Yes, you can use program power for the stage channel LF section though I tend to favor using coninious pink noise for anthing bass due to the nature of film soundtracks...that is with effect, bass tends to be a continious thing, unlike music and speech.

Unless I am mistaken, the 4638 was only available as the 4638TH with a crossover. It only has the 2035 drivers inside which are less capable than the 2226. If you are using the crossover as the input for the bass (presuming it is indeed a 4638TH) then the system tops out at 600watts AES, vs 800 watts if the woofers are connected directly to the amplifier. I would recommend two bass sections in a theatre of this size (35 meters) for digital films to be played at spec.

For both theatres, play it on the safe side. For the bass section use continious pink noise as the rating and don't take the 2/3rds power handing...remember that the 2/3 amounts to less than 3-dB...not big enough to really make a huge difference in the sound level but enough to have a driver burn up. You should keep some amount of headroom in any system (normally 3dB is a minimum for safty).

Steve

On the back of my 4638 there is just "4638 4 ohm" and anything else.
As you can see,

I have two only input for that speaker. Each speaker is connected to a Crown MT 1201, one speaker for each channel.
HF driver has its own amplifier too.

You said that for 35meters theater two LF section is recommended. But why not install just one LF section with more power?

Finally: someone told me that where 2 LF speaker are used, they should be installed one on the other and not side by side (like in my theater), what do you think?
On 4638 PDF I've noted that vertical coverage is less high than orizontal so I think that have two 4638 side by side is wrong.

Bye
Antonio

I must be drawing a mind blank....Perhaps the 4838 is a loaded 4508 cabinet with two JBL 2225 drivers...if that is the case, then you have less power handing...the 2225 was a 350 watt driver...dropping you down to 700 watts total per cabinet. With a 35 meter room, you should have about 1100 watts...even if you like to play things on the edge, you need 735 watts...either way you are coming up short. You should use two cabinets. Then you only need 550 watts for the system and each driver is only having to deal with 140 or so watts...they are loafing at that point rather than suffering from power compression....a much better design.

As to stacking the bass cabinets, yes they should be stacked one on top of the other...the directivity of bass systems is a bit wild but with a stacked array, the HF and LF directivity will be more closely matched in this case.

Steve

OK...I've traced down the 4638....yes it is loaded with the 2035...things must be different out of the states...That driver is only supposed to be used in factory-loaded systems with a passive crossover. It looks like the 2226 which is a pretty substantial driver 600 continious watts and built with a real nifty cooling system. The 2035 is supposed to be used in power limited applications wbere the passive crossover is the weak link.

Anyway, you are correct, you have 800 watts of conintious pink noise or 1600 watts of continious program. I would favor to use the 800 watt figure for cinema but you are probably safe using the 1600 watt figure...this means you would have to have an amplifier that goes to 1600 watts into 4 Ohms...I wouldn't push things too much though. Since you have a double bass system, why make it an issue? It is the smarter safer way to play it. I would be very surprised if you have any driver failures from power related issues.

Steve

Don't misunderstand me! I'm very very glad to have two 4638 for each stage channel!!!
It is just a thought: I've saw on an online price list that six 4638 cost 4128 USD. Just three 4648A will cost only 2307 instead with a difference of 1821 USD.
So, why install six 4638 instead of three 4648A?

Steve

As you noted, my english is not very good..
I really didn't understand what do you mean with:
"First off...I would only get the 4648A, myself so you don't have to sell me on it".
However, let me explain you why I'm making an inssue about my double 4638!
I'm just a projectionist, I'm not a manager. But I can realize that our techs are not "qualified" as I would.
When I've calculated the power needs for my theater, I wonder why they've installed two 4638 instead of just one unit with more power. That's all!
I fully agree with you about install speakers with adequate power, I told you that I'm very glad to know that I can project movies with no fear about overloading stage speakers.
Since "I"'ve blown up two subwoofer, I also made these calculation to realize if the problem was just in the subwoofer channel or if the entire system is inadequate.
So, in a theater 35 meters long, would you install one 4648A or two 4638?

Last thing, for all Film-Techers: sometimes I write something that is not what I think. If someone read something "wrong" or wrote in a bad way, probably is not what I ment, was just my poor english! I apologize for that in advance!

What I mean is that I would only use the 4648A, never the 4638 (regardless of the quantity). I am a fan of the 2226 driver, it has to be one of the best 15" driver I have ever seen, inside and out. The 2035 is a lesser driver made to "look" like the 2226. This is what I mean that you don't have to "sell me" on using the 4648A. If I were desiging the sound system for your theatre using JBL stage speakers, then I would use two 4648A bass sections for each channel.

So you have given me a choice of A or B, and I have given you a response of C! Don't you just hate people like that?

Steve

Thanks for the explanation. So in my theater a 4638/4648A stage channel should handle at least 722W in a Biamped system. 4648A speakers have 1200W continuous pink noise power, 2400W continuous program.
With one unit we have 478W headroom for each channel, 239W for each woofer.
With Two unit headroom raise to 2039W for the system and 1019W for each speaker (right?).
So, you think that 239W for each woofer is not enough and you prefer to install two unit. Right?

So, how much "headroom" should I have to consider when I have to project a sound system?

If you go with two bass sections it pays off in dividends, you can now use a smaller and less costly amplifier, your drivers are not being taxed and your distortion goes down overall. I think if you priced both systems out, you would find that the cost is similar (include the amplifier cost in your comparision) and the double bass system will out perform.

Heck, if you looked into JBL's 3-way systems (The 5000 series) you would find that the system for your theatre would be the 5674 and it HAS 4-2226 drivers in their "diamond-quad" configuration. This is the system that have effectively replaced the "academy speaker" for JBL The academy speaker was a JBL 4675 with two 4648A bass sections.

Steve

Ok. Thanks for all your help, it was a nice chat!