Just 2 questions:
1. Sensitivity: I didn't understand very well. For example, 4688 has 97dB, 1W, 1 meter. So if I have only 1 Watt of power on my subwoofer it will play at 97 dB at 1 meter???
2. Where can I found Dolby's manual you said? Is it on Dolby's website?

Again, many thanks! I was sure that there were some physic behind a sound system. Unfortunately I never saw any "qualified technical personnel (as DTS said!)" use that...

Yup, you pretty much have the idea....the measurement, in theory should be in an anachoic (sp?) chamber since you don't want to measure the room gain with the speaker being measured. The frequency range must also be specified for a meaningful SPL reading in dB. One thing to note is that, in your case with a 97dB speaker, a double 18" speaker of today typically has a 100dB sensitivity....that reduces the power required by half right off the bat! That is what I mean by keeping the 3-dB and 10-dB stuff in your head for quick number crunching.

As to where the manual is....they used to give them away at the training seminars and with the dealer binders...there are two that I know of...the early "draft" one had red covers and the later one was with grey covers.

I see Brad doesn't have them on the site yet...I'll check with Dolby and see if a newer version was ever made or if they have any problem with it being posted.

regards,
Christos

Thanks, it is very clear now!!

Unfortunately the subwoofer graph on c.e.s. is corrupted (!) I will ask to fix it.

Steve, I know a theater (well know around the world for its quality of presentation) in Italy, ARCADIA of Melzo, that in their biggest room have 4 Electrovoice TL880D with P3000 amplifiers.
Since that sub has 127dB sensibility, they are like 10 of my 4688, isn't it?

So, have 4 TL880D in my theater would be like have 40 JBL 4688??

I can believe, because when THX logo fall down (TEX trailer)at Arcadia you can FEEL bass frequencies and seats move itself...

Steve

EV: I have the PDF specs of TL880P (with integrated P1250). It says "Sound pressure level at 1 meter, maximum gain and amplifier at clipping threshold, 0dBu (0,775 voltsrms) into balanced input, Anechoic environment, swept sine wave 50-125 Hz: 127 dB".
Perhaps it isn't at 1 watt?

Today I enjoy myself checking my theaters sound system with diagrams on Dolby's tech sheet.
Unfortunately the SW datagram is corrupted on cinemaequipment.com link (can you post yours?).
Just to understand if I made all correctly:

Stage channels:
I have two 4638 and one 2446H for each channel.
My theater is 30/37 (average: 35) meters long (previous measures were wrong).
With datagram found on link above, I need 1250W for channel. Since my system is bi-amped, I need 834W (bi-amped system: 2/3 of 1250W) for HF since sentitivity of 4638 is 100dB.
What power should I consider on datasheet?
I have AES: 800W; Long term average per EIA RS-426A: 1000 Watts; Continuos program: 1600W.
I say that also at minimum power (800W) since I have two speaker for channel I have enough power for digital soundtrack, right?
HF: 2446 has 111dB on horn. So I need about 120W of power that in biamped system become 80W. On JBL datasheet I've found: 100W continuos program above 500Hz and 150W above 1000Hz. I'd say that they are ok...

I have trouble with surround channels.
I have 8340 (sensibility 96dB) and theater width is 16 meters.
With datagram I've found that I need approx. 200W for channel. 8340's power is 150W so two speaker should be enough (just for power needs, not for good coverage of all seats of course). Is it right?

However, Trying to calculate the same result with the equation provided I have a differen value.
I have Watts= 10^(Lp-Ls+20(logR/10)).

with Lp=100dB (SPL for each stereo surround)
Ls:sensitivity
R= distance from wall to centerline of the theater in meter.

What is wrong?

Sorry for this long post but since I was a projectionist I had the need to know the exact way to calculate the power of a sound system. You are helping me very much!

Hope that what I've wrote is clear.

You are right. I found the correct EV pdf datasheet and sensibility of TL880D (without integrated amplifier) is 96dB.

As you all can see I'm trying to understand better the "power requirement" problem!

I understand that "continuos pink noise power" is the value that has been to be considered making power requirement calculations.
You told me that I need 19200W in my theater and I need 6+, 8 preferred units. A 4688 SW is a 600W pink noise power (half than continuos program, isn't it?)
19200/8=2400W. This means that I have to give 2400W to each unit, right? But how can a 4688 handle 2400W of power if its "continuous pink noise" power is only 600W?
Again, you wrote 3600W (I suppose with 6 units). But 19200/6=3600W. Did you make a mistake in calculation or didn't I understand somethink?

Let's talk about the 4645C. It handles 800W contiuous pink noise. You wrote that I need 6, 8 preferred units. Since sensitivity is the same, power requirements are the same (19200). But 19200/8=2400W. Again, same question of above!

As you can see I'm abit confused...

Thanks for all of your help!

When you add subwoofers, you increase the sensitivity of the system when they are closely placed. As such your needed power drops. In the case of the 4688, you would only need 3600 watts with six units and you would have 600 watts x 6 speakers or 3600 watts of power handing. Similar math applies to the 4645C.

Steve

I suppose that your calculations are made over a 19200 Watts power requirement.
When I add one unit I need half of the power, right?
So:

1 Unit = 19200W
2 Unit = 9600W
4 Unit = 4800W
6 Unit = 3200W

Where I'm wrong?

Let's make an example.
I have a theater with 4 4645B and the theare is 27 meters long.
Figure on cinemaequipmentsales.com is corrupted (they'll fix it today) but I suppose that a 28m long theater with 100dB speaker sensibility needs just 6000W of power.

So:

1 Unit = 6000W
2 Units= 3000W
4 Units= 1500W

1500W/4=375W to EACH subwoofer.

4645B can handle 800W so it is correct.
Right?

Thanks for your help!

>>"1 Unit = 19200W
2 Unit = 9600W
4 Unit = 4800W
6 Unit = 3200W

Where I'm wrong?"<<

You are not wrong. You have 6 units though that can each handle 600 watts or 3600 watts of power handling combined (the 3200 watts isn't what EACH unit must handle but the total power needed given the sensitivity of the ENTIRE SYSTEM). But at this point you only have a theoretical 400 watts to spare on long peaks (like during the Apollo 13 blast off)...spread out over the 12 drivers, you would be giving yourself only 33 watts per driver of head room). Since drivers go through something called "power compression" as the voice coil heats up, it probably better to get the two additional speakers (total of 8) and run everything well within range (it also makes hook up easier). You also want to make suure all off your connection are in parallel or direct to the amplifier, no series connection (I don't want to go into the science of it at the moment).

>>"Let's make an example.
I have a theater with 4 4645B and the theare is 27 meters long.
Figure on cinemaequipmentsales.com is corrupted (they'll fix it today) but I suppose that a 28m long theater with 100dB speaker sensibility needs just 6000W of power.<<

So:

1 Unit = 6000W
2 Units= 3000W
4 Units= 1500W

1500W/4=375W to EACH subwoofer.

4645B can handle 800W so it is correct.
Right?"<<

On the nose! (Note, you could also barely make it with just 3 units).

Robert:

No, the sensurround subs were removed (and destroyed for the most part) when the theatre was renovated just before the reissue of Vertigo in 70mm. We currently have 8 JBL 4645C subwoofer and two QSC MX-3000a amplifiers (with the SF-1 subfilter with B-6 tuning). My favorite comment was that "the system now sounds like it used to!" The old sensurround subs were horn/slot loaded and quite efficient. As such, the whole system ran on 400 watts (the typical Cerwin Vega A-2200 that was used in virtually all sensurrounds that I have seen). We now have somthing like 4800 watts!

Steve

Understood!!! Great! I've checked for fun many power requirements and I think that I have fully understand the thing! Many thanks!!!!

3 questions:
1. power spare head room. I understand that powering a speaker at its rated power is not a good tecnique. So I understand why I should have 8 SW instead of 6.
But you talked about "Apollo 13 blast" PEAK. So Digital Soundtrack are capable to go further than 103dB (113 for SW)? Or you mean just that have 6 unit running at maximum power for too much time is not good?
2. Surround speakers: In that case the 3dB sensitivity increase is not applicable because speakers are not near each others, isn't it?
3. Is it possible to have the math equations to make exactly calculations abut these power requirements? Can you post it as pictures?

Steve, you helped me much in this thread: many thanks!

In a blast off like in Apollo 13, the extreme subwoofer signal is used for an extended period of time...assuming that you have set up your system to spec and assuming that you only run things at "7" or lower with "7" being the reference, then no you shouldn't exceed 113dB 2/3 back in your auditorium.

Steve