This is topic Out of phase subwoofers in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Antonio Marcheselli (Member # 373) on 04-03-2002, 04:54 AM:
Hi everyone,This is the scenario: 2 JBL MPX600 amps in bridge configuration driving 4 JBL 4645B subwoofers.
Let's suppose that two of four subwoofer are connected out of phase.
What should be heard in the auditorium? Out of phase subwoofers will kill each others completely or should I hear something anyway?
Bye
Antonio
Posted by Phil Connolly (Member # 465) on 04-03-2002, 05:47 AM:
Antonio,You will get a reduction in level but they won't totally cancel each other out, because of room reflections, distance between speakers etc..
Posted by Dave Macaulay (Member # 813) on 04-03-2002, 07:46 AM:
The level will be reduced and much of the "punch" is lost.
It does depend on placement. If the subs are close together (IMO the best/only way to install multiple subs) the negative effect of bad phasing is greatest. If they are spread across the screen it's less.Bottom line is to get all speaker polarities right! Doing a proper B-chain setup is impossible with it wrong. There will be seats where the sound is absolutely awful if the screen channels are messed up. If the subs are fighting each other the sound will be odd, plus you will be using a LOT of amp power and risking speaker damage if you manage to get the sub SPL up to spec.
Posted by Antonio Marcheselli (Member # 373) on 04-03-2002, 08:11 AM:
Dave,Polarity must be right, of course. I'm going to check it, I would have your opinion in the meantime!
Thanks!
Antonio
Posted by Steve Kraus (Member # 476) on 04-03-2002, 09:59 AM:
The out-of-phase subwoofers get connected to the sound system of the adjacent auditoria, to cancel the bleed-through sound coming through the walls!!
Posted by Antonio Marcheselli (Member # 373) on 04-03-2002, 01:05 PM:
Steve,It could be a solution... 
A
Posted by Steve Guttag (Member # 268) on 04-03-2002, 09:30 PM:
Actually Steve's solution would have a worse effect. Since one theatre would be drawing in whilst the other is pushing out...the wall would tend to flex more and transmit yet more sound. If both are in phase, then the air pressure on either side of the wall would cancel. But back to Antonio's question...if any of the subwoofers are out of phase with the others then the total output will be reduced, not entirely cancelled.
Steve
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"Old projectionists never die, they just changeover!"
Posted by Antonio Marcheselli (Member # 373) on 04-04-2002, 03:59 AM:
Steve,Thanks for your reply. I will check soon. IF subwoofer are out of phase I will cut someone's hand... 
Bye
Antonio
Posted by Darryl Spicer (Member # 711) on 04-04-2002, 10:21 AM:
Also make sure that the subs have not blown. We were having problems with subs blowing at our theater. We found that the way the subs were connected in the rack was causing them to overload. We eliminated the terminal strip that the speaker wires and the outputs from the amps were connected and wired them together. We also eliminated the grounds at these points because there were already grounds set up at other points on the rack. This was causing a ground loop to occure and was overloading the subs. We have now achieved a more smoother bass response and no more overdriving of the amps has been noticed.
Posted by Antonio Marcheselli (Member # 373) on 04-04-2002, 05:36 PM:
Darryl,Blown... Brrrr.... 
I hope not!
Ground loops that overload speakers... I didn't understand this. Can you explain me what does it mean?
Bye
Antonio
Posted by Joe Redifer (Member # 3) on 04-04-2002, 06:07 PM:
I once worked in a theater where nearly every other speaker was wired out of phase! It took Brad and I all night to check and fix them in that 16-screen complex.
Posted by Ray Derrick (Member # 1001) on 04-05-2002, 04:19 AM:
Antonio, was your original question hypothetical or do you have an actual problem? If you are lacking sub-bass it is possible that all four subs are in phase with each other but are out-of-phase with the screen speakers. Many modern screen speakers can easily be flat down to 40 or 50Hz. To check this you should do a B-Chain and check that, after equalisation of the screen and subbass speakers, that the sub bass adds when both centre and subbass are fed pink noise at the same time. Your sound processor manual will tell you how to check this in detail. It is not sufficient to rely on the colour code of the sub-bass speaker wires!
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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935
Posted by Antonio Marcheselli (Member # 373) on 04-05-2002, 05:04 AM:
Ray,Yes we are having problems with lack of bass frequencies.
A tech will come soon but in the meantime I'll check the wire polarity connecting a 1.5V battery in the booth and check on the speaker with a multimeter.
Bye
Antonio
Posted by Steve Guttag (Member # 268) on 04-05-2002, 07:07 AM:
Antonio, merely checking absolute phase (like with a battery or a genuine phase checker) is not sufficient with a subwoofer system. Most stage speakers will have some form of electronic delay on them so they will "time-align" with the HF frequency drivers. For instance, in the JBL 4675, there is a 1.7-1.9msec time delay. Furthermore, most installers place the subwoofers on the floor rather than on the level as the stage speakers. As such you will have a physical displacement. Just because you have the speakers in absolute phase does NOT mean that they will necessarily add to the stage speakers.
Do as Ray suggests, play pink noise out of your Center channel and then add in the subwoofer, your bass frequencies should go up at frequencies they both play (typically from 45Hz to about 100Hz). Set your subwoofer's polarity such that this will occur. I have yet to find a system where one polarity didn't improve the response over the other.
Steve
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"Old projectionists never die, they just changeover!"
Posted by Antonio Marcheselli (Member # 373) on 04-05-2002, 07:59 AM:
Steve,Thanks for your suggestion. Our tech has an R2 analyzer and it is THX certified, I'm sure that this time he will check bass response with attention.
In the meantime I'll have all speakers cabled correcly so that we have a known start point...
Bye
Antonio
Posted by Antonio Marcheselli (Member # 373) on 04-10-2002, 06:47 PM:
Yeeeeah!!Center Channel's LF is out of phase!!
Is there a particular THX certification for out of phase theaters? We have the standard one in the hall...
Antonio