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Author Topic: Repair or replace insubordinate subwoofer? Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 04-18-2010 12:49 AM                        But we play art movies. Not all art movies all the time, but mostly art movies. Art movies do not use subwoofer. We do get some art movies like Hurt Locker and Girl with Dragon Tramp Stamp that have subwoofer... at least I assume Girl with Dragon Tramp Stamp has some sub.  |  IP: Logged Cameron Glendinning Jedi Master Film Handler Posts: 845 From: West Ryde, Sydney, NSW Australia Registered: Dec 2005 posted 04-18-2010 01:33 AM                     quote: John Hegel Just curious what would be the disadvantages of using non cinema subs? Well the link you gave (which doesn't work) sends you to a Yamaha site which shows examples of similar P.A types of subs with similar efficiency to the JBL. The main advantage to the JBL is simply that it is a better unit that will go deeper in its bass response. Otherwise the Yamaha examples pictured should work well. Now if you wanted to try a home style of sub woofers etc they simply are not efficient enough.  |  IP: Logged Ian Parfrey Phenomenal Film Handler Posts: 1049 From: Imbil Australia 26 deg 27' 42.66" S 152 deg 42' 23.40" E Registered: Feb 2009 posted 04-18-2010 03:24 AM                     quote: Steve Guttag Gee...which do you think will cost more, a properly designed horn loaded cabinet or another subwoofer? If you factor in the extra power amps and the runs needed, I'd say the driver is more expensive. Used horn cabs are fairly cheap on eBay.  |  IP: Logged Cameron Glendinning Jedi Master Film Handler Posts: 845 From: West Ryde, Sydney, NSW Australia Registered: Dec 2005 posted 04-18-2010 04:44 AM                     Unfortunatly most standard horn cabs work best in groups of 4 for truely deep bass. A good design corner loaded might be an option or perkaps with wings added?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-18-2010 05:59 AM                     Amplifier power is relatively cheap nowadays...cabinets/drivers are designed for each other, if they are any good. It isn't simply a matter of taking an existing driver and plopping it into a horn cabinet. As I said, either turn it down or get enough to do it properly...it isn't pie in the sky sort of numbers. If the company doesn't want to spend the money, they can live with low/no subs. And, for the record, we always spec the proper amount of subs for our "art houses" too. The fact remains, the track is capable of putting out that much demand on the sub and it only takes one film, one showing, to do the damage. Steve  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 04-18-2010 10:44 AM                     Ian There is (was? Maybe that cinema has closed now) a cinema in Italy with a very powerful horned sobwoofer. The horn, made by concrete, is 14 meters wide since the lenght of the horn is proportional to the wavelenght. While it would be possible to build a horn subwoofer, it is not a solution in this case, I reckon. Steve You're right, I saw a few weeks ago that datasheets, thanks. I thought they were something customised for Empire Cinemas Leicester Square in London.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 04-19-2010 09:11 AM                     Steve, you make it sounds like the two choices are having sufficient subwoofer to cover the entire room at the specified SPL, or to have your subs blow up. [or the third choice of having low sub volume all the time] That seems kind of like a false dichotomy. Of course nobody should engineer the system such that their amps can clip and destroy the speaker, nor such that their amps can drive enough power into the speaker to kill it without clipping. This seems like a decent spot for an el cheapo limiter. Joe can get the full subwoofer volume on almost all of his films and for the once-in-a-year Hurt Locker, he'll have less base than he ought to, rather than blowing the subwoofer. What's wrong with this picture? [other than knob tinkerers] --jhawk  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 04-19-2010 11:52 AM                        There is never any reason for only ONE subwoofer. It just doesn't work. While it may be possible to have less than 4 under the right circumstances: (THX soundwall, etc.) the need for four is appropriate. There IS a JBL that has 2 drivers; it is slightly cheaper than 2 seperate units; this may be helpful. Louis  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 04-19-2010 04:00 PM                     John, simple answer to your post is this: Laws of physics limits the total SPL output of that single sub and it will NEVER be able to produce enough movement of air (i.e. sound) to adequately cover a larger room. Doesn't matter how active the bass track is, if it can't move enough air it will get blown out by even a single instance of being pushed beyond it's limits. Even with a limiter in place it will seriously compromise the sound quality and it still will NOT protect the sub from over-excursion damage. As someone who worked live sound for some years, I can tell you firsthand that a limiter will not prevent a sub from blowing out. As Steve pointed out, amplifier power has become cheap enough that it is very foolish to even think of trying to over-engineer a solution to a problem that is easy to fix. I feel for those whose owners refuse to spend less than the day's petty cash to make a long-term fix to a presentation issue.  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 04-19-2010 07:49 PM                        Tony, a properly set limiter CAN protect a sub from damage, but improperly set it will make it much easier to damage a sub. A brick wall limiter setup will greatly increase the average power going to the sub, increasing the risk of thermal damage. Since huge transients generally don't damage subs, a limiter with a fairly leisurely attack time will allow the transients to pass, keeping subjective sound quality good, but will still be effective at cutting long term high power conditions. This should obviously be used in concert with a long release time and a low threshold to preserve the average power limiting characteristic. This is a common configuration in permanently installed live sound systems, especially when you don't know what half-deaf yahoo is going to be behind the board from day to day. If you need something like this in a theatre then something is seriously wrong with your staff, and no amount of electronics will fix that. Oh, and having more subwoofers is better, especially if they're spread out along the front wall, even if you don't need more output. More cone area means less excursion which means less power compression and less distortion. Also, multiple subs are more likely to evenly excite room modes (more of an issue for smaller rooms) and makes it less likely that some seats will end up in a node.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-19-2010 08:56 PM                     Thanks Robert...you beat me to a response to John (and by extension, to Tony). A limiter in this application is just about completely the wrong tool for the job...the odds that one cooks the sub with a limiter, in this application is greater than actually protecting it. A better design for a cinema sub would be the careful use of a compressor. If you declare a portion of the output to be "normal" non effect "booms" just low frequency extension of the main channels (as is often done in a analog soundtrack), then you would leave this area linear. Then carefully select the hinge point and compression factor to extend the usable range of the sub such that only the big effects get squashed. You could stack a couple of compressors to double hinge the output so the compression gets more progressive and leave the lower level signals effectively "normal". You could then end this with a limiter as Robert describes to deal with prolonged loud sub passages though, in my experience, the limiter will do very little in a cinema environment to help. The sub channel normally does not have "stings" or other short burst noises...they are often prolonged and high powered. All the limiter can do is slam the lid down and cook the driver. Because this system is so underpowered and under subed, a limiter alone just wouldn't work, it would forever be in limit when the sub channel came on. But lets look at this again...Adding the second subwoofer, without changing the amplifier will have how much difference in gain (presuming solid state amps of typical design)? Anyone? 6dB, typical. You get 3dB from mutual coupling and another 3dB from the amplifier through more efficient power transfer. This is one of the chief reasons the second sub in any system yields a lot of "boom-for-the-buck." It is more efficient all of the way around. To put it in more perspective, 6dB difference is the difference between 300-watts and 1200-watts...it is huge. By the time you buy the compressor/limiter and pay someone qualified to set them up, the 2nd sub was cheaper and better. As stated before, with the right size amplifier(s), the 3rd sub puts you over the top in terms of power handling/sound producing. The trick there is getting amplifiers with that much power and have it be cost effective. Odds are, the 4th subwoofer will allow one to use a more reasonable sized amplifier and lower the effort of the entire system. Steve  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 05-10-2010 03:12 PM                        What is everyone's general opinions on the JBL 4642A dual sub? I found a few used for $850 and am currently considering pitching them to the theater if they are any good. One 4642 would be better than one 4645, yes?  |  IP: Logged Demetris Thoupis Phenomenal Film Handler Posts: 1240 From: Aradippou, Larnaca, Cyprus Registered: Apr 2001 posted 05-10-2010 03:57 PM                     The 4645c is an 8ohm sub and you need either a bridged amp or a really powerful one to give it justice. Jbls dual subwoofer is 4ohms impedance. I prefer the 4645c over the dual. Dual sub I prefer the KCS or the QSC ones. But considering any properly eq sub will give excellent results Demetris  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 05-10-2010 08:28 PM                     In your situation, a single 4642A will serve you better than a single 4645C. You will gain 3dB of sensitivity, 400-watts of power handling and due to the lower impedance of running at 4-Ohms, better power transfer to the subwoofer and gain another 3dB of potential power from the amplifier. Frequency response wise, the 4645C will still be flatter and go a little lower...BUT if you only have one...one 4642A, in your room is better than one 4645C. Steve  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 05-10-2010 09:22 PM                     Short answer Joe, get the 4645C and you'll be a lot happier.  |  IP: Logged

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