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Author Topic: speaker distance behind screen Matt Hale Expert Film Handler Posts: 123 From: Vancouver, Canada Registered: Dec 2002 posted 06-20-2003 12:17 PM                        In the construction of our new mixing stage, a question has arisen that I have no answer for. How far back from the screen should the speakers be? We are using JBL 4675C-8LFs, the same as the Film Tech screening room. According to the description with the photos, the horns are only about 1 inch back, but this was caused by a mistake with the placement of the door (or something like that) So what is the ideal? Do our friends at THX have an opinion on this? I have not been able to find anything in the documentation. Thanks  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 06-20-2003 12:38 PM                        Check out the JBL Sound System manual: http://www.jblpro.com/pub/cinema/cinedsgn.pdf On page 32, they specify horn flanges should be 5 to 7 centimetres from the screen.  |  IP: Logged Paul Mayer Oh get out of it Melvin, before it pulls you under! Posts: 3836 From: Albuquerque, NM Registered: Feb 2000 posted 06-20-2003 12:50 PM                         |  IP: Logged Bob Koch Expert Film Handler Posts: 183 From: williams ca Registered: Nov 2001 posted 06-20-2003 01:32 PM                     Some things never change and remain correct even before THX. Speaker systems should be as close as possible, without touching the sheet.  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 06-20-2003 01:36 PM                        Bob Koch wrote: quote: Speaker systems should be as close as possible, without touching the sheet. I guess allowing for some "sag" in the screen material, the 5 - 7 centimetres specified by JBL is in line with that practice. You don't want the speaker to ever actually touch the screen.  |  IP: Logged Matt Hale Expert Film Handler Posts: 123 From: Vancouver, Canada Registered: Dec 2002 posted 06-20-2003 06:29 PM                        Thanks, that is exactly what I wanted to know. I assume the woofer can be a bit farther back than the horn - screen attenuation not being such an issue at lower frequencies - and besides, the horn protrudes beyond the front of the LF cabinet by quite a bit anyway.  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 06-21-2003 05:14 AM                        Yes, but remember that the horn driver is actually much farther back than the woofer in the cabinet. You have to take that in account when you design a sound system. If you have a 2 or 3 way system that can cause the contributions of the woofer and tweeter be out of phase.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 06-21-2003 03:39 PM                     If you biamp then use a time corrected crossover (guess what THX makes one) a(as does Smart Ultra Stereo QSC) the driver offset is set to match the delay at that frequency  |  IP: Logged Philippe Laude Film Handler Posts: 79 From: Longueville, Belgium Registered: Jul 99 posted 06-23-2003 04:07 AM                     Thank you for posting, Paul, your answer clarifies the situation Philippe  |  IP: Logged Paul Mayer Oh get out of it Melvin, before it pulls you under! Posts: 3836 From: Albuquerque, NM Registered: Feb 2000 posted 06-23-2003 12:05 PM                        Clear as crystal isn't it? I did say something there but then decided to delete it. Something about a John Eargle paper and demonstration I attended at a joint AES/SMPTE meeting in 1996 about the five new JBL tri-amped screen channels for the AMPAS Samuel Goldwyn Theater. Something about graphs in said paper that showed increasing combing and diffraction effects with increasing speaker to screen distance. But I deleted the post because I couldn't remember sufficient details to make the post worthwhile...  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 06-30-2003 11:51 AM                        Has anyone heard of the problem of the acoustic output of the high powered sub-bass bins behind the screen actually causing the sheet to vibrate and make the picture look out-of-focus while the sound is pumping? Frank  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 06-30-2003 12:48 PM                        The screen usually vibrates when you play really loud stuff. I never noticed that until I ran the self-playing dts demo disc without the film that belongs to it ("Buzz and Bill").  |  IP: Logged Robert Throop Master Film Handler Posts: 412 From: Vernon, NY USA Registered: Jun 99 posted 06-30-2003 01:14 PM                     In Imax Dome theatres (at least the one in Syracuse) the speakers are several feet from the dome. Sonics doesn't think this is a problem.  |  IP: Logged Adam Martin I'm not even gonna point out the irony. Posts: 3686 From: Dallas, TX Registered: Nov 2000 posted 06-30-2003 08:55 PM                     Due to the nature of the Imax theater geometry (both flat screen and dome) it is necessary to actually hang the speaker from the screen tower rather the place the cabinets on platforms. The speakers are aimed at specific places in the auditorium and will sit at awkward angles to the screen, as can be seen in these photos of two different left channel cabinets. Acoustic treatment on the cabinet and behind the screen and equalization of the sound system accommodate for the cabinet placement, as well as the reflection of sound from the vinyl sheet of the flat screen or aluminum panels of the dome screen. And, yes, I've sent ripples across a few sheets in my day. To help prevent this ripple effect, the subwoofers in Imax theaters are placed in pairs facing each other so that the sound is reflected out rather than aimed directly at the screen. Remember that it is more difficult to localize on a low frequency sound, so placement of the subwoofers is not as critical as other speakers.  |  IP: Logged

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