Film-Tech Forum ARCHIVE: Laser fixture for aiming HF horns

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Posted by John Walsh (Member # 168) on 12-23-2003, 01:16 AM:

I noticed that at theaters our company had previously built, the high-frequency horns were usually poorly aimed. We mostly use the JBL 4675’s and the usual procedure to aim them is to look through the horn throat prior to installing the HF driver, and aim the horn at the center of the auditorium, 2/3’s to the rear of the seating area. When looking through the horn throat, the aperture is very narrow, but tall. This makes it easy to correctly position the horn left-to-right, but leaves a lot of room for error when tilting the horn up-down.

Whenever a new theater is in the process of being constructed, the trades really do want to do things the right way, but are pressed for time. I felt it could help if there was a tool to help them aim the HF horns. I had an idea using a laser, but I needed help making such a tool. I needed a machinist, who knew about lasers, and would understand about theater sound. Where would I find a specialist like that? Well, for once I had an easy answer; Greg Mueller of course!

I sent him a drawing I made, and in no time I had a working laser fixture, just in time for a new theater we were building. I had originally thought about just a sleeve I could put an Align-O-Tron into, but Greg went the extra mile and not only made the whole thing, but also put a battery and switch inside.

I added a lanyard so it could be hung around a person’s neck to leave their hands free. The boss on the laser end fits into the opening in the back of the horn. I would imagine this would fit any of the JBL horns that used the 2446 driver.

The rear of the HF horn before the driver is installed.

We set up a cardboard dude and placed him on the seat in the ideal location. One person would stand in the auditorium to warn away people, so they wouldn’t get “shot” and also tell the “aimer” where to move/tilt the horn. It’s hard to see when aiming; the large horn and the baffle wall block the view.

The 9v battery lasted the whole time; about 3 weeks of “turn on-turn off” use. So, a battery-operated Align-O-Tron or this “speaker Align-O” might be a good option. Greg already thought about turning down the face of Align-O-Trons to match different driver openings.

Obviously, this is not much good if the screen is already hung. I would think the holes in the screen might deflect the laser beam as it passed through, but I haven’t tried it. For new theaters, I felt it was well worth the time using this fixture; it made the job go quicker with more accuracy.

Many thanks to Greg for a very handy tool!

Posted by Richard Fowler (Member # 893) on 12-23-2003, 01:38 AM:

Nice [Cool]

....we did something similar with a maglite and wooden plug for the horn and a clamp for small horns with existing drivers. The Laser set up would be more precise and work under a higher light level. Our "kit" did not solve the occaisonal paper safety cover on the opening of the driver that was not removed when the driver was bolted on by a rushed crew [Confused]

Posted by Adam Martin (Member # 641) on 12-23-2003, 07:51 AM:

Is there any reason why you would only want to align the HF horn and not the cabinet as well? I understand it's more difficult to discern directionality of bass, but why not align the whole thing?

Imax speaker cabinets are all self-contained, horns and all, and are aligned with a laser when hung from the screen frame (or I-beams in the case of surrounds).

Posted by Richard Fowler (Member # 893) on 12-23-2003, 08:34 AM:

Bass is not as directional in dispersion compared to the high frequency. The bass cabinet should be moved slightly horizontal to favor a focal point 2/3 back or to follow the arc of a curved screen frame or to minimize wall slap on the side walls. All in one cabinets generally are aimed based on the high frequency components contained within.

Posted by Mark Gulbrandsen (Member # 72) on 12-23-2003, 09:52 AM:

All of the THX systems I've installed were set so the horn C/L hit just above the rear center seat. Thats the way THX designed these particuluar installations. They were not all similar shaped rooms either. One thing I noticed and you may want to check on your align-o-tron is that it actually points straight. Mine was not pointing straight and I had to shim the diode inside. Probably more of a reflection on the diode than Greg's fine machine work though. The Alogn-O-Tron hyas saved me countless hours aligning lamphouses!! Put yours in a V block and rotate it to see if the spot goes around or if it stays in one spot(pun intended). If it goes around then its off a bit and the further out you go with it the more off it'll be.

Greg are you going to offer the Horn-O-Tron as an off the shelf piece of equipment?

Mark

Posted by Manny Knowles (Member # 1171) on 12-23-2003, 11:02 AM:

Very nice! I'd buy one.

Can it sit by itself in the HF throat or does someone have to keep a handle on it so it doesn't slip out and fall to the ground?

I'd like it if this new Align-O-Tron (please keep the same name) could be dual-purpose for aligning lamphouses and speakers. That way we don't have to carry both the old one and the new one around. The battery feature is a very good idea.

I don't know whether I would keep the old one as a backup or sell it to help finance the purchase of one of the newer gizmos. Hmmmm.

Posted by Greg Mueller (Member # 17) on 12-23-2003, 03:25 PM:

I might be redesigning the snout so that I can screw a plate to it for a variety of different horns. This is long range planning and I haven't really thought about it a lot and I still have inventory of the current design.

Any time there's a problem associated with one of my gizmos, I'll be glad to fix it (in this case remount the laser) at no cost.

I didn't know about this problem or I would have offered to fix it. Currently I use a jig for aiming the laser that has a target 20'+ away. With the average lamphouse (working distance) at 3' or less I figured it should be pretty close.

Damn, guess I'm not perfect after all

Manny
On that one you have to hold it against the horn manually

Posted by Mark Gulbrandsen (Member # 72) on 12-23-2003, 03:40 PM:

Greg, Don't worry about it. The one I ahve is one of the forst ones you made. I only posted about it it here so others can be aware of it and check theirs if they are concerned for long distances. Even I know you don't wanna be like DTS and sweep things under the rug so to day.
Actually a bit of bubble gum wrapper solved it!!
Mark

Posted by Alexander Alves (Member # 1746) on 12-26-2003, 05:49 AM:

What would one have to do with the newer JBL Screen Array speakers coming pre-assembled? Unscrew the HF driver and then use the jig?

Posted by Richard Fowler (Member # 893) on 12-26-2003, 07:07 AM:

On Screen arrays we where clamping our aiming fixture on the horn edge. The horn vertical dispersion is not symmetical and is more downward firing to work with stadium rooms. This has to be considered when aiming which we plotted out on our field shop drawings. We where the first company to install the Tri-amplified Screen array system and have sold many 3632 / 4642 systems.

Posted by John Walsh (Member # 168) on 12-26-2003, 04:53 PM:

I set up the laser and put some screen material in front. Although the light was reduced, it still seems like it will work, although having to take the drivers off would still be a major pain. I could make some kind of large fixture to hold the laser on the outside, but the baffle wall blocks much of the area. As with most of these kind of things, I'll have to wait until some special circumstances force me into designing a solution.

To hold the fixture up to the horn, we just wrapped the lanyard through the mounting hole on the top, above the holes for the driver. The boss could be extended more like a "tube" to stay in the throat, but the inside of the throat flares inward so the tube would have to be cut inward also.

THX wants the bass cabinets to be placed flush with the outside of the baffle wall, so you are forced to point them on the same plane. We usually curve any screen over apx. 30 ft wide, so the "faceting" of the baffle gets the bass cabinets generally pointing in the right direction.