of that movie..
How will pay the bill of the broken HF drivers?

Kurt

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Marcel Birgelen
Film God

Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012

posted 01-06-2014 12:07 PM

quote: Kurt Werbrouck
How will pay the bill of the broken HF drivers?

You, or rather the company you work for [Wink]

But, shouldn't a THX certified theater be somewhat immune to such problems? [evil]

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 01-06-2014 12:22 PM

If it did indeed meet THX specifications, was in calibration/good repair...then yes, you should not have an issue in a THX room. Note, if the room was designed/calibrated in the film-era (most of them)...then you may have a 2dB deficiency. Also, if you have a company that got THX to approve non-qualifying theatres...well, you get what you got.

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Scott Norwood
Film God

Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99

posted 01-06-2014 12:30 PM

I am confused as to why a D-cinema track would have more headroom than an SRD track. Both are recorded so that 85dBC == -20dBFS, correct (sub excluded)? If so, then the maximum achievable SPL per channel in an ideal room at a fader setting of 7.0 should be 115dBC (in that magical seat, 2/3 back, slightly off-center), right?

What am I missing?

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David Zylstra
Master Film Handler

Posts: 432
From: Novi, MI, USA
Registered: Mar 2007

posted 01-06-2014 01:17 PM

I find it interesting that the memo specifically mentions a common denominator of JBL 4632 speakers.

I am curious if anyone has had issues with other models?

quote: Steve Guttag
Another thing that will zap the inexperienced are passive crossovers. First, most suck nowadays...they do the minimum, it would seem, to have the various sections play the frequencies that they are supposed to play but very little matching seems to be going on. Also, with passive crossovers, any distortion is going to likely take its vengeance out on the higher frequency drivers because that is where the distortion is going to end up and the passive crossover is all to happy to send it that way.

When one looks at the response of a speaker...if you have a speaker that needs a lot of inherent EQ, particularly boost...that boost is on TOP of the normal power going to the driver. If you boost 1.3KHz by 3dB to make things look flat on the analyzer...well you just doubled the power going to that driver at that frequency. If the crossover isn't well matched you are going to be doing a lot more EQing...that all comes off of power handling.

When I started my current position I had 2 auditoriums with 4632s that regularly popped HF drivers, after fixing a bad EQ (in a 6AD with a lot of HF boost) the issues dropped from every few months to every couple years . . . . . those racks have now been rebuilt with all new amps (swapped Crowns for QSC) and new processors (1 CP850 and 1 DCP300) - they perform much better now and without issues.

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

posted 01-06-2014 01:59 PM

If I recall, Dolby decided on +18 from 85 dB as the headroom. Don't recall the reasoning except perhaps caution.

This was early '90's so there were a lot of cinemas having trouble making these numbers.

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Marco Giustini
Film God

Posts: 2713
From: Reading, UK
Registered: Nov 2007

posted 01-06-2014 02:14 PM

quote: Steve Guttag
I think it is incompetent to not design a system that is capable of playing the program material.

I agree 100% with that.
But what happens when the customer asks for a smaller sound system? It may be different in the US but in Europe I know very few sound systems properly spec'ed to the room. Most won't break so easily and many are protected by the amps' limiters.

Most cinemas run film at the stellar fader of 5.0 (many lower than that) so it does not really matter.

I may know one or two sites running films at fader between 6 and 7. And I've seen many cinemas, trust me.

Glad that in the US, sticking to the math does not take you out of business in a blink.

I see why Dolby Digital is 2dB light than PCM, but why 5dB for the sub? I thought DD was 113dB and PCM was 115dB?

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

posted 01-06-2014 03:18 PM

Our customers are just as cheap as in the UK. We have many underspec'd cinemas for sure. There may be more competition here so keeping up with the others may be more of a factor here but just guessing.

The subwoofer spec is hard to meet in a room of any size and continues to be a problem as the customers just don't believe the # of boxes required so end up skimping.

Since they often run the fader too low it does not blow up subwoofers as often as one would think.

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Marco Giustini
Film God

Posts: 2713
From: Reading, UK
Registered: Nov 2007

posted 01-06-2014 04:38 PM

Sam, that sounds more like the world I'm used to! [Smile]

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