Film-Tech Forum ARCHIVE: Blown Subwoofer

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Posted by John A. Parker Jr. (Member # 1719) on 07-09-2005, 09:43 AM:

I hope everyone is enjoying War of the Worlds. I did for the first four days until my sub got blown by the lo-frequencies they have put in this soundtrack. Had turned down the sub by three dbs when it went(that is half power for you non tech types). Have heard from another chain that some of his theatres are complaining about their sound. So check your subs, they might be blown. And yes brad, it was set up right. [Big Grin]

Equipment type are QSC MX 1500A amp in bridge for 1000 watts driving a JBL 4645c sub.There is no way this should have happened (1000 watt amp on a 1600 watt driver).

John A. Parker Jr.

Posted by Gordon McLeod (Member # 33) on 07-09-2005, 09:58 AM:

Well how big is the room
remember that it is possible that to get the sub setup to the correct level which is done referenced to the 50% mod point at full output you maybe over driving your sub
If the room is of any size I would normaly use multiple subs remember also that the digital sub requires 10db more headroom than for analogue

Posted by John A. Parker Jr. (Member # 1719) on 07-09-2005, 10:25 AM:

30 feet wide by 65 feet long by 16 feet high.
239 seats, single screen 1933 box. Sub and Amp should have been more than enough to handle the theatre.
John A. Parker Jr.

Posted by Gordon McLeod (Member # 33) on 07-09-2005, 10:33 AM:

You are incorrect the 4645c is rated for 800watts and with a 99db 1watt/meter rateing I would have used 2 of them in that large of room

Posted by Brad Miller (Member # 2) on 07-09-2005, 01:06 PM:

quote: John A. Parker Jr.
I hope everyone is enjoying War of the Worlds. I did for the first four days until my sub got blown by the lo-frequencies they have put in this soundtrack.

The soundtrack didn't blow your subwoofer, your system was not properly designed in the first place. I know VERY FEW auditoriums small enough to be properly serviced by one 18" sub, and yours absolutely requires more than one. Two would've probably been passable, three would've been a better choice.

Posted by Dominic Espinosa (Member # 2122) on 07-09-2005, 01:12 PM:

Don't forget too that the soundtrack for this feature is louder than most.
I stand behind the correctness of Aaron's design and alignment of our auditoriums sound system and we currently run our SDDS at -1.0 for this feature and it's almost too loud there too.

Posted by Gordon McLeod (Member # 33) on 07-09-2005, 02:07 PM:

"Don't forget too that the soundtrack for this feature is louder than most."
That may be true but still not the root of the problem the system wasn't properly designed if it can not operate at the limits that the track can deliver otherwise some form of limiter should be used

Posted by Dominic Espinosa (Member # 2122) on 07-09-2005, 02:24 PM:

quote: Gordon
That may be true but still not the root of the problem the system wasn't properly designed if it can not operate at the limits that the track can deliver

Absolutely no argument there.
I just wanted to point out the importance of actively monitoring the volume.

Posted by Louis Bornwasser (Member # 3063) on 07-09-2005, 03:59 PM:

Subs are a special case. I use the 4645c almost exclusively. One of them by itself is "futile." When you add another, it seems be be 4 times as solid/loud.

I agree with Brad. "Failures" on this unit occur MUCH more frequently in single installations than in multiple unit installs. (still better than the 4645B.)

Good luck!! Louis

Posted by Steve Guttag (Member # 268) on 07-09-2005, 06:36 PM:

Okay lets work the problem...

First off...as Gordon pointed out...the JBL 4645C, which uses the JBL 2242H driver, is rated at 800-watts continious pink noise...this is the figure that you want. "Program" ratings are typically 2-4 times that but are meant for very short durations that may be encountered in music or speach, not for ANYTHING sustained as the subwoofer channel in a cinema soundtrack. For subwoofers, only the continious pink noise specification should be used. Thus, you have an 800-watt subwoofer.

Next, your amplifier...the MX-1500a amp (going by memory) is a 350-watt into 8-Ohm amp...bridged it is 700-watts into 8-ohms (my memory was wrong here, John is correct..the MX-1500a is rated at 1000-watts into 20-20KHz...the rest of my text is being left in since it true of many amplifier spec sheets). That is all the power you got. But wait you say...the spec sheet claims it is 1000-watts....check that spec...is it 20-20KHz or just the 1KHz spec that most amps use? It is the 1KHz spec...are you playing just a single tone? Is that tone 1KHz? That does not sound like a subwoofer channel to me...in fact, it isn't representative of any signal (other than a tone). Real program is in the 20-20KHz range and for the subwoofer...it is 15Hz - 250Hz, depending on processor and application. Thus, you want the 20-20KHz spec whenever determining amplifier power...NEVER the 1KHz spec...just throw that one away. If all you have is the 1KHz spec, then multiply by around .7 to determine real power in the audible range (this is a rule of thumb and is not applicable to all amps but if the amp manufacturer is being truthful, they will publish both specs (they have to use 1KHz since everyone else does).

Okay...how loud can the subwoofer be 2/3rds of the way back from the screen in a movie theatre running a digital sound track? If you go with Dolby, that is 113dB (this presumes you set up your system properly such that the stage channels are at 85dBc and the subwoofer channel is 10dB higher in the pass band referenced to the center channel for digital). Note, DTS uses the spec 115dB but lets go with Dolby's 113.

Okay. Back to the subwoofer...what is its sensitivity...this should be rated in 1-watt at 1-meter. Don't be fooled by some manufacturers that list a voltage rather than wattage...do the math and those that use voltage are boosting their numbers.

Gord gave an optimistic number...the 4645C is around 97dB 1-watt@1-meter. (Gordon's number is for the frequency range of 50Hz-500Hz. For 40-100Hz...the sensitivity drops to 97dB).

You listed your room as 65-feet long.

Based on that information, you need: 6,945-watts

Looking over your equipment...you don't have that! Your subwoofer should have blown up!

Where did you get it into your head that

quote: John A. Parker Jr.
There is no way this should have happened (1000 watt amp on a 1600 watt driver).

You clearly don't know what you are doing or what you are talking about.

Lets work the problem again...add just one more subwoofer (4645C). That will increase the sensitivity by 3-dB for you non technical types and increase power handing to 1600-watts. The MX-1500a amp will run bridged into 4-ohms at right around 1300-1400 or so watts.

Now how much power do you need? As you pointd out in your original post...3-dB is half power. Running the actual numbers you get 3,481-watts. Hmmm still not enough. Now if you run your fader at say 5 or so...you will get away with that because you will be 3-5 dB down. 5dB down from 3481 is 1101-watts. The system is still poorly designed.

So lets add a 3rd sub...sensitivity goes to? Anyone? Anyone? Bueller? Anyone? 101.8...but lets be optimistic...call it 102dB

Run the numbers again and we get: 2,196-watts. Well at last. Subwoofer wise we have enough power handling. But how to hook up a single stereo amplifier to this...you can't just parallel them all. Three subs at 8-Ohms is 2.67-Ohms...when an amp is in bridge mode that would half the impedance. The MX-1500 is not rated into 1.33 ohms a side nor 2.67 ohms bridged. You could, at this point, go using another amp to get the power and impedances to work out.

Alternately lets go to four subs. The sensitivity goes to 103dB of the 4-subwoofer system. The power drops to 1,745-watts. Still more than your single MX-1500a can handle but there are amps available that can provide 900-watt/channel into 4-ohms or with two amps running bridged one can easily achieve this rating without using particularly large amps. Note too, when running large amps, if you are nearing 2400-watts...you can't do that on a single 20-amp circuit at 120 VAC. If you are crossing the 2000-watt range on an amplifier...consider a 30-amp circuit or runing it at 208 volts (normally you must specifiy that voltage in the USA since 120VAC is presummed).

Let nip any idea you may have on how can an undersized amp destroy a speaker rated for more power. Many people are under the misunderstanding that the amplifier can effectively limit the power to the speaker...it can...kind of...however, it is rather nasty about it. When an amp runs out of power...it clips. To the speaker...this is nasty. The cone is forced all of the way to one extreme, held there...then sent to the other extreme, held there and then add harmonics to the signal and you get a driver that cooks. A woofer depends heavily on its motion to provide its cooling...when a driver is driven by a clipped signal...it isn't cooling its voice coil it is just cooking...this will kill a driver faster than over driving it. Most drivers are pretty good about it until they "bottom out" or until the voice coil is driven out of the gap (common issue with the JBl 2245H).

In non-biamped systems, when an amplifier clips, the distorted signal will get processed by the passive crossover and send that high-power nasty crap to the poor HF sections of your speakers and smoke them too.

The moral of that story NEVER underamp.

Sorry if I sounded cross but your statement in your original post was just plain wrong and stupid. It showed absolutely no understanding of power ratings and sound.

Digital soundtracks require a lot out of the subwoofer channel and the surround channels....MUCH more so than optical sound ever asked. Where one sub can work in a optical sound theatre...it is not uncommon to go to 4 subs in the same theatre for digital.

Again, all of the above presumes that you tuned your theatre to spec at fader setting of "7" and actually run your theatre at "7". Whereas most cinemas I've encountered do not run their theatres at 7, they can cheat the numbers a bit but it is still bad design.

BTW...Louis ... the JBL 4645B is functionally the same speaker as the 4645C...only the port design was changed. I think you were meaning the original 4645 that used the JBL 2245H driver...it is rated at a mere 350-watts. The 4645 has had a hard life in the Digital world...then again, its foam surround has worked against it for long term durability. What the original 4645 did have was an inherent very low bottom end. The Driver easily went down to 20Hz (the foam surround helped out with that). The current 4645C really needs a B-6 tuning to get down to 22Hz flat. BTW...the B-6 tunning comes off of the power handling [Smile]

Steve

[Edited to correct amplifier mistake and to clarify the difference in subwoofer sensitivity between mine and Gordon's number]

[ 07-10-2005, 09:02 AM: Message edited by: Steve Guttag ]

Posted by John A. Parker Jr. (Member # 1719) on 07-09-2005, 08:42 PM:

First of all, I am not stupid. I have been in theatre, both live and cinema for 40 years! The entire point of this post was to alert you to the fact that there could be a problem with this soundtrack. It seams funny that this system has been running for 5 years and has not encountered any problems. If you want to gripe at the original installer of the system, call Frank Worthington of Goldsboro, N. C. as that is who designed and installed this system in 1999. (end rant) As for levels, the master was on -2.0, and also had lowered the output to the amp(software control) by 1.5db. I was about to invoke the high pass filter (48 hertz) when things went south. Before this post had been made, the decission had been made to go to two subs to handle this. Thank you for all your comments, since I am too stupid to stay, I bid you goodbye. Brad please cancel my password.
John A. Parker Jr.
your stupid audio tech.

Oh yeh, The spec sheet for the mx-1500A says 1000 watts in the bridge mode. The sub IS rated at 800 watts pink noise and 1600 watts program. since what was playing was program, I would expect it to handle those peaks. Another reason I was trying to bring the level down was that I did notice the amp clipping and knew that this was not good.

Posted by Brad Miller (Member # 2) on 07-09-2005, 09:53 PM:

Stupid was a poor choice of terminology. The proper word would have been ignorant (look it up if you think those two words mean the same thing).

Simply put, Steve's answer is right on the money and clearly took him a bit of time to calculate and compose to benefit others for LEARNING. This site is about the spreading of knowledge. No one person here knows the most about everything. There is ALWAYS someone who knows more about a specific topic. You shouldn't take it personally. Steve's answer was helpful to your dilemma and could save you problems down the road if you will listen to his advice.

Posted by Steve Guttag (Member # 268) on 07-09-2005, 09:56 PM:

Well you proably won't be reading this since you are choosing not to belong anymore....but...

quote: John A. Parker Jr.
First of all, I am not stupid

I did not call you stupid...only the statement you made in your original post. It is... just look at it:

quote: John A. Parker Jr.
There is no way this should have happened (1000 watt amp on a 1600 watt driver).

I have explained why the statement is wrong...very wrong. Your premise that all was set up well and that with a 1000-watt amp you can't blow up a 1600-watt subwoofer is just plain wrong. I have explained that in my previous post.

quote: John A. Parker Jr.
The entire point of this post was to alert you to the fact that there could be a problem with this soundtrack.

I think we all realize the point of your post however, what it showed was your lack of understanding rather than alert people of a problem.

Digital sound is pretty well defined. If your system is set up properly, as you claim and that with pink noise in a cinema system your speakers are set to 85dBc at your nominal fader setting. The output can not be more than 20dB above that since nominal level in all cinema digital audio system is at -20dB. The subwoofer channel is set at 10dB above the stage channels...this allows the subwoofer channel to reach as high as 115dB in a properly set up sound system running at nominal level (in your processor's case 0dB which corresponds to "7" on a Dolby Brand processor and most others).

As such, all the information you need is there. Based on this information, how you can make a statement that there may be a problem with the film? You system is GROSSLY underpowered and under subwoofered. The problem isn't with the film, it is with the system as it is installed.

I stand corrected on the MX-1500a's specification, you are right that it is rated at 1000-watts into 8-Ohms from 20-20KHz.

Even using the number of 1000-watts. You are nearly 8.5dB deficient. Lowering your fader by 2dB (-2dB) still leaves you nearly 6.5 dB light. What choice did your system have but to blow up?

quote: John A. Parker Jr.
Before this post had been made, the decission had been made to go to two subs to handle this.

Why? How are you coming up with your numbers. With two subs, you are still underpowered and don't have enough subwoofers. You are still nearly 3.4dB light. With your fader at -2dB, you are still over a dB short and all else being equal, should still clip the system.

Misinterpreting a cut sheet is normal and deliberate. The manufacturers always want their equipment to look better than it is. "Program" levels are based on nothing more than a multiplication of the continous pink noise level. In a subwoofer system, the signal does not resemble "program" but more like a continious noise stream as long as the the effect is needed. It isn't like kick drum in music where you will get a "whomp" and it is over...that is what the "program" spec is really for.

Amplifiers notoriously use a 1KHz spec even though that is not practical for actual program use. QSC is good in that they list both (though they typically don't list the 20-20KHz for the lower impedances)

quote: John A. Parker Jr.
Thank you for all your comments, since I am too stupid to stay, I bid you goodbye.

No one, besides yourself, said anything about being too stupid to stay. What did do was boast an incorrect statement as matter-of-fact.

Brad, you are correct...the word stupid was not the best choice though accurate as applied to the statement made. However, ignorant is clearly the best and most accurate choice, even as applied to the statement. My apologies.

Posted by Jim Ziegler (Member # 86) on 07-09-2005, 10:26 PM:

Actually, I think the word "mistaken" would be much more friendly, polite and tactful... Really guys, do you think saying "You clearly don't know what you are doing or what you are talking about." is, in any way, going to encourage people to ask questions, or cause anything but hostility?

Posted by Tim Reed (Member # 137) on 07-09-2005, 10:34 PM:

quote: Brad
No one person here knows the most about everything.

Except me. [Razz]

Posted by Steve Guttag (Member # 268) on 07-09-2005, 10:40 PM:

Thanks Jim...I'll stick with the word ignorant on this one. The original post was not in the form of a question but stated as if one is coming from a base of knowledge which they clearly didn't have. Mistaken if for someone that knows better but merely made a mistake. John professed knowledge he did not have.

Posted by Bill Enos (Member # 440) on 07-10-2005, 04:08 PM:

It appears that Mr. Parker thought that he and his tech knew it all. One thing I've learned at Film Tech is that if your knowledge isn't complete you will get shot down pretty quickly, best to shut up and learn something.

Posted by Tim Reed (Member # 137) on 07-10-2005, 04:31 PM:

That's a two-way street, Bill. In the ultimate arrogance of those who DO profess to have all the knowledge, I often find gaping holes in information or practice. No one should close their mind and assume they know everything, yet there are many here who carry themselves as if they are the final word in all things. When challenged, they rationalize and offer circular arguments, but NEVER acquiesce. I submit that this is far worse than someone who simply speaks from ignorance. We're not talking brain surgery here; no one's life hangs in the balance.

I can't say that I blame Mr. Parker for baling out. This could've been handled with a great deal more tact and sensibility, and with less conceit.

Posted by Gordon McLeod (Member # 33) on 07-10-2005, 04:49 PM:

"This could've been handled with a great deal more tact and sensibility, and with less conceit. "
I didn't see much conceit except from the original poster
steve offered a very fair and honest explaination of the facts

Posted by Steve Guttag (Member # 268) on 07-10-2005, 05:04 PM:

quote: Tim Reed
When challenged, they rationalize and offer circular arguments, but NEVER acquiesce. I submit that this is far worse than someone who simply speaks from ignorance.

Tim, there are those that offer opinions as facts and I can understand your dislike of that. "I like brand ABC projector and it blows the rest out of the water...yadda, yadda, yadda." People can debate such a statement forever since what goes into what one likes/dislikes is unique to the individual.

What I don't agree with you on here is the harmlessness of one that speaks from ignorance. They do a disservice to everyone else that reads the post and tries to apply truth to it.

In this post, we have an individual that backs up his statement with "40-years of experience". To many people, that would be considered pretty credible. However to make a statement that is beyond debate wrong (the math does not back it up) and hence is misleading to those that don't know better. So this gets chalked up as a "print problem" and others now feel that it is a fluke and that a single 18" sub is all one needs in a 65' deep room. What a disservice that is to everyone that reads it.

Many people in this biz are weak in the higher mathematics department. To calculate power is not that hard but it does require working with logarithms and anti-logarithms.

How many people here on Film-Tech select amplifier power based on actual need and how many choose it based on "gut feeling" or past experience?

Again it is one thing to post something to the effect that "we just blew out our subwoofer on such and such a film and I don't understand why" but this guy came from a position of experience that it couldn't be the amp and subwoofer combination. It is like saying 2 + 2 = 5 and saying it has to be true...that is not life and death either but is quite wrong as well.

If the guy had posted that with a 1000-watt lamp his 50-foot wide picture looked dark while running THE VILLAGE and there is no way that 1000-watts was not enough...after all that is over 16 TIMES the size of his Soft-white lamps he uses in the ceiling and that is pretty bright. Other films didn't look that dark. The math is easier there and many at least have some formula they use to justify their lamp size versus screen size (then again, there are those that continue to figure that the throw factors in there).

If someone professes knowledge they do not have, they should be challenged, in my opinion.

Posted by Jim Ziegler (Member # 86) on 07-10-2005, 05:36 PM:

quote:
I didn't see much conceit except from the original poster
Steve offered a very fair and honest explanation of the facts

Along with some less than tactful comments and a condescending tone that most people would take as an insult... If the original poster learned anything at all from this, he learned that asking questions will get you flamed. What he does or does not know, or think he knows, is no reason to flame him and lecture him for daring to have a gap in his knowledge.

Steve, rather than teach someone something that they could use to improve their presentations, you did nothing but make them think of you as some pompous SOB. I can assure you that, in the eyes of the original poster, your explanation was completely lost in the condescension that surrounded it.

Posted by Gordon McLeod (Member # 33) on 07-10-2005, 05:44 PM:

I guess if you can't handle the heat then get out of the kitchen
The original poster made a claim and then went on to blame the film and then offer incorrect facts to back it up.
one takes there knocks

Posted by Bruce McGee (Member # 131) on 07-10-2005, 06:29 PM:

I think I know the theatre that Mr. Parker spoke of. It is a tiny room, but even I would have used more than one subwoofer. I've seen one film there, and there was definitely something lacking in the sound. I'm sorry that a fellow North Carolianian will not benefit from the vast knowledge of the people here on F-T. I'm about 45 minutes from Brevard. I would have liked to work with these people in this theatre.

Posted by Steve Guttag (Member # 268) on 07-10-2005, 06:58 PM:

Jim, you missed it...he didn't pose a question...he made a statement of fact. He wasn't looking for knowlege...he was posting to profess knowledge (there was something wrong with the film). He didn't have a gap in his knowledge...he didn't have the knowledge yet posted as if he had.

BTW...a few years ago I made a post regarding Altec speakers and power handling...the same rules apply...here is the original post:

Altec 802 Power handling

Posted by Jim Ziegler (Member # 86) on 07-10-2005, 08:13 PM:

Steve, you are missing the point. I am not questioning speaker placement/quantity/make/model/color etc... What I am saying is that you could have said exactally what you said, without the following phrases - "You clearly don't know what you are doing or what you are talking about." and "Sorry if I sounded cross but your statement in your original post was just plain wrong and stupid. It showed absolutely no understanding of power ratings and sound."

Omitting those two phrases would have taken nothing of the substance away from your post, but would not have made you come across sounding like a self righteous jerk. So what that he was mistaken on an issue? You can say, "Wait a sec, you have something wrong here - here is how all this is supposted to work" and you probably would have gotten some follow up questions not had him get pissed off and leave.

Steve, people will be hesitant to ask questions if they are going to get flamed - which defeats the entire purpose for this forum being here.

Posted by Mark Gulbrandsen (Member # 72) on 07-10-2005, 09:17 PM:

Steve was dead right in ALL of his comments. I see alot of this happen and in my service area and its mainly due to digital systems being installed onto older SVA type "a" capable systems without any additional upgrades when the dts was slapped in place. I think way back to the Jurassic Park days and when DTS was new how many of those systems never got any upgrade..... back then many cinema owners were just way too cheap to upgrade amps/speakers and many techs were not qualified to spec system upgrades anyway. Still I alert the customer to this problem and get ignored even though I sell and repair alot of drivers.....

Mark

Posted by Brad Miller (Member # 2) on 07-10-2005, 09:19 PM:

Jim, as owner of this board I believe you have said more than enough. These forums aren't here to play happy talk to a bunch of kiddies and more importantly, you are not their father. If Steve's wording was too harsh for you to handle (or your belief that others can't handle it), then you may leave or keep your comments to yourself. In the future if you have a problem with something on the board, you should bring it to my attention and that marks the end of your involvement. You are not a moderator and have no business telling other people how to post. Posting this sort of crap again WILL get you removed.

Steve's posts were completely fair and accurate...every single sentence. The only "flaming" was due to flat out incorrect information attempted to be passed off as fact, which does this forum a TERRIBLE disservice. This is no different than the guy "with 25 years experience" who thought all films should be framed to where the frameline was just barely hidden above the edge of the top masking...and that is 100% flat out dead wrong. Steve didn't just respond with how many subwoofers that size room needed. He actually took the time out of his day to EXPLAIN how and why he got to that number. That's quite admirable, as most of us don't have tons of spare time to be writing out extended length posts to try and educate others on a regular basis, so knock off the Steve flaming.

Posted by Bobby Henderson (Member # 840) on 07-10-2005, 09:58 PM:

quote: John A. Parker Jr.
If you want to gripe at the original installer of the system, call Frank Worthington of Goldsboro, N. C. as that is who designed and installed this system in 1999.

quote: Brad Miller
Stupid was a poor choice of terminology. The proper word would have been ignorant (look it up if you think those two words mean the same thing).

I think the real term to apply to that sound system design is "cheap."

The auditorium John Parker described at his theater is larger than the two houses at the Carmike 8 in my town that were originally built for THX certification. Those auditoriums originally had just over 200 seats each. Each house has four JBL subwoofer enclosures. Just one subwoofer enclosure for a 239 seat theater, likely with stadium seating, is being really cheap.

It's just another example of how a great deal of movie theaters are installing (and later not maintaining) weak sound systems. Sure, they'll put a Dolby Digital capable cinema processor in the rack or install a DTS unit. But do they put in the sound systems beefy enough to do the job? For the most part no. It seems like the digital sound processor is just there to get the "digital" moniker onto the marquee and in the stack ads. Funny thing is most of the marquees and ads don't feature that stuff anymore either.

Posted by Randy Stankey (Member # 64) on 07-11-2005, 01:10 AM:

I've noticed that, sometimes, peoples' replies can be a little curt. Frankly, most of the time I just think to my self, "F*** him!", then let it roll off.

Sometimes I'll reply by saying, "I thought...", then wait for the explanation. By that time, I've probably gained about 10 times more information than I could have dreamed of learning in the first place.

By that time, if I still don't like the guy's tone, I just go back to the F.U. thing. You know the saying: "If they can't take a fuck, JOKE EM'!" [Big Grin]

Back to the topic:

I can empathize with the people whose sound systems are under gunned and over worked. I have to deal with a real "Frankenstein" of a sound system! I spend all my time trying to make my shows as good as possible only to have a crappy sound system make me feel like I'm shoveling shit!

I'm shooting a 31 foot wide screen with a 2000 watt lamp from 109 feet away. The picture is just about as bright and clear as what the local Cinemark does with the same size screen and half the distance and twice the wattage. (I'd like to see it a LITTLE bit brighter... Maybe I can squeeze a few more photons out of it... Someday.)

But, anyway, the picture is as good as I know how to make it. Then we play some British costume drama where 2/3 of the movie's plot is in the dialogue and they all speak with heavy accents. It's like trying to pull hen's teeth to convince people to even THINK about fixing the sound system!

Some nights I go home feeling like I want to spit bullets!

Posted by Frank Angel (Member # 248) on 07-11-2005, 08:43 AM:

quote: Steve Guttag
Many people in this biz are weak in the higher mathematics department. To calculate power is not that hard but it does require working with logarithms and anti-logarithms.

I say screw the math. You simply can't have too many subwoofers. [Big Grin]

quote: Steve Guttag
(then again, there are those that continue to figure that the throw factors in there).

Funny you should mention that -- my General Manager and I just had this argument a week ago. He insists that the greater the throw, the more light is needed. I keep telling him, if the lens speed stays the same and the image size stays the same (by virtue of the correct local length lens), then the light intensity that produced nFtL at that given image size when the projector was 50' way will produce the same nFtL at 75' or 100' or 150' for that same image size, assuming a controlled indoor environement, not an outdoor DI where atmospheric light attenuation.

He refuses to believe me. I think it is because most people's experience comes from unscientific observation, where when they move a lamp closer to a surface, the reflected light on that surface gets brighter; move the lamp away and it gets less bright -- bring a flashlight close to the wall, the light gets brighter & vise a versa. Of course this observation is missing the lens AND the size of the lighted area is not remaining constant. Yet it seems that this practical experience or "gut feeling" is one powerful thing to overcome, even when facts to show otherwise.

Posted by Brad Miller (Member # 2) on 07-11-2005, 02:39 PM:

Frank, use that to your advantage. The next time your GM wants to do an outdoor show with 4K lamps, remind him that you must have new equipment with 7K lamps because of that whole throw issue. [Wink]

Just like it is a good thing to have extra subwoofers, having extra light to spare is also a good thing.

Posted by Steve Guttag (Member # 268) on 07-11-2005, 09:22 PM:

quote: Bobby Henderson
I think the real term to apply to that sound system design is "cheap."

I didn't respond to John's remark about Frank Worthington but probably should have. The problem with "blaming" him is, 1, he did not post and claim anything and 2, we do not know the circumstances that lead to someone deciding on the current set up. Sure, it could have been due to cheapness, as you say. But did Frank recommend more sub/amp and the customer said "no way!"? Did Frank, knowing his customer, only spec a single 18" sub (one of the most powerful) so as not to lose a sale and "please" his customer. Or did Frank tell his customer that this set up was proper and all he needed...the customer trusting his consultant (Frank) just did what was recommended by a trusted contractor. It could have been a combination of the above too.

Unless one knows what transpired, one can not assign blame (or flame) to anyone.

quote: Bobby Henderson
But do they put in the sound systems beefy enough to do the job?

The problem there, I've found, comes in two forms...one, the exhibitor doesn't want to spend the money and just wants to boast "digital", as you say. The other, and seemingly most prevelent is a complete lack of understanding of the science of sound systems.

When selecting lenses, they go to one of the "slide rule" devices or the venerable Schneider Lens Calculator (many Kudos to Dwight Lindsey and the people of Schneider for developing this valuable tool and GIVING it away). People don't just order an 80mm lens because it is a 200-seat theatre. You'd laugh at someone that ordered lenses just based on seat count.

As mentioned...people tend to have their formulas on xenon lamp size versus screen size. I did know one guy that speced 2000-watt lamps for 35mm and 4000-watt lamps for 70mm. It may be a rule of thumb for him, but it makes no sense in reality.

When it comes to sound systems what is the exhibitor to do? Speaker manufacturers don't make it easy either. They classify their speakers in terms of generic room sizes (Large, Medium and Small)...sometimes they assign seat ranges. Then how is one to know how big an amp to use?

Then there is the fact that sound moves in a logarithmic way...it isn't intuitive to most. The room is only 20-feet longer but it takes much more power to amplify. 3-dB doesn't sound that much louder but DAMN it takes twice as much power...and what is this dB business? there are suffixes and it seems to change depending on contex. Digital added 12dB to the output requirements of the stage channels and full bandwidth to the surrounds. The subwoofer has to deal with 22dB more than analog and most didn't have subwoofers in the Dolby-A era. 12dB means that if you needed 100-watts in the analog era, you now need 1,585 watts for digital. It is a huge change. A JBL 4675 speaker in a 65-foot room only running Good old Dolby-A could get away with about 22-watts. Many people ran on Kelmar mono amps that had 50-watts or 25-watts for many years in theatres without blowing things out. Dolby SR added another 3dB to the mix so stage channels needed 94dB. 50-watts still did you in a small theatre. There wasn't much head room there but it could work. I remember the QSC 1400 starting to go in just about everywhere around here. It was 200-watts a channel, reasonably priced 3U tall and worked for just about every room. Take that same room and install digital and the power needed jumps to 348-watts. Whoops, the good old QSC 1400 doesn't cut it anymore, except bridged. 348-watts seems rather massive...especially to people that used less than 100-watts even in the old barn sized theatres. But where are the rule of thumbs? What is layperson to do?

About the only publication I've seen that tried to explain this to exhibitors/technicians was Dolby's little known "Technical Guidelines for Dolby Stereo Theatres." It didn't even have digital in its name. There were two "draft" publications. The red cover version in 1992 and the gray cover version in 1994. In this document there are the math formulas for those that want work their calculators (or slide rules if you are game) and there are "nomographs" that allow one to look at the distance, the sensitivity of the speaker and then figure the wattage needed. Dolby even listed the popular models of speakers (in the three main categories of Stage, surround, and subwoofer). The subwoofer nomograph is flawed in that it only has one figure on 110dB instead of 113dB.

The rational, as I recall (Sam Chavez would know better since he was with Dolby at the time) was two fold. Room gain due to the reverberation of the bass bouncing around the room would shave some needed power and also that the sheer size of the power was just so damn huge it would seem cooky to the average technician/exhibitor.

That aside...the Dolby document is the single best one I've come across for exhibitors/consultants to use to ensure a reasonably well designed system. How many Film-Techers have seen it or read it or use it? From what I can tell it is one of the best kept secrets and wasn't updated beyond 1994 (Too bad).

It is also too bad that no one ever made the "Speaker slide rule" like the various lens calculator slide rules...it probably would have done a lot to allow people to choose the right amount of speaker and power. Perhaps a manufacturer out there could make one. I have worked on an Excel spread sheet that will do the math but it is very primitive at the moment.

quote: Randy Stankey
The picture is just about as bright and clear as what the local Cinemark does with the same size screen and half the distance and twice the wattage.

Randy? Are you baiting me on this one? Do you really think there is a relationship between throw and light for a projection system?

Posted by Gordon McLeod (Member # 33) on 07-11-2005, 09:44 PM:

Steve said "It is also too bad that no one ever made the "Speaker slide rule" like the various lens calculator slide rules..."
actually JBL made one

Posted by Steve Guttag (Member # 268) on 07-11-2005, 09:46 PM:

Got one of those Gordon? I'm just a bit curious there! They would be the type of company to make such a thing...they have speakers AND amps to sell!

Posted by Gordon McLeod (Member # 33) on 07-11-2005, 10:00 PM:

Yup it lists the MI line name and then sensitivity
Also Smart has a software calculator for download [Smile]

Posted by Randy Stankey (Member # 64) on 07-11-2005, 11:01 PM:

quote: Steve Guttag
Randy? Are you baiting me on this one? Do you really think there is a relationship between throw and light for a projection system?

No, not really.

Yes. The brightness of a picture has nothing to do with the distance between the screen and the projector. However, if the screen is the same size but a longer distance away you'll need a different lens. It's that difference in the lens that might change the brightness of the picture. (ƒ-number)

Help me out for a second on this question: Is it not possible that increased throw works FOR me in this case? Since the screen is the same size, a lens with a longer focal length is needed. That lens will have a lower maginification factor. And, less magnification produces a clearer picture. (Clearer? Sharper? What?)

So, talking about throw IS useful. It's just not useful in the way people think it is.

At a drive-in, increased throw means that there is more dust and fog in the air between the screen and the projector to scatter light. No?

Posted by Bobby Henderson (Member # 840) on 07-11-2005, 11:51 PM:

quote: Steve Guttag
Unless one knows what transpired, one can not assign blame (or flame) to anyone.

I didn't mean to specifically flame Frank Worthington, I just put up the quote for context to let people know what I was blabbing about. The truth very well may be as you say: that the customer probably chose something that was thought to be good enough.

Still, overall, the sound system does come off as being cheap with having that little of subwoofer power.

quote: Steve Guttag
That aside...the Dolby document is the single best one I've come across for exhibitors/consultants to use to ensure a reasonably well designed system. How many Film-Techers have seen it or read it or use it?

I'd like to see it. I may not be able to fathom all of the math in it, but it would still be interesting to view. Would it be possible to post that kind of document on Film-Tech, or at least get Dolby to display it on their website?

Some of this discussion makes me a bit sad to see how the intention of THX has been arguably marginalized over the years. Has THX required more careful speaker system and amplifer combinations according to specific room size, height and shape? I've been under the impression they did. At least it would seem that way in respect for subwoofer use with digital sound -coming from my own personal observations with how THX auditoriums seem to have a lot more of what I call "beef" happening in them than non-THX houses.

Posted by Steve Guttag (Member # 268) on 07-12-2005, 01:34 PM:

quote: Randy Stankey
Help me out for a second on this question: Is it not possible that increased throw works FOR me in this case? Since the screen is the same size, a lens with a longer focal length is needed. That lens will have a lower maginification factor. And, less magnification produces a clearer picture. (Clearer? Sharper? What?)

You can't really draw any conclusions there. There are too many variables once one changes focal length. Not all lenses are created equal, even within a model line. When one is talking about a significant distance difference..say between 60 and 120-foot throws...one is almost always talking about a different lens design entirely.

For instance...if you had a 16' tall screen...for scope at 60-feet away, you would use a 65mm lens. From ISCO, that would be an Ultra Star PLUS (from Schneider a Super Cinelux)...if you were 120-feet away it would be part of the "Ultra" line from each manuafactuer. A much older lens design. The new short lens may be inherently sharper and collect more light than the longer lens. Then again, the longer lens might be more efficient due to its design.

Next, the screen comes into play. How much does it really behave like a matte-white screen (essentially 180-degree light scatter). Many "matte-white" screens are not matte-white in nature...I've measured gains as much as 20% and still termed "matte-white". Once directivity comes into play...the point of projection has an effect on how the light reflects off the screen. The longer throw will have the light come back towards the audience better, the shorter throw will have the light reflect off to the side walls giving the appearance of a hot-spot.

However, all things being equal (perfect screen, identical lens except for focal length, no significant dust particulate)...then no; projection throw has zero effect on brightness.

Posted by Pete Naples (Member # 761) on 07-12-2005, 04:57 PM:

Dolby UK do or did give the publication away to those who attended their training courses, I have a coupel of copies and it comes in handy sometimes, often as not to be able to quoet chapter and verse to an architect or consultant who ought to know these things but invariably don't.

Posted by Brad Miller (Member # 2) on 07-12-2005, 05:32 PM:

Barry Ferrell from QSC has been nice enough to send in a powerpoint presentation regarding the specification of amplifier power and quantity of woofers. It will be on the front page at midnight tonight.

Posted by Andy Summers (Member # 3207) on 07-12-2005, 09:25 PM:

Hallo there all

Thank you so very much for inviting me to the forums Brad, however I have been unemployed from the game foe awhile and I can’t get my foot in the door and get back on the horse, anyway please bear with my spelling will you I’m not word perfect nor will I ever be.

Blow sub bass, eek not good there is another possibility the fact that motion picture films today are getting louder in there potential usage of there dynamic ranges, as for the JBL 4645 the Jedi blaster Lightsaber as I call her she’s sat on her side in my home cinema alone with smaller three-screen JBL control 5, as for the JBL 4645, you mentioned early Brad, about one could do the task in a cinema, and is this without any strain what so ever put on her, as its my total understanding that the more the better and with some custom made design implemented into this program and the projectionist advised on this as must don’t care a least bit about the sound side.

As one projectionist said to me many years ago you’re the only one that I know, that knows what to look for and listen for.

“Event Horizon” 1997 I remember this playing at the MGM cinema as it was called back in 1997 which is now under the ABC name and in partnership with Odeon Uk cinemas, anyway this loud popping sound was the sine of something’s wrong very wrong hear is came in during the LFE sub bass tracks, and to me it stuck out like a sore thumb and for years the ABC just let this carry on giving the cinema experience a let down, and one sub bass is just not going to cut the ice.

The peaks of popping are within the 25Hz and 40Hz ranges mostly, and maybe it was down to poor technical engineering that was rushed and not tested out properly, and the amplifier is under powered as well, what I think it really comes down to is budget and all the money had been spent on the Dolby DA 20 Dolby Digital decoder which is inline with the classic Dolby CP-200.

So I think its down to experience not ignorance Brad its not his fault we are only human and life is short, all I can say it was just bad luck, I read on the in-70mm life with THX that the HF gave up the ghost on the film “Eraser” and also I remember reading an article on home THX that someday the amplifier is going to clip and give up the ghost, it like what NASA says a rocket has to explode at some point it’s the odds on it, though that may sound crawl it the same that can be said about cinema and the home cinema market something as to give sooner or later.

However with my home THX I always do a rehearsal before playing to friends, and sometimes making customized EQ adjustments if the mix is to bright sound on the mid highs and high frequencies, but that is very rare, anyway with the 7 years I’ve been running the JBL 4645 with a different sub bass driver with edge ribbon surround and not the edge foam surround, which will give in time as you may know, and is a pain in the a$$ to put right and sticky too, if you get my meaning, but doable.

Going back to the ABC I saw “War of the Worlds” there last week and may go and see and hear it again this time I’ll remember to take the SPL with me as it’s a question of science, those tripods are loud and mean, I guess they looked at the sub bass with envious eyes and slowly and surely drew their plans against the JBL 4645 KABOOM, sorry my poor attempt in humour.

Though the ABC screen #1 sub bass was taken out with the Matrix which does not surprise me at all, the JBL 4675-A are still intact and sound fantastic for there age the JBL 2360-A HF sensitivity is around 130db if I’m not totally mistaken the edge it as is good the front row gives around 105dbc weighting though I believe she can reach 110dbc weighting at best.

Anyway that’s that I sound worse than a Hollywood actor picking up his award.

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[ 07-13-2005, 01:08 AM: Message edited by: Brad Miller ]

Posted by Steve Guttag (Member # 268) on 07-12-2005, 09:43 PM:

The JBL 2360A horn only has a sensitivity of 113dB, 1-watt@1-meter.

Posted by Andy Summers (Member # 3207) on 07-12-2005, 10:00 PM:

Hallo there all again back for a second Oscar!

Steve Guttag

I remember reading that article and I can’t find it, it was regarding Tomlinson Holman and a comment he made in the interview that Dolby laboratories require that spot frequencies test are done at different Sound Pressure levels the highest was 110dbc weighting, knowing me I’ll came across this in the flat tomorrow there’s dozens of articles surround me, so not sure if you know which person at Dolby stressed the need for this usage in the cinema, the test was done one the three & five-screen channels…

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[ 07-13-2005, 01:08 AM: Message edited by: Brad Miller ]

Posted by Brad Miller (Member # 2) on 07-13-2005, 08:10 PM:

Special thanks to Ioan Allen and Ken Jacquart from Dolby for providing the document Steve Guttag has been referring to in this thread "Dolby Stereo Technical Guidelines for Dolby Stereo Theaters".

It will be available for download at midnight tonight.