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Author Topic: Stage speaker strange noise Muhammad Husni Film Handler Posts: 6 From: Petaling Jaya, Malaysia Registered: Jul 2007 posted 05-04-2008 08:45 AM                     Hello, there were actually 3 different matter that I would like to ask. 1. There were humming sound on the stage speaker. What could be the actual cause and how to overcome it? 2. Hall #1 is now showing a movie and the output can be heard on stage speaker of hall #6. The distance is about 3-4 halls and the sound can be heard clearly, only the level is a bit low. How can this happen? 3. While running a DTS trailer in SR mode, there were popping sound on the subwoofer. But if we were running it on a DTS mode, there's no such sound. These were the details of the sound system used for reference: Processor: CP500 Amplifier: QSC MX 1500A, MX 3000A Speaker : JBL, KCS (ALL WERE USING 2-WAY SPEAKER) Hopefully U guys can help me on that. Regards, Husni  |  IP: Logged Mike Olpin Chop Chop! Posts: 1852 From: Dallas, TX Registered: Jan 2002 posted 05-04-2008 09:12 AM                     quote: Muhammad Husni 1. There were humming sound on the stage speaker. What could be the actual cause and how to overcome it? This is probably a ground loop. It tells you that one of the components is not properly grounded. Make sure all equipment is plugged into a 3-prong outlet and no three-to-two prong adapters are being used. If you only hear it during the non sync, you can purchase an inexpensive RCA ground loop isolator. quote: Muhammad Husni 2. Hall #1 is now showing a movie and the output can be heard on stage speaker of hall #6. The distance is about 3-4 halls and the sound can be heard clearly, only the level is a bit low. How can this happen? That is very strange indeed, and if true, would indicate an epic wiring problem. But I have seen this illusion occur if two theatres are positioned directly across from one another. The sound passes through the door of the auditorium, across the hall, then into the door of the theatre across from it. The way the sound funnels in and then reflects of the screen really makes it sound as though a faint bit of the other theatres audio is coming through the speakers even though it is not. Theres an easy fix for this, just close the auditorium doors. quote: Muhammad Husni 3. While running a DTS trailer in SR mode, there were popping sound on the subwoofer. But if we were running it on a DTS mode, there's no such sound. DTS is a digital format, and not subject to much extra noise beyond what is present in the soundtrack. SR is analog, and is subject to noise, especially if the print is damaged or warn out, contains improper splices (make sure your splices aren't leaving a small gap) and alignment issues. Make sure you clean your sound head with a dry q-tip periodically and have your technician check its alignment.  |  IP: Logged Muhammad Husni Film Handler Posts: 6 From: Petaling Jaya, Malaysia Registered: Jul 2007 posted 05-04-2008 09:24 AM                     Thanx for the #1 answer. But the #2 answer is quite funny. Actually the hall is quite far from each other. Even the sound from hall #1 can't be heard from hall #2 which was only beside it. And the sound comes from the center, right low frequency and on the left hi frequency. I've checked it few times to confirmed it. For #3, its not about the film or sound track or the splices. And DTS is quite different from digital, if I;m not mistaken. DTS is 6-channel output, am I right? SR a.k.a. analog has only 2-channel (Lt,Rt). So, I'm still puzzled how does the popping sound came out. Its not all the way from the start till the end of the trailers, but only when the CP500 LCD indicates that the subwoofer level almost hit the +20 level. Hope u guys get a clearer picture regarding these matter. Husni.  |  IP: Logged Sean McKinnon Phenomenal Film Handler Posts: 1712 From: Peabody Massachusetts Registered: Sep 2000 posted 05-04-2008 09:55 AM                        Sounds to me like the issue with the sound from one house playing in another that you have a case of "crossed wires" or signal being interjected somewhere. Could possibly be the non-synce wiring. Do you have a video set-up? If you have one DVD player in a rack that can be played in multiple houses the signal could have gotten in there somehow, or maybe through the speaker lines in the walls, or ceilings? I think you are going to need to have a good tech come out for this one. There is no way to get enough information about this problem on here.  |  IP: Logged Muhammad Husni Film Handler Posts: 6 From: Petaling Jaya, Malaysia Registered: Jul 2007 posted 05-04-2008 10:11 AM                     Thanx, sean. But for this cinema there is no DVD or any playback that can play on multiple halls. Actually I'm the tech that suppose to handle this problem and till now I still can't figure out how's this happened.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 05-04-2008 11:52 AM                     Hi, Husni, welcome to Film-Tech! I'd suggest you'd be better off posting seperate questions in seperate threads (maybe not all on the same day!), otherwise some of your questions may get lost amidst the discussion of the others. 1. Hum can have many causes. Grounding problems can be one, but there can be others. Usually it is because unbalanced wiring is not robust over long distances, and most equipment uses unbalanced wiring. The absolute best answer is to convert your unbalanced signals to balanced signals, ideally with a transformer. How far are your amps from your processor? Does the hum happen when the processor is off? Does it happen when you change sources? Does it happen if you turn off your analog sound power supply? 2. This is definitely weird. If the wiring runs go nowhere near each other, and it's not crosstalk, when perhaps you are having some kind of RF-related problem. Are you near a powerful radio station transmitter? In any case, you should debug the problem scientifically. Which channel of Hall 1 is causing the problem? Can you hear the problem with Pink Noise? Is it frequency dependant? Is the signal originating before the amplifiers or after them? Turn off the amplifiers in Hall 1 and see if the sound continues to travel to Hall 6. Where in Hall 6 is the sound being picked up? Is it in the cinema processor? Is it between the processor and the amps? Is it between the amps and the speakers? Does it happen in non-sync or just in film? In DTS as well as SR? Turn off and disconnect individual components in the system until you isolate the source. Then report back and we can hopefully give you some better recommendations on where to procede. You mention the sound appears in the Center (LF *and* HF sections?) the Right LF and the Left HF. Tell us more about your crossovers. Also, are these problems recent? Did they start when other changes were made? Maybe there is a hint as to what is wrong based on other factors. 3. DTS is absolutely a digital format. When we talk about "digital," we mean many things, but usually we mean any of Dolby Digital (aka SRD), Digital Theatre Systems (DTS), or Sony Dynamic Digital Sound (SDDS) -- three ways of encoding film sound without using the analog optical track of the film. But the point really is that SR and DTS use a different signal path for sound to get to the processor. So if you have a problem with noise in the subwoofer in SR, it tends to suggest that the problem is not with the amplifier or the subwoofer itself, but in the cinema processor or beforehand. But it's strange that you would see analog problems manifest only in the subwoofer, and not in the other channels, because as you correctly point out, SR is not discrete. It is misleading to say that SR is only two channels. There are two channels encoded on the film, but your cinema processor decodes those channels into 4.1: left, center, right, mono surround, and a subwoofer channel. Are you sure that your subwoofer is actually working in DTS? Do you hear the problem if you play the DTS Empiral Test CD? Do you hear it with pink noise on the subwoofer channel? --jhawk  |  IP: Logged Paul Mayer Oh get out of it Melvin, before it pulls you under! Posts: 3836 From: Albuquerque, NM Registered: Feb 2000 posted 05-04-2008 12:57 PM                        Regarding the sound showing up in another house, we had an issue like that at a newly built theater here. Turned out to be a improperly constructed common return air duct in the air conditioning. The return air registers were at the back of the affected houses, and I swear it sounded like the adjacent house sound was getting into the rear surrounds. Took quite a bit of extra sheet metal work on the part of the building contractor to make that right. If your return air was built like this and the registers are over the screen perhaps it could sound like adjacent sound getting into your stage speakers.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 05-04-2008 11:17 PM                       I think John is on the right track regarding how to isolate the audio from one screen ending up on another. I would guess it's electrical crosstalk and not acoustic leakage. One electrical path can be through the wiring of a common intermission source feeding multiple screens. I've also seen places where two screen's sound systems were built into one rack. Almost always leaks back and forth.  |  IP: Logged Danial Simmonds Expert Film Handler Posts: 107 From: Kota Damansara, Selangor, Malaysia Registered: Jan 2008 posted 05-06-2008 09:33 PM                        Could it be due to improper grounding that causes the buzzing sound from the speakers?  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 05-06-2008 11:57 PM                        For question #3, what type of projector are you running? I know Simplexes (some people simply call them "Strong" now) can emit a thumping noise in analog if the sound gate is left closed overnight and the little rubber thing develops a flat spot (I highly disapprove of that part, by the way... it is a horrible design). It works itself out after a small amount of time, but it's always a good idea to open those things up when the show is not running. Not matter what, something is definitely wrong with your optical A-chain.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 05-07-2008 12:14 AM                       Figure out which problem is the most important and chase it one at a time. If the main problem is the hum, disconnect the audio input to the power amp. of the channel that is humming and work backward or forward from there. For instance, if the hum stops when you disconnect the amp input, go back a stage, etc. Figure out where the hum starts as in DTS, or soundtrack reader. You need to do some sequential analysis here or you will never be a good technician. Asking whether poor grounding could be the cause does not show much insight. Of course, poor grounding could be a problem, much like a flat tire could be a problem to driving a car down the road. The final word is concentrate and focus on one problem at a time.  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 05-07-2008 01:12 AM                        Off topic: Hey Sam, I like your Dolby Digital readers.  |  IP: Logged

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