Film-Tech Forum ARCHIVE: how close behind the screen do you place you JBL subwoofers ?

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Topic: how close behind the screen do you place you JBL subwoofers ?

David Baum
Film Handler

Posts: 90
From: Brussels, Belgium
Registered: Aug 2000

posted 11-09-2000 04:35 PM

have compared different positioning ? JBL recommends the main speakers with horns at a distance of 2 to 5 inches maximum but what about vented woofers like the 4645B, 4645C etc and their huge port ?

thanks
PS: if anyone can advise on the watts power you use for the subs, I'd appreciate. I suppose 800W, 8ohms seems appropriate for the 4645C.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 11-09-2000 06:17 PM

David,
I personally never put subs up with the regular screen channels. I always put them a foot or so from the floor. If this is a fairly conventional or stadium type theater this will work very well. If you do not have a baffle wall like a THX system does then the woofer will couple to the room better if placed down low.
Mark

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David Baum
Film Handler

Posts: 90
From: Brussels, Belgium
Registered: Aug 2000

posted 11-09-2000 06:56 PM

Hi Mark

thanks for reply. But what about the distance between the sub and the screen?
any recommendations on the power to feed them? (I like QSC DCA amps). I have two 4645C and 2x 3677 (a third one later on). I can't really make wall baffles now so I'd rather leave the subs on the floor (with rubber pads probably).
I must say this is my home theater and the screen is now 16ft wide (diagonal), microperforated. (Harkness Hall P1M)

regards

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 11-09-2000 09:11 PM

Generally speaking, the lower the audio frequency, the less directional it is, so placement is less important.

Woofers and tweeters would be together, located within the screen frame. Subwoofers can go on the floor, underneath. I don't think the distance between the "regular" speakers and the subs is that critical.

800 watts seems plenty for a home system (better than some "real" theaters I know of!)

Usually, if you have two subwoofers, one is placed on top of the other to get a gain (2dB?) from acoustic coupling.

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Stefan Scholz
Expert Film Handler

Posts: 223
From: Schoenberg, Germany
Registered: Sep 1999

posted 11-10-2000 06:01 AM

Generally, with low frequency, direct radiating speakers effency is increased, if placed on the floor or in a corner.
Efficience of those cabinets is more than 10 dB lower, than of horn loaded systems, round 98 dB/1Watt/1meter. The power required is dependend from some factors. Requested SPL at a given distance, room absorption at low end, and linear power handling capability of the speaker/ enclosure combination.
Generally a standard power amp (ratings are somewhat limited to 600 W for safety regulations) per enclosure will do. To determine the No. of enclosures required with digital sound, tables and recommendations have been published in numerous articles and papers in the past.
E.G. Length of house from screen to rear wall 80 feet to reach 113 dB SPL @ 2/3 way back: 4 units close coupled with 2400 Watt amp power. (See graph 2.10 Subwoofer Power Requirement)

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David Baum
Film Handler

Posts: 90
From: Brussels, Belgium
Registered: Aug 2000

posted 11-10-2000 05:40 PM

Thanks for the info.
JBL recommends, ideally, a power twice of the speaker IEC (for the 4645C, thus 1600Watts), to avoid as much as possible distorsion or dangerous low harmonics and to have ample headroom available, so isn't it better to bridge the amp for instance (a QSC DCA 2422 bridged will give a good 1100watts at 8ohms for instance) ?
On top of the power matter, another factor that seems as important is the damping factor, especially the one that really reaches the subwoofer, after the loss due to the cable. Without going to extremes factor like those of the CROWN K serie, I wonder how low you go , especially when, as stated earlier, the bridging will cut in half the damping factor outputed by the amp.
bottom line: bridging for maximum headroom, minimal distorsion but compromise on damping factor OR, non bridging and maintain of the highest level of damping factor ?
thanks again if anyone has an opinion on that. of course, the other BIG factor is the cable length, its AWG size and the cost factor evidently.

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Rick Long
Jedi Master Film Handler

Posts: 759
From: Toronto, Ontario, Canada
Registered: Nov 1999

posted 11-10-2000 11:09 PM

Perhaps another solution might be to place the amplifier near the speaker, provided it was a relatively secure area, sufficient a.c. was available, and the amp fans are fairly quiet.

This would require running a two-conductor plus shield cable from the processor sub-woofer output to the stage, but would certainly be less expensive that running 10-guage (or heavier) cable at that length.

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Steve Guttag
We forgot the crackers Gromit!!!

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

posted 11-11-2000 08:52 AM

>>"On top of the power matter, another factor that seems as important is the damping factor, especially the one that really reaches the subwoofer, after the loss due to the cable"<<

In the technical audio circles, the damping factor signifigance has been pretty thoroughly debunked as merely an advertising ploy by amp manufacturers. The signifigance of the damping factor when compared to v.c. winding is next to nil. In short, don't worry about it. Use a good gauge wire for the power and length of run.

Placing the amp away from the other electronics, more often than not, gets you new problems associated with different ground levels. Also, many processors are unbalanced which will make it even more challenging. I'm not saying it can't be done, just that it is more trouble and will often cost more to do it right than just running #8 or so wire, again, depending on the power and length of run.

Steve

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"Old projectionists never die, they just changeover!"

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