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Author Topic: Subwoofer requirements Jeremy Weigel Phenomenal Film Handler Posts: 1062 From: Edmond, OK, USA Registered: Mar 2007 posted 01-10-2011 07:06 PM                     I have researched many threads on the forums as well as read the QSC power point in the manual section and have come to the conclusion that we may be under spec'd on subs. The following are the auditorium lengths and the current equipment in place. Aud# 1 66ft 2- Smart single 18" 1-QSC MX2000a 2 60ft 2- JBL 4645 1- QSC RMX 1450 3 54ft 2- Smart single 18" 1-QSC MX1500a 4 62ft 2- Smart single 18" 1-QSC MX1500a 5 62ft 1- JBL 4645 & 1- 4645B 1- QSC MX1500a 6 72ft 1- JBL 4642A 1- QSC RMX 1450 7 80ft 1- JBL 4642A 1- QSC RMX 1450 8 88ft 2- JBL 4645B 1- QSC MX1500a All subs are parallel with amps in bridge mode. One forum thread suggested that the Smart subs are 400w, but I can not confirm this. Do any of the auditoriums meet spec? On the ones that don't, what would be your suggestion(s) on regrouping subs and/or adding more units as well as grouping/replacing amps?  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 01-10-2011 07:32 PM                        At the very least, single subs should be doubled up (or more.) Louis  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 01-10-2011 08:41 PM                     the only way is to actually do the math for the room size/amp powert/ and SPL of the speaker against the desired SPL that is the only "good" engineering practice  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-10-2011 09:12 PM                     Theatre #1: Needs 3600-watts...has about 800-1000 watts of capacity and not enough amp to achieve needed power. Theatre #2: Needs 4700-watts and has 700-watts of capacity and also does not have enough amp. Theatre #3: Needs 2400-watts...has 800-1000 watts of capacity and not enough amp to achieve needed power (see a trend here). Theatre #4: Needs 3200-watts...etc. Theatre #5: Needs about 5000-watts as the 4645 is 3-dB less sensitive and also has about 350-watts of power capacity in a cinema subwoofer mode...it will explode long before the 4645B...you are mixing non like sensitivities, frequency response and power handling. Theatre #6: Need 3400-watts and have 1200-watts of power handling capability and not enough amp. Theatre #7: Need 4200-watts and have 1200-watts of power handling capability and not enough amp. Theatre #8: Need 6400-watts and have 1600-watts of power handling and not enough amp. So yeah...you don't have enough speaker or amplifier to play the system at reference (85dBc = -20dBFS). -Steve  |  IP: Logged Jake Spell Master Film Handler Posts: 294 From: Johns Island SC Registered: May 2009 posted 01-10-2011 09:46 PM                     So how could you get the required amount with only the auditorium lengths? I always thought that you had to get a volume of the space for that, but I could be wrong.  |  IP: Logged Jeremy Weigel Phenomenal Film Handler Posts: 1062 From: Edmond, OK, USA Registered: Mar 2007 posted 01-11-2011 04:02 AM                     Steve, What would be the best reconfiguration of the subs/amps already on hand and then acquiring new subs and amps to re-equip the sub-less houses? For example: If I group all 6 (set in 3 pairs paralleled) Smart subs together powered by 3 MX1500a's bridged; would this be enough to meet spec on one the houses? Or better yet what would spec these houses at if you were starting from scratch?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-11-2011 06:27 AM                     Jake...it is just the length of the room and the sensitivity of the subwoofer that is needed to determine power required. What do you care if the ceiling is getting bass information? With bass you are also not going to be getting as much room build up at the lowest frequencies as they are going to cut through the boundaries rather than reflect around them. As such, it is a simple matter of what the direct path is between the speaker and the listener. At some nodes in the mid/upper bass, there will be a build up due to room construction...and hence why most processors have at least one or two parametric EQs to knock that down (not to build any frequencies up). Jeremy, I'll look into that when I have some more time. -Steve  |  IP: Logged Jeremy Weigel Phenomenal Film Handler Posts: 1062 From: Edmond, OK, USA Registered: Mar 2007 posted 01-11-2011 01:31 PM                     Thanks Steve. I look forward to your further input. I have already advised the theatre owners that we are under spec'd and they open to the idea of bringing them up to spec. I just want to have really good information to put together estimated costs. I know for sure that more wire will need to be pulled. There are only 4 pairs pulled.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 01-11-2011 05:33 PM                     Jeremy If you look through the forum you will find all the explanations and you can create a simple excel file to calculate the power needed depending on the size of your room and the speakers used.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 01-12-2011 10:53 AM                        Simply put: more is better. Too much can be reduced. Louis  |  IP: Logged System Notices Forum Watchdog / Soup Nazi Posts: 215 Registered: Apr 2004 posted 01-15-2016 09:36 PM                 It has been 1829 days since the last post.  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 01-15-2016 09:36 PM                     Steve, What formula did you use to calculate the power requirements above?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-16-2016 03:54 PM                     My own (honest answer) however it is based on published principles of sound. I suggest downloading the Dolby Technical manual from the warehouse. It has nomographs in it that will let one figure out the power requirements based on speaker sensitivity and length of room. Note, it is based on Dolby digital's 103dBc Max, not the current 105dB and, as I recall, the subwoofer level is fudged by another 3dB or so. http://www.film-tech.com/warehouse/manuals/DOLBYTG1994.pdf If you look on the printed page 18 (not the PDF page...which is 21), you'll see a formula that will get you started. Note, when you are working with sound, you have to be comfortable with logarithms.  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 01-16-2016 04:03 PM                     Steve, Yes I am very familiar with that field bulletin. Since the requirements have changed, would doubling the subwoofer requirements shown on the graph be fairly accurate?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-17-2016 01:42 PM                     I'd say it will get you in the ballpark. Doubling power is 3dB so that will cover the potential extra power between Dolby Digital and DCinema (2dB difference there) and there is the fudge factor in there. Then there is the reality check, do you set your theatres to spec (85dBc from the main channels with pink noise at -20dBFS)? And if so, do you run your fader at 7.0 (or 0.0dB)? If the answer is no to either question, then you already are lowering the requirements some.  |  IP: Logged

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