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This topic comprises 3 pages: 1 2 3
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Topic: Blown Subwoofer
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 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]](smile.gif)
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 ]
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John A. Parker Jr.
Film Handler
Posts: 21
From: Brevard, N.C., USA
Registered: May 2003
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posted 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.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 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.
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