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Topic: Crossover issue on JBL 8330A surround

Scott Norwood
Film God

Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99

posted 10-22-2008 09:08 AM

Wow. Thanks Steve and Richard!

I don't design theatres and have no intention of doing so. I'm mostly just curious as to what the theory is behind it and why some systems sound amazing and some...well..don't.

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John Hawkinson
Film God

Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002

posted 10-22-2008 10:04 AM

Steve, Scott's question started out with "So, how does one determine how many surround speakers of a given type are needed in a given room?"

Beyond the simple math of sensitivity and desired SPL and soforth, I was under the impression that there was a largely-unanswered question about uniformity of sound fields, with the only real agreement being that "yes, more speakers are better."

Is there now a consensus on how to determine the number of surround speakers?

--jhawk

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Steve Guttag
We forgot the crackers Gromit!!!

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

posted 10-22-2008 04:46 PM

John,

Continuing on the theme of this thread...I answered Scott's question as it relates to keeping the speakers from blowing up. You can do EASE type modling and/or use math to compute what the coverage will be...however, you will find that, unless you are using atypical surround speakers that when you satisfy the power handling requirements you are going to be pretty darn close on satisfying the coverage requirements.

EASE type modling (and there are others like LIRA) is probably the best way as you will get a visual indication of the coverage by frequency. It does depend on the manufacturer to have made EASE models. You can do the math/CAD providing that you have good dispersion specifications of the speaker(s) desired.

Whereas you are trying not to have any one speaker being decernable in a surround field, yes, the "more is better" statement is accurate. There comes a point of expense versus perceived benefit. The same holds true for such things as frame-rate in movies too. Is 30fps better than 24fps? Yes...same with 48fps versus 30 or 60fps versus 48. Emperically, through study, 60fps was determined to be the point when adding frame rate didn't justify the added expense versus any perceived improvement. To the best of my knowledge, the same sort of study hasn't been done for surround speakers...though THX did do some testing on the subject and have their own formula. In a THX certified theatre...you will tend to see about twice as many speakers as you would in a normal theatre.

Steve

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Cameron Glendinning
Jedi Master Film Handler

Posts: 845
From: West Ryde, Sydney, NSW Australia
Registered: Dec 2005

posted 10-22-2008 04:59 PM

I am sending some new speakers to Adelaide as we speak. Im not sure if its the same cinema, but 5 out of 12 seems similar, the 8" JBLs are being replaced with 15" Cinepros

One factor is speaker efficiency, the 8" JBLs are 91 db for 1 metre and 1 watt, the 15" speaker replacing it is 97db/ 1 w/ 1 m

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Louis Bornwasser
Film God

Posts: 4441
From: prospect ky usa
Registered: Mar 2005

posted 10-22-2008 06:18 PM

I remember when Dolby suggested one speaker for each 3 seats! No power handling problem there!! Louis

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Scott Christopher
Film Handler

Posts: 69
From: Adelaide, South Australia
Registered: Apr 2001

posted 10-29-2008 08:38 PM

Cameron, does 'PalaceNova Eastend' ring any bells?

I realise that auditorium size and total amount of surrounds does alter the final outcome, I was just referring to the JBL documentation as a starting point. You'd have to way up the costs of installing more of the same (plus repair) vs installing higher power units if you're burning out the voice coils.

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Cameron Glendinning
Jedi Master Film Handler

Posts: 845
From: West Ryde, Sydney, NSW Australia
Registered: Dec 2005

posted 10-30-2008 08:43 PM

Yes Nova, former Imax and 18 metre screen

quote: Scott Christopher
Admittedly our installer put them in a 290 seat auditorium which JBL do not recommend. I suggest, if possible, installing the 8340A if you wish to keep the same brackets. The power handling is also better and the driver is 10inch.

That ws my first suggestion as well, the JBL 10 inch has a 96 db/1w/1m efficency, and is pretty much twice as loud per watt. Another option was to double the amount of 8" surround speakers.

The 15 inch surround speakers ended up being the safest option.

I think you will be impressed with the sound quality of the units. The brackets that are being sent with the units will allo the recomended 15 degree tilt.

Please note that these speakers feature different internal components than the ones Hoyts are now using.

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