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Speaker systems should be as close as possible, without touching the sheet.

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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.
 

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Posted by Matt Hale (Member # 1518) on 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.
 

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Posted by Michael Schaffer (Member # 1204) on 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.
 

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Posted by Gordon McLeod (Member # 33) on 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
 

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Posted by Philippe Laude (Member # 79) on 06-23-2003, 04:07 AM:
 
Thank you for posting, Paul, your answer clarifies the situation

Philippe
 

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Posted by Paul Mayer (Member # 355) on 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...
 

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Posted by Frank Angel (Member # 248) on 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
 

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Posted by Michael Schaffer (Member # 1204) on 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").
 

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Posted by Robert Throop (Member # 61) on 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.
 

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Posted by Adam Martin (Member # 641) on 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.
 

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