Film-Tech Forum ARCHIVE: Subs placement

This is topic Subs placement in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Antonio Marcheselli (Member # 373) on 06-09-2002, 01:51 PM:

Hi everyone.

In our theater probably we will add two more subs soon.
However since physic says that our present subs should be enough for the room, I was wondering if the actual placement of the subs is not adeguate and if we can move them to a better position plus, eventually, two others.

Actually we have 4 4645B subs, coupled toghether exactly under the center channel, at 3 meters from the floor.
In a first moment we thought to add other two subs under the screen, at, say, 50cm from the floor. What about move ALL subs at 50cm from the floor? However in that position we cannot couple toghether the enclosures but we have to put them "side by side" under the screen.

What do you think is the best? Left our 4 4645B where they are and add two others under the screen or move all them under the screen?

Barry Ferrel from QSC sent me some very useful documents where I discovered that the best placement is on the floor but for layout resons we cannot put them directly on the floor...

More: the subs wwill not be the same model. We thought to cut the two different speakers into two different bandwitdh like, say, 20-40 one type and 40-100 the other type... Opinions?

Thanks for your suggestions!

bye
Antonio

Posted by Chad Souder (Member # 343) on 06-09-2002, 02:56 PM:

Can you give any specs of the auditorium?

Posted by Antonio Marcheselli (Member # 373) on 06-09-2002, 04:48 PM:

Of course.

23 meters long, 17 meters wide. Screen is 15 meters.
Stadium seating. THX certified.

Bye
Antonio

Posted by Steve Kraus (Member # 476) on 06-09-2002, 05:28 PM:

3 meters above the floor? What is underneath the sub? In particular is there a solid wall extending downward to the floor directly under and even with the front of the sub? That would help as would solid panels on either side of the sub.

Posted by David Favel (Member # 1125) on 06-09-2002, 05:40 PM:

We have ours on the floor as the sub vibration gives a wonderful added bonus to a movie. (esp the Matrix slo mo scenes)

Posted by Paul G. Thompson (Member # 655) on 06-09-2002, 07:19 PM:

I threw mine on the floor behind the screen, which was the best way I could make it fit.

David is correct. It can shake the seats better for some reason.....

Posted by Gordon McLeod (Member # 33) on 06-09-2002, 10:14 PM:

Well THX will specifie the speakers required the amps and the placement if you want to keep the certification

Posted by Antonio Marcheselli (Member # 373) on 06-10-2002, 05:52 AM:

Steve,

There is a sound wall.

Gordon,

THX specs are absolutely incorrect, in our theater. ALL speakers are underpowered.
However subs actually aren't where THX said... Let's see this photo:

Pls excuse the wrong perspective!!
As you can see I cannot put them directly on the floor, there are the seats just in front of that platform.

Originally THX said to put the subs where I draw, on the platform. It is a bit hard now to have them all coupled on that platform (we'll se what we can do) but what about to have them all side-by-side under the screen, behind that black "curtain"?

Thanks for your replies

Antonio

Posted by Gordon McLeod (Member # 33) on 06-10-2002, 10:04 AM:

Odd if anything THX usually err's on the side of bigger than required amps

Posted by Antonio Marcheselli (Member # 373) on 06-10-2002, 02:12 PM:

Grammar question: do you mean that usually THX doesn't errs on power needs?

If so, I though too. But just yesterday I tried the DTS empirical test disk on the system looking on the power amps.
At 7.0 there is clipping on all channel.

I can assure you that sound in the theater is good, no issues with EQ.

Bye
Antonio

Posted by Brad Miller (Member # 2) on 06-10-2002, 03:45 PM:

I must interject here, as I have a really hard time believing THX did not design the room properly.

Antonio, do you know if the SPLs are correct? Maybe your meter is not calibrated properly. What are you trying to set the subwoofer to? Are you using an RTA to set the sub, or just a handheld SPL meter? Could one of the subs be out of phase? Are the drivers of different vintage? Is thin wire being used from the amp to the subwoofers? Is there just one line ran to the screen and then split between the two? What make/model amplifier are you using? Are you sure everything is set properly (bridge/parallel and so forth)? Are the speaker leads properly connected to the amplifier per the mode you are running it in? Are you sure the amp itself is operating properly?

There are a LOT more possibilites than just these few I rattled off. If THX designed the room and at one point certified it, such a problem would have turned up during the certification process. Somewhere along the way someone has been screwing with the system that does not know what he is doing. You need to get a THX tech or at least someone who knows what he is doing out to the theater to run through that auditorium from top to bottom. Clearly there has been a LOT of tweeking by someone who does not know enough about proper sound system calibration.

There is just no way THX would have certified a theater that had clipping on all channels. That system has been tampered with by someone who doesn't know what they are doing, plain and simple.

Posted by Antonio Marcheselli (Member # 373) on 06-10-2002, 05:25 PM:

Brad,

Thanks for your message.

Let's take apart the subwoofer channel, because our tech usually set it with R2 SPL meter instead of RTA. We are talking about it, he is talking with his souces and we will reach a solution. Oh, our tech is THX certified.

I've checked everything on that theater. And I've found many issues, but nothing important or critical.
I've checked the subs polarity many times, I'm VERY sure that are all in phase.
Amps-Speakers cables are right. I found, before last eq, the LF center channel out of phase, has been corrected.

I've checked the impedance of the surround arrays, is about 7,8Ohm for all arrays.

For subs, there are two different lines from booth to screen, amps are JBL MPX 600 bridged (2), subs are JBL 4645B (4) without B6.
Calulation says that are enough, however sound in the theater is "normal", no excessive bass frequencies and amps are clipping.

I've saw one of our surround amp (4 speaker for each amp channel) clipping so hard and then going into PROTECTION. AND, despite the power, surrounds in the auditorium are not so clear as in other I've heard...

All stage's LF driver goes into "soft" clipping at 7.0.

Perhaps Subs has to stay on the floor, perhaps surrounds are too high. But the theater is now as THX certified it 4 years ago. If these problems are here now, were there at the time.

As I told, sound in the theater is good, hi quality.
There are not strange settings in the EQ, levels are not forced.
I've personally followed that tech during last EQ. Since sound was not good, he start from flat, and have the theater completely re-aligned. He used an R2 RTA.

Brad, unfortunately THX doesn't mean nothing for me anymore. You should see that theater: first raw of seats is just 3/4 meters far to the screen. Try seating at 4 meters in front of a 15 meters long screen... There are seats next to the lateral walls, not covered by surround because are 4.3 meters high...
Why don't they could have made a "mistake" in Sound System?

Antonio

Posted by Gordon McLeod (Member # 33) on 06-10-2002, 11:04 PM:

I think you have some sick amps
typically THX provides more power than required and if the clculations you have done verify the power reqirement there is something else wrong
Also there is no R2 SPL meter The R2 is a RTA

Posted by Antonio Marcheselli (Member # 373) on 06-11-2002, 05:14 AM:

Gordon,

Our amps are not showing any signs of fault. All are working in the same manner, front's LF amps clips all in the same way, just a bit.
Surround amps clips, all channels (4), at the same level, same way.
Same for subwoofer.
If I had just one amp clipping so I can think that one amp has failed.

R2: is there an SPL indication on the R2 screen. Is not that an SPL meter?

Bye
Antonio

Posted by Antonio Marcheselli (Member # 373) on 06-11-2002, 05:15 AM:

None has answered to my original question!

Where is better for the subs?

Bye
Antonio

Posted by Gordon McLeod (Member # 33) on 06-11-2002, 09:39 AM:

Because there is a stage lip I would tend to go higher
Also where does the baffle wall end
Also I find it very odd that you have all this clipping as that I hve never seen in a room that THX designed

Posted by Antonio Marcheselli (Member # 373) on 06-11-2002, 12:21 PM:

Gordon,

1. Tha baffle wall end on the stage. Is this answer to your question?
2. Clip: do you want a photo? This time THX erred... I'm absolutely sure that EQ and all levels are correct.

Posted by Peter Hall (Member # 695) on 06-24-2002, 12:24 PM:

The R2 does not measure subs on a numeric basis - regardless of what weigthing you apply you should always set subs on a RTA - you know the score, flat for optical and +10 for Digital or Mag. The R2 (in THX mode) only looks at the sub between (I think) 40 and 100hz, and then decides a pass or fail. OK, practically this always reads around 91db as it does on an Abacus.