Ok, so it is just a safe precaution!

My theater is an old style theater as you saw. Sound system was completely rebuild in 1994 with SR.D added and speakers changed.
Behind the screen I have 2 4638 JBL LF speakers for each channel.
Assuming that my theater is 35 meters long, I need 1250W for channel, 833 for LF units (It is a biamped system).
One 4638 can handle just 800W, with two unit the power requirements drop to 400W so that each LF should handle only 200W.
So, no problem. But why they've installed two 4638 instead of, say, one 4648? Perhaps in 1994 there were no JBL speaker with more than 800W for channels?
Having two 4638 instead of one (for example) 4648 could improve bass frequency response?

Perhaps you missed part of my message: can you post (or send me by email) the formulas for exactly calculate the power needs of front, surround and subwoofer channels?

Lets work through one....

Lets say you have a JBL 4675 stage speaker...it has a sensitivity of 100dB/watt @ 1-meter

Lets also say you have a 30-meter long room.

Therefore, you would need

103 - 100 20Log(20)
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10
10 = 798 watts.

Why did I use 20 meters? Because the power requirement is for 2/3 of the way back, not the full 30 meters.

For subwoofer, it is a bit more tricky...since sub tracks are normally sustained enough to have room gain contribute to the total sound, you can normally fudge thigns by about 3dB...more so in larger rooms where it is bound to be reverberant....therefore instead of 113dB for the subwoofer, Dolby only has you compute for 110dB...in most rooms this is ok...if the room is dead in the lower frequencies...you should use the 113 dB figure.

Steve

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"Old projectionists never die, they just changeover!"

I have told them before that we abused their products, but they warrant them anyway. JBL is a good company.

I still have one sub that I have never re-coned.

It is made by SRT. They are not THX approved, but they look fine on an RTA.
http://www.hydrosonic.com

The added water weight also makes then un-attractive to sub thieves.

Antonio, you can show your picture with arrows to JBL and they will honor their warranty.

I've received subs back that had been blown out of their baskets with a thank-you note for using JBL.

JBL stands behind their drivers (no matter what we do to them!)

Regards
David

Since no one from JBL is jumping in...

Thanks! Now I have all that I need!

Russ,
Do you really believe that JBL will repair my 2440 in warranty????
The speaker is 7 years old, but has been reconed less than 12 months ago...
I will ask our Italian agency immediately!!!

So cool to talk to someone from JBL. So, in ideal conditions, what amp power would you feed a 4645C ? Let's say you have 4 of these, stacked 2 by 2. connecting bridged amps so as each woofy gets a clean 1500w for instance ?
I use 2 4645C in my home theater with a "small" 2x150w amp ( real watts ), forced cooled and with the signal cut at 20hz, AWG 8 wires, 1meter long ( very short wires thus and I can hear the subs like that!!!). the bass are fabulous. room is 6.5*2.45*12meters

Ok, power needs of that theater is not high so 400W is also enought, but I think that the bridge solution is better than THX one.
What do you think?

We always recommend the max.power rating of the speaker. Like the JBL 4645C, our SB184C handles 800 watts, so we recommend an amp that can deliver 800 watts into 8 ohms. As a loudspeaker manufacturer, it is the most practical answer we can give - to recommend 1600 watts is unrealistic (although more power is ALWAYS BETTER), and to say you can get away with, for example 400 watts, without knowing the demands of the actual installations (room volume, dynamic range of the soundtrack, digital or non-digital) risks amp clipping and speaker and/or amp damage.

So basing on Dolby formula a theater 45 meters long needs 2 HF drivers?
I saw on JBL data sheets that their most powerful HF drivers can handle 75W above 500Hz.
I've calculated that for my theater (35m) I need 57W on biamped system.
So for a 40/45meters long theater we should install two HF drivers?

Nope....JBL's specifications for drivers like the 2446 (used on the JBL 2360 Horn which in turn is used on the JBL 4675 stage speaker) is rated for 100 watts continious program.

Furthermore, looking at the JBL 2360 spec. sheet, the sensitivitiy is 115dB/watt at 1-meter above 1KHz....it is slightly less at 500Hz BUT at 500Hz the HF section is 3-dB down so that the LF and HF section will join seemlessly (both LF and HF being -3dB will sum to 0dB)...therefore essentially pick up the 3dB.

Therefore, using the formulas....

We see you need just over 57 watts. This will put you to less than 3dB of headroom but it should play fine at reference level. Note, you can use the "program" level on a stage channel whereas continious pink noise should be used for subwoofers due to the different types of singals sent to each channel.

As to the issue of adding a second driver....that is a VERY tricky issue. Altec was one of the only sucessful people (I am aware of) to do this with a Constant Directivity (CD) horn. They offered a dual throat on their MR94 (90 x 40 degree) and the MR42 (40 x 20degree). They had the drivers share the same horn (it was unique in that they had one driver working the top half of the horn and the other working the bottom half such that they worked like one BIG driver)....this was important.

When you add an additional horn/driver system....you WILL get lobing since your sound is now coming from two points and will have time alignment problems and the problem change through out the shared spectrum. In short, this is something you really don't want to do unless you HAVE to. It is like EQ...it is an addmission of failure, that is, your speakers and room are not perfect so you must fiddle with it and mess up the phase response to overcome frequency problems...it is a better of two evils situation. Same with going with a second (or more) horn/driver combination. Note, the need for the additional units may be for adequate coverage, not just power handling.

On the up side, we now have computer modeling available to us so in the event that multiple systems MUST be used, the results are much more predictable and can be optimized (just like EQ) to provide the best solution for a given situation.

But back to YOUR theatre. As I recall, you have an overhanging balcony. This might give you a good reason to use two HF sections without the nasty artifacts previously mentioned (lobing). One thing about the CD horns is that they have a pretty sharp cut off outside of their coverage area. You could use a horn like the 2360 (90 x 40) aimed to cover just the lower level (stall) and a horn like the 2365 (60 x 40) to cover the balcony. Since the balcony is further away, using the narrower width horn will not have you waste power (and sound reflections) on the walls getting the sound to the balcony. You could then tune the two sections independantly to optimise both rather than what most people do (make the lower section tune to spec whilst the balcony gets what it gets).

Steve

Steve

Ok for HF power.
Continuous program can be used also for LF stage speaker?
I have 2 4638 for each channels.
My theater is 35m long. So I need 1083W for each channel. 2/3 of 1083 (my system is biamped) is 722W.
If continuous power can be used, one 4638 should be enough, isn't it?
If two adiacent 4638 have the same +3dB "coupling" power increase, to have 103dB in my auditorium each speaker is powered by only 180W... I can uncover the theater...
Another THX theater of my chain has 4648A-8 and it is only 21 meters long. Should I consider 2400Watts for these speaker???
In that case that speakers are powered at 65W maximum!!!