Film-Tech Forum ARCHIVE: Subwoofer Re-Installation/Screen Questions

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Posted by Dominic Espinosa (Member # 2122) on 09-02-2005, 03:51 AM:

Well, in the interest of time I've agreed to reinstall our big theater's newly repaired subwoofers.
I didn't do the removal nor have I ever done anything this large.
The closest experience I have is changing out old ones at the 2 screener I worked at.
It was pretty straight forward with taking the old one out and putting the new one into the cabinet/connecting the leads.

We're using JBL 2245H, 18" subs and unfortunately there's not enough room to access the cabinets without first getting the screen out of the way.

So, what I need are any last minute tips or things to worry about when doing this job. Just to be safe.
Any tricks to lacing the bottom of the screen back up quickly and easily?
Anything to watch out for when reinstalling the drivers in the cabinets?

Basicaly, I just don't want to get half way in and find out I'm missing something crucial.

Thanks, guys.
--Dom

Posted by Monte L Fullmer (Member # 2797) on 09-02-2005, 04:57 AM:

Can you swing the sub cabinets to the side for driver replacement, or is the room just to tight back there for such a task (also, can't the sub be located underneath the screen on the floor instead of behind the screen on a platform? Or, do you have baffling behind the screen).

Is your screen hung by screen springs, or has it been laced? If by springs, just undo all of the bottom springs and halfway up the two sides. Then, have two helpers, each with a long pole to lift up the lower part of the screen, about 10ft apart with a nail on the end of the pole to engage into a screen lace eyelet.

Good luck in hunkering that cabinet back up on the platform being a heavy beast as it is. (unless you can take the drivers out first, then hoist up the cabinet. Then replace the drivers and pay attention to the wiring polarity on reconnection..)

-Monte

Posted by Steve Guttag (Member # 268) on 09-02-2005, 06:47 AM:

Monte hit the nail on the head, if the screen is sprung...I normally use a spring or two on each side to hold the screen up (hook the spring(s) into the corner grommet and then attach the other end to something that will hold the screen up while you are working back there.

As for lacing...that is more difficult, especially if you have not dnoe it before. A properly laced screen will have a loop of the lace go through the grommet to its hook. The quick and dirty way of lacing is the "barber pole" method where one goes through the hold and then to the hook...etc in a spiral fashion. Some will do this and go around the frame. This sort of lacing requires the complete removal of the lace to get behind the screen. I believe it is actually legal to kill the person that has done this to you in all 50 states!

Posted by Dominic Espinosa (Member # 2122) on 09-02-2005, 12:14 PM:

It's all lace, no springs.
The lace is on each edge is not connected, it's a separate stretch for each.
Anything besides polarity I should be on the lookout for when reinstalling the subs?
I should note, I didn't do the removal. One of our companies techs did. Unfortunately they're all tied up with a much larger project elsewhere.
So, I have no idea how they were originally mounted in the cabinets or the condition of wiring inside them.
I assume them to be in good shape.

Thanks again for the help.
--Dom

Posted by Monte L Fullmer (Member # 2797) on 09-02-2005, 01:29 PM:

You can be gutsy enough to cut the cords, install the drivers in the cabinet(s),then head to the hardware store to find the right size of cord that was used for the lacing, since they are looped individually.

Just start at the bottom corners and lace towards the center in trying to keep a straight edge of the screen and frame. Then, undoing and retying a lace here and there to get the wrinkles out when completed...or find someone who has a box of springs that they can give you...

..that's a tough call, especially if you don't want to mess things up...and it's not your theatre.

-Monte

Posted by Dominic Espinosa (Member # 2122) on 09-02-2005, 03:21 PM:

quote: Monte L Fullmer
Just start at the bottom corners and lace towards the center in trying to keep a straight edge of the screen and frame.

I was under the impression that you should begin from the center and work outwards? Thus stretching the slack in the screen to the edges?
Or is this not the case with the bottom lacing?

Posted by Monte L Fullmer (Member # 2797) on 09-02-2005, 03:57 PM:

Whoops, right on that one. We would put up springs starting at the center in as well as the edge then work outwards. As for lacing, honestly I'm not familiar with since springing is the only way I know how.

Posted by Dominic Espinosa (Member # 2122) on 09-04-2005, 04:47 PM:

Ahh. Thanks guys. I don't think it'll be too much of a problem. I had to unlace it partially once before to check them (this is when I discovered both were blown) and didn't have a problem lacing it back up. Hopefully it will go smoothly.
One last thing...I checked with the tech that removed them prior to repair and he's fairly sure he left the fasteners in the bottom of the cabinets, if for some reason he didn't is there a generic bolt I can purchase just in case?

Thanks again, guys.
--Dom

Posted by Dominic Espinosa (Member # 2122) on 09-07-2005, 03:39 AM:

Now the update!

The reinstall went fairly smoothly. Those babies are heavy.
ended up relacing the entire bottom of the screen, but it came out pretty even and without wrinkles.
They sound great!
Another job well done.

[Cool]

Posted by Monte L Fullmer (Member # 2797) on 09-07-2005, 12:50 PM:

quote: Dominic Espinosa
Those babies are heavy.

..how did you hunker them things up to the platform? Used a hoist or a bunch of guys?

thx-Monte

Posted by Dominic Espinosa (Member # 2122) on 09-07-2005, 12:59 PM:

Monte, I had an assistant hand them up to me.
Mind you, these were just the speakers, not the whole enchilada with a cabinet and everything.

Posted by Monte L Fullmer (Member # 2797) on 09-07-2005, 01:31 PM:

Oh, that's right. Sorry, I forgot that they were just the drivers themselves that you had to put up there - not the complete units.

But, congrats anywho of taking care of your own theatre instead of outside help to do this.

Posted by Dominic Espinosa (Member # 2122) on 09-07-2005, 03:34 PM:

Thanks, Monte.

I try to earn my keep. That way when I ask for raises I deserve them.
Though, our DM asked me if I could do it since our techs are busy upgrading another theater.
Naturally I'm more than happy to do this stuff though.
Just like working on my car, if I did it then I know what's in it and such.

I'd encourage anyone who takes their job seriously, no matter what it is, to dig in and get your hands dirty. It can be an incredible learning experience as well as teaching you valuable information.

Posted by Frank Angel (Member # 248) on 09-10-2005, 12:05 PM:

Is there ever an issue with subs behind the screen where the volume of air that the sub displaces actually causes the screen to vibrate? Our subs are on top of the screen. We may replace the screen in the near future with a perf screen (the screen we have now is non-perf) Will four bins with four 18in subs in each cause the screen to vibrate when that large amount of air is slammed against the back of the screen? I would think if it does, it would make the picture look like it is slightly out of focus (ever look out your rear-view mirror when you've got your car sound system cranked? -- it makes the image vibrate). So I am thinking with hefty subs behind the screen, that amount of air moving can't ALL get thru the perfs, SOME of it is going to impact the sheet...no?

Posted by Monte L Fullmer (Member # 2797) on 09-10-2005, 12:25 PM:

Hi Frank - In all of the theatres I've witnessed, I've never seen this condition of screen vibration due to the subs since with the perfed screens, the hole density is high enough and screens tight enough on the frames where I've never seen this condition. Also, some theatres have their subs located below the screen on the floor and behind the curtain valance.

Yet, interesting enough, last night I had to do an emerg service call with some sound issues and with doing sound checks.

(which included the sub and this sub was located behind screen yet far back from the rear face of the screen where the other speakers are about 3 inches from the rear face of the screen. If you have speakers far back from the face of the screen, you lose quite a bit if highs due to the surface reflection of the rear screen and you have highs bouncing all over back there...).

With all speakers driving, I didn't notice any screen vibration from the thumping of the sub.

And I had that sub thumping pretty good. Sounded good though.

thx-Monte

Posted by Dominic Espinosa (Member # 2122) on 09-12-2005, 08:52 PM:

quote: Monte L Fullmer
Hi Frank - In all of the theatres I've witnessed, I've never seen this condition of screen vibration due to the subs since with the perfed screens, the hole density is high enough and screens tight enough on the frames where I've never seen this condition. Also, some theatres have their subs located below the screen on the floor and behind the curtain valance.

I've seen both and in the theater with the subs behind the curtains the curtains dind't really move either. The subs were however about a foot and a half back from it.

EDIT: Forgot my new signature [Wink]

--dom

Posted by Frank Angel (Member # 248) on 09-12-2005, 10:57 PM:

hmmm, thanks, guys. It was just a thought. I figure asking is better than mounting those monsters only to find out my screen is acting like the top of a kettle drum! We have insisted that the subs be at bottom of the screen level so we can get more spl in the bass region out to impact the audience more. Having them 30ft in the air looses that really filling rattling bass presence, eventhough acoustic guys insisted that the low end couples to the room better when they are in the corner. I say, but yah, they are coupling to the ceiling....the people are on the floor! We tried them out on the stage on either side of the screen and even THAT made a huge difference. Floor level they stay.