Film-Tech Forum ARCHIVE: JBL 3677 screen speakers: do you cut them (30hz or 50hz ), and do you

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Topic: JBL 3677 screen speakers: do you cut them (30hz or 50hz ), and do you

Darryl Spicer
Film God

Posts: 3250
From: Lexington, KY, USA
Registered: Dec 2000

posted 01-09-2001 12:39 AM

Thanks for clearing that up Gordon. I think I was confusing myself.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 01-10-2001 12:32 PM

Personally I would cut these speakers at 60 to 70 hertz. If you do this,the system will have much lower harmonic distortion in the LF. This is assuming that you have a good subwoofer to take over the low end.
Mark

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 01-10-2001 01:58 PM

I wouldn't be too worried about low frequency distortion as these speakers are designed for small room applications and they are also designed around the use of a baffle that will effectively lower the amount of energy required at extreme low frequencies and at the same time reduce the distortion

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David Baum
Film Handler

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

posted 01-11-2001 09:04 AM

thanks for the comments. currently cut at 80hz ( I have 2 4645C subs ) but will try with the QSC a cut at 50hz ( which gives a nice boost around 80-100hz) and see how they do. they are on a platform but maybe I'll try some sort of bafle wall.

thanks again!!!

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 01-12-2001 09:51 AM

JBL has an on-line "Cinema Sound Systems Manual" that give guidelines on power requirements, which are dependent upon a variety of parameters including room size, speaker type, speaker placement, etc.:
http://www.jblpro.com/

JBL also has a professional "white paper" on "Theatre Systems: A Quick Guide to Specifying Cinema Loudspeakers" by John Eargle and Mark Gander.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

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David Baum
Film Handler

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

posted 01-12-2001 03:55 PM

thanks John!
I don't seem to see the second file you mention on their website. is this the "Product Application Guidelines" , which I have also ?

best
David

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 01-13-2001 07:33 PM

The "white paper" was distributed at the presentation JBL gave at last week's ITEA Seminar in Los Angeles. Assume you could write them and ask for a copy.

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Mark Mayfield
Film Handler

Posts: 12
From: Whitinsville, MA, USA
Registered: Jan 2001

posted 01-18-2001 12:49 PM

Couple comments. (Going back a ways) The JBL 3678 has foam in the port tubes to prevent light from the light-bulb protection circuit from leaking out to a darkened theatre, causing a visual distraction. It causes no impediment to low frequency transmission; you should leave it in. Also - I agree with previous comments that you should band pass ANY cinema screen system at 50 Hz. Let the subs do the VLF work, and minimize your screen LF amp clipping.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 01-18-2001 09:44 PM

There should be no amp clipping is the amp sizeing vs speaker efficiency are matched to the room volume vs the desired SPL. So the rolloff should be a mute point if they are installed in the baffle recomended and they are used in the room volume they were designed for

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 01-19-2001 04:57 PM

Gord,
Why push a speaker to put out low frequencys that it physically can't? Why not cut it off at a given low frequency and end up with much cleaner, lower distortion bottom end, baffle wall or not?
Small cabinets simply cannot produce extreme low frequencys at low distortion if pushed beyond the design limitations
On the THX systems that I have done they cut the mains at 50 hz, or didn't give them any boost at all. They let the sub take over and things stayed nice and clean. Pushing a cabinet beyond its design capabilities will only leads to listening fatigue, and a muddy bottom end.
Mark

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Steve Guttag
We forgot the crackers Gromit!!!

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

posted 01-19-2001 10:51 PM

Even worse...Once the frequency falls below the design of the cabinet/driver system, the driver will unload and slap itself silly since the enclosure isn't helping it out.

Since one of the LFE crossover points is 50Hz, let the subs roll in there so there isn't an overlapping of the two system causing an unnatural boominess.

Steve

P.S. Mark Mayfield, it is a real pleasure having you here! Anyone who doesn't know who Mark is should do some homework!

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 01-20-2001 11:18 AM

I can remember hearing one of Mark's talks at Showest way back when he was with E-V.
Mark @ GTS

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