This is topic Repair or replace insubordinate subwoofer? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.


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Posted by Joe Redifer (Member # 3) on 04-16-2010, 08:17 PM:
 
I don't have much experience with blown subwoofers, but the few I've seen have had noticeable damage. But maybe that's because they used paper drivers.

Anyway, over the past few months or whatever I have not noticed any magnificent rumbles coming from our large giant screen auditorium. So I put my noise generator in the CP65, nothing. Disconnected the speaker lines from the amp and put my meter on 'em... wide open. Climbed up behind the screen, sub was still connected and looked in perfect condition. I disconnected it and put my meter on the terminals... wide open. Removed the cone and put my meter on that... wide open.

I don't remember the model of the JBL sub itself, but the cone's model is 2242 HPL. Should I order a new one or is this something that I could fix myself? Keep in mind that this one sub powers ALL of the large giant screen auditorium, so it works pretty hard, but I've never heard it clip nor seen the amp clip during playback.
 

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Posted by Tony Bandiera Jr (Member # 2365) on 04-16-2010, 08:38 PM:
 
There are folks out there who can recone your blown sub (But my last price check had a recone so close to the cost of the whole driver it was a toss-up.)

But, if that house is more than say 100 seats you need to have at least two subs and if it's over 300 seats 4 subs would be better.

Back in the analog-only days you could get away with one sub but not with digital sound.

Oh, and it is very easy to overdrive a sub and blow it out without clipping the amp.

 

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Posted by Gordon McLeod (Member # 33) on 04-16-2010, 08:40 PM:
 
only one sub you need more than that
 

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Posted by Steve Guttag (Member # 268) on 04-16-2010, 09:28 PM:
 
The 2242 is a phenomenal driver...it can handle 800-watts continuos pink noise....all day.

As Gordon mentions...if you only have one for an auditorium, it is WAY under speced. Up until last month, the only system that used the 2242 was the 4645B or 4645C. Tell me how big your theatre is (distance from the screen to the back row of seats) and I'll tell you how many of those 4645Cs you SHOULD have. I suspect, the number will be around 4, not 1.

I would definitely recone the driver...the cost of the recone will typically be half the price of a new 2242HPL or perhaps even less.

JBL prohibits selling the recone kits to end users.

Steve
 

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Posted by John Walsh (Member # 168) on 04-16-2010, 10:01 PM:
 
I have seen several JBL subs where the flexable wires going from the cone itself to the terminals have broken off. It was as if there was too much solder, and after many 'cycles', they broke off.
 

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Posted by Steve Guttag (Member # 268) on 04-16-2010, 11:21 PM:
 
Too much solder can indeed be a problem. As can too little...it is quit tricky to get right. Sometimes, the bond where the v.c. lead to the tensile lead (the flexible wire John speaks of), also fails.

But still...it would almost be impossible for one 4645C to be sufficient for most any sized theatre in a digital audio application.

Steve
 

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Posted by Joe Redifer (Member # 3) on 04-16-2010, 11:49 PM:
 
Those wired seemed fine to me.

I really don't know how big the theater is, but it has 237 generously-spaced seats. I would guess the throw is somewhere between 60 and 70 feet maybe.
 

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Posted by Bobby Henderson (Member # 840) on 04-17-2010, 12:44 AM:
 
The two largest, originally designed for THX, houses in our local Carmike theater have 4 sub drivers each. These auditoriums have just under 200 seats each. They originally had a little over 200 when the seating was standard slope. The #3 and #6 houses flanking #4 and #5 have around 150 seats. They have 2 sub drivers each. The 4 smallest houses have 1 only sub driver; these are rooms with barely over 100 seats. Those subs were added in 2002 when the theater was upgraded to DTS on all 8 screens. That's arguably a conservative configuration for subs in an 8-plex.

So only 1 sub driver for a room significantly bigger than anything in our local Carmike? From my perspective it seems grossly under-equipped. Gotta have at least 2 or more subs.

Incidentally, what's the most sub drivers anyone has seen in a modest sized commercial movie theater auditorium? Private, pimped out screening rooms do not count.
 

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Posted by Steve Guttag (Member # 268) on 04-17-2010, 07:22 AM:
 
Okay...lets split the difference...with 65-feet.

Using the JBL 4645C's sensitivity of 97dB at 1W/1m...you need 6945-watts to play at digital spec. The 4645C only has 800-watts of continuous (pink noise, the figure you SHOULD use for subwoofer sizing due to how subwoofer tracks are played) power handing.

Seems a bit impractical. Go up to two subs...you need 3481-watts but only 1600-watts of capability. 3 subs bring you down to 2300-watts needed with 2400-watts of capability. This is the tipping point for a 65-foot room with this subwoofer. You would need to use amplifiers with 800-watts/channel (20-20KHz power, not the stupid 1KHz power spec most amp companies throw about). Or use one with 1600-watts bridged into 4-Ohms and one bridged into 8-Ohms for the other 800-watts.

A better design though is to get the 4th subwoofer. It brings you down to neading 1745-watts. Two 500-watt/channel amps or one big 900-watt/channel amp (into 4-Ohms). Something like the DCA-3022 by QSC will do it fine.

Steve
 

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Posted by Joe Redifer (Member # 3) on 04-17-2010, 02:21 PM:
 
But ideally I'd need 9, right? No prob, I'll place the order Monday. I'll also call the electrician and have him install another breaker panel for all the amps we'll need. Let's see, I'll need another sound rack to install the amps. Actually I'll just order all new amps for every channel... the newer thin QSC DCA series and just throw the old fat QSCs away. That way they will fit into the existing rack. Might as well get a bunch of Monster Cable® to go with that, because without Monster Cable® everything sounds like crap.
 

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Posted by Phil Hill (Member # 371) on 04-17-2010, 03:02 PM:
 
Joe...
 

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Posted by John Hegel (Member # 628) on 04-17-2010, 04:43 PM:
 
Just curious what would be the disadvantages of using non cinema subs?

yamaha 218v

I have 5 of the yahaha 218v's on one of my live music stages/bar. We run playback music 24 hours a day and live shows 10 hours a week on these without any problems. In my opinion these offer a lot of bang for the buck.

[ 04-18-2010, 01:57 AM: Message edited by: Adam Martin ]
 

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Posted by Marco Giustini (Member # 4544) on 04-17-2010, 05:01 PM:
 

quote: Steve Guttag

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Up until last month, the only system that used the 2242 was the 4645B or 4645C

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quote: John Hegel

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Just curious what would be the disadvantages of using non cinema subs?

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quote: Steve Guttag

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Gee...which do you think will cost more, a properly designed horn loaded cabinet or another subwoofer?

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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.
 

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Posted by Cameron Glendinning (Member # 3516) on 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?
 

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Posted by Steve Guttag (Member # 268) on 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
 

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Posted by Marco Giustini (Member # 4544) on 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.
 

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Posted by John Hawkinson (Member # 1135) on 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
 

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Posted by Louis Bornwasser (Member # 3063) on 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
 

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Posted by Tony Bandiera Jr (Member # 2365) on 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.
 

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Posted by Robert Minichino (Member # 3495) on 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.
 

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Posted by Steve Guttag (Member # 268) on 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
 

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Posted by Joe Redifer (Member # 3) on 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?
 

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Posted by Demetris Thoupis (Member # 833) on 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
 

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Posted by Steve Guttag (Member # 268) on 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
 

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Posted by Tony Bandiera Jr (Member # 2365) on 05-10-2010, 09:22 PM:
 
Short answer Joe, get the 4645C and you'll be a lot happier.
 

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Posted by Joe Redifer (Member # 3) on 05-10-2010, 10:06 PM:
 
I already have a 4645 and just having one isn't good enough. I was hoping to find a couple of used 4645s but I can only find used 4642's.
 

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Posted by Demetris Thoupis (Member # 833) on 05-11-2010, 01:13 AM:
 
I just Specd a 14m wide screen with 4 4645C subs. They handle better than the JBL duals. Joe try to find at least a second one.
Demetris
 

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Posted by Steve Guttag (Member # 268) on 05-11-2010, 06:02 AM:
 
Another thing that can be done is if you have multiple auditoriums with single 4645Cs is to put one of the 4642As in an then combine the removed 4645C with another theatre's 4645C and thusly improve two theatres with one purchase.
 

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Posted by Louis Bornwasser (Member # 3063) on 05-11-2010, 06:21 AM:
 
I like Steve's solution.

I once had a 6 plex with one sub in each. I put 2 subs in three theatres. That't right, 3 have no sub. About the same sound as having one. Did fix the other three, though. Louis
 

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Posted by Tony Bandiera Jr (Member # 2365) on 05-11-2010, 12:26 PM:
 

quote: Joe Redifer

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I already have a 4645 and just having one isn't good enough. I was hoping to find a couple of used 4645s but I can only find used 4642's.

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Sorry I should have said 4642...typo. Damn JBL and their goofy numbers.
 

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Posted by Joe Redifer (Member # 3) on 05-11-2010, 02:42 PM:
 
Steve, the inoperative 4645 is currently the only subwoofer in the entire building.
 

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