All houses are running JBL 4675 stage speakers, 8340 surround speakers(8-per house), and 4645B subs. Each house is between 300 and 350 seats.

 |  IP: Logged

 |  IP: Logged

4645B is listed at only 97dB

You say your auditoriums are either 61' or 63'

You have 3 units...

You will need somewhere between 1000-2000 watts

To be precise...

You have 3 97 dB speakers...doing logrithmic addition yields a system efficientcy of 101.77dB for closely spaced speakers.

Now, using that with a 63 foot room and a desired max spl of 113dB in the middle of the room, we get:

1223 watts

For the 61 foot room the power reduces to 1148 watts.

This presumes you are indeed running your fader at 7.0 for all films. Your QSC 1400 only has only 550 watts into 2 Ohms at a whopping 1% distortion and only 1KHz tone, unlike real world conditions. Since you are only at 2.67 Ohms (nominal)your real world wattage is closer to 350-400 watts maximum.

Thus, yes you are under driving your subwoofers. The 4645B uses the JBL 2242 driver, probably one of the best drivers made at the moment. It can handle 800 watts of continious pink-noise or 1600 watts of "program power"...always use the pink noise spec for theatre subwoofer applications.

To properly power you subs, sticking with the QSC line up, You would need something like a DCA-3022..run two off of one channel and one off the other or, use two DCA-1222 amps , both bridged and again two subs on one amp, one on the other. Cost wise, it is a wash so I would favor the two-amp scenerio. Alternatively, if you have the depth and the weight capacity, QSC's new ISA amplifiers can offer a lower cost solution using a pair of ISA-450s in bridge mode.

One last thing to consider for your 4645B (and "C") subwoofers is to B-6 tune them to yeild a substantially improved bass response. Use the QSC SF-3 module on either the ISA or DCA line. Using other amplifiers, RDL makes the CX1S that will also allow B-6 tuning.

BTW, you are fine on your surrounds but are a bit difficient on your stage amplification if you run your system at 7.0. Ideally, you should have just over 300 watts and you only have 200 watts available.

Steve

One other thought -- placement is always a way of improving bass efficency. Placing subwoofer bins in the corner of the room if at all possible will show a marked SPL improvement, as will placing bins as close to each other as is physically possible so they "couple."

You are always precise and clear!! I've amazed many of my colleagues with our last "chat" on power needs.
You said:
--
One last thing to consider for your 4645B (and "C") subwoofers is to B-6 tune them to yeild a substantially improved bass response. Use the QSC SF-3 module on either the ISA or DCA line. Using other amplifiers, RDL makes the CX1S that will also allow B-6 tuning.
--

What is "B6"?

Also: in one of our theater, 4 4645B Subwoofer has been placed directly under the center channel, on the centerline of the auditorium. Can this improve the resonance of the room?

Think of "B-6" tuning like you would slit loss for optical soundheads. A speaker designer can purposly design the low-end fall off of their speaker to conform to the B-6 criteria (kind of a gradual roll off actually). If a B-6 filter is applied to the signal, it will apply a complimentary boost to the signal and high-pass the signal above the unload point of the speaker. As such you will get an extremely flat response down to the cut off point of the speaker and then a sharp drop (again, just like on your slit loss of an a-chain). As such, a subwoofer like the JBL 4645B, and the 4645C can be ruler flat down to 22Hz rather than starting to roll off below 30Hz. The 4641 and 4642A also can take advantage of the B-6 tuning

Another popular theatre subwoofer than can take advantage of the B-6 tuning is the EV TL-880D.

Some speakers call it "step-down mode." The older TL-880D had a movable port plate so you could choose wether or not use use it with B-6 tuning or not. That way if you didn't have the appropriate boost, you would get a more extended bass response if you moved (I think removed) the plate.

Now to the powered subwoofer stuff. There really is no advantage to them and in fact a disadvantage. There isn't any trick to matching amps to their application, subwoofer or otherwise. A good speaker manufacturer may not be the best qualified to make amplifiers and vice-versa. The damping factor arguement has long been exposed as sheer marketing. What is the biggest resistance in virtually any amplifier/speaker run? Why the voice coil, naturally (unless you are using 22awg speaker wire over 100 feet). Now if an amplifier has very poor damping then yes it can be heard as mushiness in the bass (anyone wanting to experiment, take a series resistor on their speaker line, some actually prefer the tubbiness).

If the amplifier is down by the screen and the processor is in the booth, you have a grounding situation to consider. It is unreasonable to expect that the path to the building ground rod is identical between the two desperite power sources (now you could have two ground wires go from the ground rod, one to each system and make sure both are of equal length, irrespective of how far they are away from the ground rod). As such you WILL have some form of ground loop if the grounds of the two systems are tied in any way. Since most balanced systems nowadays are active balanced, their will be a ground reference so even if you lift the shield at one end (as most people do) you will still have two different points on the ground system tied together. So now, if you want to keep the grounds interacting, then the best solution is to have a transformer in line. Now one should have a good audio tranformer to keep the frequency response good.

Then there is the service aspect, who would want to climb behind a screen to service electronics? Also one must power it and switch that power to avoid whomps and such as the system is powered on and off.

I trust your explanation of adding units until the power requirements were met was a joke. Since both powered and passive subwoofers follow the same laws of physics and performance can be predicted without too much hocus pocus.

As to placement...there are advocates of corner placement as well as directly center. The corner people try and take advantage of the interaction with the boundries so as to get an exaggerated response, the center people seem to go for "equal." Most of the theory I've read and experienced is that placing subs slightly off center will yeild the most uniform response. I don't run away in terror from corner placement nor center placement as if all is lost if it is done. My personal preference is to use two subwoofer arrays (or merely two) such that they are a little off center to the left and a little off center to the right. I have always found that if there is only one sub location, I can localize on it...I much prefer distributed subs, despite the omnidirectional claim. I think in theatres where the sub frequencies can get above 100Hz allows for perceived directionality.

There are programs such as EASE that will allow a designer to design subwoofer (and any other speaker) placement for optimal uniformity.

Steve

Thanks for the response. We are currnetly only running one sub in those houses and not two. Im hoping to convince the new tech to add two. Sorry for the confusion. Im not sure what amps we will be using probably the QSC MX series amps. Can you explain series and paralell wiring configurations and how this effects the amp. As Antonio explained your explanatons are clear and concise and this has always confused me.

I guess I have just been listening to much to all the marketing hype about the ease of the packaged subwoofer system. And although BGW has been making high powered amps for decades, you are right, that doesn't mean that any company will necessarily excels at both speaker and amp design. Then, all you really needed to do was to say the words "ground loop" and all of a sudden I am running in the other direction. I know how difficult that nemisis can be. In our big house, the amp racks are unfortunately in an amp room off the side of the stage -- a good 200 feet from the booth. Grounding issues were a nightmare when we first started and ikt took lots of money for installation of a single point earth ground and lots of transforer swapping to get good sound and no loops. Now whenever I hear "active ballanced" or "electronically ballanced," I think, yah, that's fine, but buy the REAL transformer accessory anyway.

When I first started working in a small studio in another part of our complex, I found a ground potential difference WITHIN THE SAME ROOM of 17v between grounding pins on outlets with less than five feet between them. Try and get an active ballanced piece of equipment to null THAT out.

Glenn, I misread your post, you have 3 auditoriums each with only ONE subwoofer, not three, right?

If so you have a 97dB speaker...in the 63 foot theatre you will need 3670 watts! Got that much? Your amplifier, the QSC 1400 using just one channel into 8 Ohms has only 200 watts/channel. Hmmm sounds like it isn't going to work! Not to worry, you only need 3441 watts in your smaller 61 foot rooms! Again, this all presumes you run your fader at 7.0 and have tuned your system "to spec." If you are like most theatres, you probably are running at about 5 or so so you may take off 5-6dB...that brings you down to 860 watts for the smaller theatres and 917 watts for the larger one....hmmm still not enough.

The biggest bang for the buck any subwoofer system can get is the 2nd subwoofer...it brings with it normally a 6dB increase in output. This is due to the mutual coupling effects of the two units, and that most amplifiers will give you nearly double the power into half the impedance. So in your case, adding just one subwoofer to each system, will take you from 97 dB Sensitivity to 100dB and your power will shoot up from 200 to about 400 watts. You power requirements with two units will be cut in half (1835 watts, and 1721 watts respectively). And if you only run at 5.0 on the fader, you are practically there! This isn't the way to set up a system. You should set it up so at 7.0, everything is just fine. Since you have 800 watt speakers, the two sub senerio really isn't sufficient if you want to set it up to spec (though it will probably work fine). You would want to have at least 2000 watts into 4 ohms though. With the MX line of amps, that would be the MX-3000a (an excellent amp by the way, one of my favorites). Add the SF-1 subwoofer filter card to get the B-6 tuning and things would be nice. If I were designing the system, I would probably go to 4 subwoofers (using the 4645C). This would drop the power requirement to a reasonable 922 watts and using the MX line of amps a single MX-1500 would do the job with two units on each half of the amplifier. Now if cost is a concern (and when isn't it?) you can consider using the JBL 4642A...it is a 101 dB sub (double 18") you would only need 1461 watts in your larger house and it can handle 1600 of continous pink noise. A single MX-1500a bridged could barely handle it (1500watts @ 1KHz 1%THD). The 4642A's bass response is not quite as good as the 4645C's (or 4645B)...with speakers, one is always trading off efficientcy, response, sensitivity and size. The 4645C is JBL's best subwoofer at the moment in terms of performance (response/power handling). The 4642A is JBL's most efficient and often the less expensive solution though not quite as good bass (same with the 4641).

Now ignorance is bliss, I believe the Dolby nomographs cheats the subwoofer chart by 3dB (only requires 110dB instead of 113dB). I've never blown a speaker using the 110dB figure.

Antonio,

The B-6 filter can be realized by a variety of manufacturers of electronics. For instance, QSC offers a subwoofer filter module for their cinema amplifiers (SF-1 or the SF-3 depending on the model of amp), they also incorporate the option on their DCM line of monitor/crossovers. Radio Design Labs (RDL) also makes a cute little piece called the CX1S module that will provide the required boost (JBL used to sell it under their name briefly when the 4645B came out). I'm sure there are others out there as well.

Back to Glenn,

Wiring anything in series with the amplifier to the speaker, will kill it's damping factor (the amp's ability to control the drivers)...as such it normally will show up as a mushy bass response. Amplifier manufacturer's often use this spec (expressed as a ratio with the higher number being better) to show off their superiority. So even speaker wire can be considered a series resistance. However, it often is negligable in the over all scheme of the system. The driver's voice coil which often has impedances of 8-16 Ohms is much more significant unless you use really tiny wire over a long distance (to wich current limiting may come into play, if not fire hazards). Now an often overlooked aspect of this is in the surround arrays...many of us (me included) have used series/parallel connections on the surrounds to present a usable load to the amplifier. Not too long ago, anything below 8 Ohms was pushing it. Nowadays, most good amps will go down to 2 Ohms. Whenever possible, one should never wire speakers in series. The solution? Get more amplifier channels or an amplifier that can handle the load. With modern amps, a 5.1 sound system can use a single amplifier channel for up to 4 speakers (presuming 8-Ohms). What to do with a 10 speaker surround array (5 each side)? This is a sticky one and comes up a bunch. QSC makes the DCA-1222 that WILL run into 1.6 Ohms (5 8-Ohm speakers) all day long and not mind...it was designed to do that. Even series/parallel doesn't work well for 10-speakers since one group on each side will be louder.

If you must wire surrounds in series/parallel, minimize your series connections. For instance, if you have 6 speakers on each side (12 total) wire four groups of 3 parallel sets (two groups each side). This way each group is only 2.67 Ohms and that is the equivalent series connection to the other group. This will present a 5.3 Ohm load to the amplifier (each channel).

Lastly, I didn't want to imply that the powered subwoofers are necessarily bad, some, for instance, really think that the BGW unit is the cat's meow. I was just stating that just because they are packaged that they are somehow better is not necessarily true, and often isn't. I do firmly believe that in cinema applications, powered subs present more problems than they solve. The types of people that want the ease of powered subs don't normally think about the consequences.

I hope I answered all of your questions.

Steve

------------------
"Old projectionists never die, they just changeover!"

Thanks for the info. That subwoofers are in a (Ted pls don't hear!) THX theater. I wonder if THX crossover will provide the B-6 filtering.
Can be these filters be applied to any kind of amplifiers? We have two JBL MPX600 driving 4 4645C.

Steve

The ironic thing about this situation at my theatre with the subs is they have now spent more money on replacing drivers than they would have if they would have just put in the right amps in the first place. What is it with corporate thinking, it drives me up the wall sometimes !!!! Well i better stop now before i get on a rant.