Film-Tech Forum ARCHIVE: STAR WARS LFE SOUND ISSUE?

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Posted by Christopher Lani (Member # 7952) on 01-02-2016, 03:37 PM:

Has anyone else experienced any overloading or Sound distorations during the scenes were the bad guy is using the force to get info out of the characters. I'll give some exact times, but it only happens a few times every showing. I do have a older system but I have not experienced this kind of distortion before.happens at 1:23 and 2:04. Thats how's and minutes. Speakers have been checked and otherwise the sound is fine.

Posted by Sam D. Chavez (Member # 1841) on 01-02-2016, 04:13 PM:

Define "older". Dolby, USL, Smart? Model too.

Film, SR.D or analog? Digital, etc.

Posted by Christopher Lani (Member # 7952) on 01-02-2016, 04:31 PM:

Jesus. I accidently posted this question in 2 forums and I get spanked? Anyways, Dolby CP750 sound, QSC Amps and re coned EV Speakers.

Posted by Stephan Shelley (Member # 8587) on 01-02-2016, 04:36 PM:

Bi-amped? What crossovers?

Posted by Jim Cassedy (Member # 4115) on 01-02-2016, 04:46 PM:

It's very interesting that you should post this question.

The first weekend the movie opened, I got a call from a theater that some
people were complaining about a "sound problem' in parts of the movie.

But they weren't specific. Nonethless, I went down there early the next
morning, went completely through their sound system and could find
nothing wrong.

And now, the theater where I'm running HATEFUL 8 in 70mm has has
had numerous complaints about some sort of 'sound problem' since
they moved the Star Wars flick into a different auditorium last week.

The complaints were too vague to pin down, and spot checks of the audio
by the theater staff, and even the manager, didn't expose any problems.

Yesterday, the manager got an e-mail from some guy from LucasFilm who
went to one of the shows here and he complained that it sounded like
"the subwoofer was blown"

Just because it's been causing them problems and I wanted to help them
out, I came in early this morning with signal generators, DCP Low Freq
tests and an RTA- - and everything checked out OK. I even crawled
under the screen and physically checked the subwoofer, and all the other
speakers and they were all OK and in good shape.

I was beginning to suspect that perhaps something in the room or backstage
was resonating at low freqs and causing a 'distortion effect' or something.

But now that I read your post, I'm beginning to think that mabye there's
some weird low frequency harmonics or other anomaly in the sound mix
that some systems just can't handle, although they are otherwise working OK.

System here is digital (DCP). Processor is a CP-750. I forget what type
of subwoofer was back there, but the amps are QSC 1400's.

>>One Question about your time points:
Is that the actual running time from the start of the movie, or do those
timse include any traiers or pre-show stuff you had in your playlist

THANKS! -jc-

Posted by Stephan Shelley (Member # 8587) on 01-02-2016, 05:37 PM:

Were both the 15" woofers as well as the subs reconed? EV cone type drivers have a bad habit of coming unglued from the outer ring. It can be subtle and hard to spot.

Posted by Monte L Fullmer (Member # 2797) on 01-02-2016, 06:19 PM:

Sounds like the diaphragm in the HF horn is clipping, and could be starting to crack which would create the distortions.

Why the question on bi-amp since the horn should be only getting high freqs where the lower speakers gets the other. If a crossover system isn't doing its thing, the low freqs are heading up to the horn making that driver go insane which it's not designed for such and can easily take a HF driver out.

Two weeks ago, I lost a center horn. The diaphragm had indeed opened up. I moved the driver from left channel to center channel to get my sound back to normal. When I got the replacement driver, the new one went to the left channel.

Posted by Stephan Shelley (Member # 8587) on 01-02-2016, 07:08 PM:

Monte, just to get a complete idea of the system. I don't think it has been isolated to the HF drivers yet, but certainly a possibility.

Posted by Jim Cassedy (Member # 4115) on 01-02-2016, 08:10 PM:

I checked their system here with frequency sweeps, pink noise, etc, and
everything looks absolutely normal on the RTA. I ran about 20min of
the movie this morning, louder than normal, with scenes that included
both quiet dialogue, as well as a few 'battle scenes'- -
and I
heard nothing out of the ordinary. But they've gotten too many complaints
to ignore this. And I'm aware of three different theaters now that have
seem to be experiencing the same issue.

That's why I say I think there's some weird thing in the sound mix in
some scenes that some systems just can't handle for some reason.

They've never had any sound complaints in that auditorium until they
moved Star Wars there last week.

Posted by Steve Guttag (Member # 268) on 01-02-2016, 08:11 PM:

Remember, when you are doing your testing, that you are using a reference signal that is at -20dB, typically so a problem that doesn't show itself at that level might be a real problem at 0dBFS.

I've found MOST subwoofer systems underspecified in my career. That is, if you do the math, if the theatre is played back at reference level (and that is a big "if") then there is no way the subwoofers/amps can handle what might be requested of them. If you have less than a double-18" system in all but the tiniest of rooms (and I mean something like 30-foot deep rooms) you are going to run out of sub. It is not unusual to need four or eight 18" subs to properly handle a typical "large" house in a normal sized room. Remember too, more subs equals more efficiency. It is often better to add more subs than more amplification.

When judging subwoofer power handling, you HAVE to use continuous pink noise not the bogus "program" power or even more bogus "peak" power. You can use program for stage and surround channels but never subs. Sub's signal more closely resembles pink noise and you don't know what its duration is. Think "Apollo 13"...the sub signal can be quite protracted and if your subwoofer runs out of mechanical range, it will kill itself. Likewise, either your amplifier(s) can or can't handle the demands on them.

Posted by Marlon Martins (Member # 8823) on 01-02-2016, 09:43 PM:

i saw the SW-TFA 3 times now, and the "force" scenes really pushed the subs. feels like its giving too much variance on low frequencies and the sub is distorting a bit. but after a sub died a while ago, we split the power between two of them, and the audio is passing on a compressor, using only the limiter function. so it's much more "safe" now.

but i can see how would be easy to distort on that scenes, as look like the sound "intensity" are more than usual. (special on Rey and Kylo Ren interrogation scene)

Posted by Frank Cox (Member # 6258) on 01-02-2016, 11:51 PM:

Jim: Why don't you sit down and watch the WHOLE MOVIE in that auditorium instead of bits and pieces? That way you'll find out exactly what the audience is hearing and that will take a lot less time and effort than fiddling around with test equipment and whatnot. After you have heard the problem for yourself and know where it's located in the movie, then you can drag out the test equipment and you'll know what to look for. If you watch the whole movie by yourself late at night or early in the morning when nobody else is around you'll be able to find out what's really going on instead of making guesses.

Posted by Jim Cassedy (Member # 4115) on 01-03-2016, 12:42 AM:

quote: Frank Cox
Jim: Why don't you sit down and watch the WHOLE MOVIE in that auditorium instead of bits and pieces?

That would be very nice Frank, if running Hateful 8 wasn't taking up
almost 14hrs of my day already. I wish I had an extra two hours.

Don't get me wrong, I'm not complaining about the long H-F-8 shifts,
I'm just saying that it currently occupies all my time.

I don't even really work for this theater, but they were having a problem,
and they've been very nice to me while I'm here, so I thought I'd take
a little spare time to help them out and at least try to track down if it
was something obvious like a blown/damaged speaker or an amp going
bad, etc.

I did them a big favor today by coming in extra early and lugging
test equipment down here, in hopes that I'd find something "obvious".

But, unfortunately, it's going to take more time than I have available
right now to track down this apparently obscure problem.

- - - and yes, even < I > had told the manager that the best thing for me
to do would be to sit down & watch the entire movie, but he understands
that I just can't work that into my schedule at the moment.

There's only so many hours in a day that I can be productive, and right
now my focus (no pun intended) is on running the H-F-8 shows.

Posted by George Germisashvili (Member # 6597) on 01-03-2016, 12:41 PM:

I heard some sub distortion during show at IMAX cinema, 2 days before, the system is only 4 month, every day sound calibration status "pass OK", so idea is that Walt Disney made not only bad "Star Wars" movie, but also something wrong with sound. Once the movie will end at auditorium, the sound problem with sub will go away [Smile]

The better way to check- record the amp output by oscilloscope with record function or use USB oscilloscope, and after check the waveform peaks.

Posted by Jim Cassedy (Member # 4115) on 01-03-2016, 08:50 PM:

They moved Star Wars here to another auditorium.
Problem solved. (sort of)

Posted by Louis Bornwasser (Member # 3063) on 01-03-2016, 09:00 PM:

Wire RTA across each channel and feed pink noise. Might as well do them all. Verify that crossover is, in fact, running correctly. Then look at subs and see if they are restricted to below 100 Hz as expected.

I cannot tell you how many times things "blow up" when fed with full range sound. (This problem was really bad with the old CP-45.) If not driven too hard, system might last a long time before failure.
\

Posted by Christopher Lani (Member # 7952) on 01-04-2016, 02:44 PM:

The Subs were indeed reconed as I was made aware of the cones pulling away by the great staff that installed our digital system. We are also running the QSC but the sub and center channel run off a 1700, not the 1400. I will try to post a video of the distortion as well exact times that it is happening. The problem seems to appear when Kylo is using the force. There are some weird harmonic that are probably state of the art but are more designed for Dolby 7.1, Atmos, or other newer systems. Dropping the volume doez not seem to make the problem go away, that is why I am wondering if it is problem that is occurring in other, older theaters that have sound systems similar to mine. I wonder if just getting a couple self powered JBL 18 INCH SUBS would solve the problem. On a side note, with all of theater closures happening, is there a good place to pick up surplus speakers and chairs?

Posted by David Buckley (Member # 2600) on 01-04-2016, 10:27 PM:

quote: Steve Guttag
Remember, when you are doing your testing, that you are using a reference signal that is at -20dB, typically so a problem that doesn't show itself at that level might be a real problem at 0dBFS.

How close to 0dBFS does digital cinema go?

I hear what you are saying about under-powered subs though, its takes lots of cone area moving lots of distance to get the same sort of SPL a HF horn can do on a few watts, and it still isn't "loud".

Posted by Bill Brandenstein (Member # 7758) on 01-05-2016, 02:06 AM:

David, my educated guess is that movies such as this do reach 0dBFS. From having looked at mix levels in DVD releases (before a home re-mix was the norm) as well as interaction with at least one mix engineer, full scale gets used. Go back a few years to "Master and Commander" and the cannon shots are clipped pretty hard, so (technically) greater than 0dBFS.

Posted by John Thomas (Member # 6509) on 01-09-2016, 07:34 PM:

The digital IMAX near me had to replace 4 (of the 16) sub drivers due to Star Wars.

Posted by Marco Giustini (Member # 4544) on 01-10-2016, 05:31 AM:

quote: George Germisashvili
every day sound calibration status "pass OK"

I doubt the calibration routine is testing the system at 0dB at all frequencies. As it's been said before on this forum, using pink noise you can easily EQ a broken cone and don't realise that.

I've now watched SW and there are no issues with its soundtrack. The point is that when the bad guy uses the force, they used a massive quantity of very deep low frequencies and nothing else. During an explosion any rattle or foreign noises would be covered by the HF content. During the "force" sequence any deficiencies of your system would show up.

During my show one amplifier had a gain potentiometer issue. One of the sub was making a crackling noise which could only be heard during the 'force' scenes. Once the trimpot was cleaned, the noise went away.

There is nothing wrong with the LFE in Star Wars. It's just pushing the subs and there are very few systems around with properly spec'd systems.

Finally, this is usually something that goes overlooked. Nobody notices issues like that till:
a) the auditorium gets full of people
b) there are scenes like the 'force' which clearly show a problem.

Ideally we should test our systems with pink noise AND sweep tones regularly and catch those issues before the customers do.

Posted by Christopher Lani (Member # 7952) on 01-10-2016, 09:25 PM:

Very interesting. Is there somewhere I could go to learn how to clean the trim pots on my QSC 1700? Thing is, I went down and observed the speakers during those scenes and the sound was coming from excessive movement from the 2 15' speakers in the center channel
Under the horn. Maybe cleaning the trim pots still may be the solution. I agree, it is the massive use of the entire acoustic spectrum of the lower and lower mid range that is showing the deficiencit's in our system. Thanks. I'll let you know if cleaning the trim pots helped. I did notice this one other time in a Disney film. During the credits for "TOMORROWLAND" there is a scene where a rocket ship takes off. That was the first time I ever heard this phenomena.

Posted by Marco Giustini (Member # 4544) on 01-11-2016, 01:57 AM:

If you have it, enable the high pass filter on stage speakers - 30 or 40hz.
You may have a 30hz filter on your amps.
You may also be over eq'd at those frequencies. Then check the movement of the cones again.

Posted by Tony Bandiera Jr (Member # 2365) on 01-11-2016, 11:53 AM:

A quick (but often temporary) fix for the noisy pots is to first, mark where they are currently set (and make sure you put them to that mark when you are done) and with the amp OFF, rapidly turn them back and forth over the full range several times, then reset them to the mark.

If you are skilled enough, you can open the amp up and spray the pots with a cleaner (some use WD-40 which sort of works but will attract dirt later on) and again rotate the pots as you clean them.

Or better yet, replace them. Use pots of the same ohms rating and shaft dimensions (QSC may be able to still supply them, but I did not see them on their parts listing), which is the best long-term solution. They can be found online at places like Digi-Key and Mouser. You do need to be able to do good solder removal and soldering though.