Film-Tech Forum ARCHIVE: Speaker Position

This is topic Speaker Position in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Amanda Mundin (Member # 3386) on 11-08-2005, 03:27 PM:

I have a few queries regarding speaker position, how far out (or in) do people position their Left & Right stage speakers with a common height screen?
Is it a good idea to bisect the 1.85 picture edge?
Regarding subwoofers, what are peoples opinions on seperating the subs as in the first picture, would this spread the sub-bass better across the width of the auditorium & provide a less pronounced sub position, or do you think the mutual coupling of the second picture would be a better idea?

Posted by Mike Blakesley (Member # 26) on 11-08-2005, 04:09 PM:

First, you better edit your thread title so it doesn't SHOUT or the moderators will be all over you like a cheap suit.

Second -- ours is like your bottom picture. Our L-R speakers are all the way to the edges of the screen because our auditorium is fairly narrow anyway and I wanted to get the best separation possible. Since most of the L & R material is music/efx, there is no weirdness from the sound coming from "outside" the flat picture that I have ever noticed.

As for the subs - we've never tried them spread apart, but the guy who installed the system recommended them sitting next to each other. Bass being nondirectional, I'm not sure if it'd make much difference but I'm sure the experts here will weigh in.

[ 11-08-2005, 05:13 PM: Message edited by: Mike Blakesley ]

Posted by Richard Fowler (Member # 893) on 11-08-2005, 04:59 PM:

Subwoofers are more efficient coupled ( touching ) cabinet to cabinet. Assuming you are using sound transparent masking material, you final location of L and R horns may be cheated in or out depending if your masking has a hard edge created via pipe or wood that would interfere with the high frequency output when in the 1:85 position.

Posted by Steve Guttag (Member # 268) on 11-08-2005, 05:07 PM:

The lower picture is the better choice for both screen speakers and subwoofers. Note, you can turn the subwoofers on their ends if there is a space issue too. For efficientcy, mutual coupling needs them to be together acting as one big subwoofer. However, do not discount distributed bass...I can amost always localize the source of the subwoofer when there is just one location so as the subwoofer systems grow in size...I will start splitting them up to distribute the bass. One of the better systems I've heard actually had subs all around the room.

Steve

Posted by Monte L Fullmer (Member # 2797) on 11-08-2005, 11:46 PM:

Yea, we have our HF/LF setup just within the 1.85/1 masking edge so not to lose the HF's when playing in flat. Plus, the subs are standing on end next to each other, Yet, I've seen other cinemas with subs on each side underneath the H/L bins.

(yea, edit your title as such ...> *Speaker Position* )

-Monte

Posted by Thomas Dieter (Member # 2507) on 11-09-2005, 01:12 AM:

I've yet to learn this, but what would be the reason for placing the subs off center? Also, would it not be best to have them doubled up, and have it on both sides? Although I can see that being to much power behind the screen. I guess it would just depend on your budget.

I don't know if anyone here went to see Derailed at ShowEast, but there was one part in the movie that literally scared the [bs]

out of me cause of the bass, coupled with the picture. I don't know AMC's traditional setup behind screen, but it sounded like they had subs throughout the entire theatre.

Posted by Michael Schaffer (Member # 1204) on 11-09-2005, 01:34 AM:

This may have been pointed out before, but there is some useful information about aspects of cinema sound system design in JBL's Cinema Sound System Manual which can be found here.

Posted by Sam D. Chavez (Member # 1841) on 11-09-2005, 01:34 AM:

The reason the subs are shown off center is that there is usually a platform holding up the center speaker that is not shown in this drawing.

Posted by Alex Grueneberg (Member # 2652) on 11-09-2005, 03:34 AM:

In my best theater I have the lower picture set-up but with 2 subs (not bad for a 180 seat house!). As unfortunate as it is my 2 JBL 4641's subs are so badly deteriorated on the outer cone and I had to disconnect them. I've requested over and over to have them repaired but my company doesn't seem to care or have the money to get them fixed. As Thomas mentioned above there are those moments in movies when the subs just make the experience. You guys are lucky you have working subs! Worship them!

Posted by David Buckley (Member # 2600) on 11-09-2005, 04:00 AM:

You're almost certainly advised to have your subs together, not only to take advantage of coupling effects, but to avoid the creation of a "power alley"

Posted by Brad Miller (Member # 2) on 11-09-2005, 04:30 AM:

quote:
I have the lower picture set-up but with 2 subs (not bad for a 180 seat house!)

That's still quite possibly not enough for the room. [Wink]

You would have to post the room dimensions and amplifier make/model driving it to calculate.

Posted by Steve Guttag (Member # 268) on 11-09-2005, 05:37 AM:

JBL 4641 with deteriorated surorunds (compliance)? Doesn't sound too likely since they use the 2241 driver with a pleated cloth surround. Perhaps you have the orignal 4645? It used the 2245 driver which does have a foam surround that will rot away over time.

If one really wanted to locate their subwoofers well, then one can obtain the "EASE" program (and it isn't free or cheap) and then use the EASE model of the suwboofer (most speaker manufacturers have EASE modeling parameters of their speakers). It takes the guess work out of a sound system design in that you will KNOW the coverage you have.

We did a complicated surround system in an Historic theatre where the surrounds were not to be visible and with hard surfaced walls...the EASE modeling system allowed us to choose speakers and and placements that were predicted to work before going through the tremedous effort and expense. The sysem performed exactly as predicted and the customer is quite happy with the results.

As to subwoofer placement...being on dead center of the room could set you up with weird resonances and being in the corners will normally set you up with exaggerated lower response.

I've also noticed that with the prevelent use of curved screens and only allowing something like 2-feet at its apex for behind screen space...a double-18 sub will typically have to be off center just to fit entirely behind the screen too!

Posted by John Walsh (Member # 168) on 11-09-2005, 08:08 AM:

I would go with the lower drawing also, but I would probably go with Mike and move the L & R speakers apart farther such that the edge of the speaker was at the edge of the 2.39 line. This assumes the use of acoustial material for masking and (as Rich pointed out) nothing blocks the speaker, like a screen frame strut or masking edge. Also, are you planning on baffles, or 'bafflettes?' Highly suggested to do so. Don't forget rubber feet under all the speakers.

Try to design the screen frame such it avoids blocking the speakers. Below is a picture of a bafflette with speaker where a screen frame strut was in front of the speaker; I had them cut it out and reinforce it, but it's better to avoid the problem in the first place.

Posted by Gordon McLeod (Member # 33) on 11-09-2005, 08:57 AM:

At one point in time some engineers felt that 2 subs should be placed apart by the distance equal to the 1/4 wavelength of the room peack in that band
Also in many olderlocations LE and RE were used for the bass enhancement and often the subs were placed under them so they coupled to the LE and RE speakers

Posted by Sam D. Chavez (Member # 1841) on 11-09-2005, 09:58 AM:

Good point Gordon.

Posted by Amanda Mundin (Member # 3386) on 11-09-2005, 01:36 PM:

I ask about Subs as using Dolby Tech Guidelines i figured out 1 4642A would be enough for my auditorium as its quite a small house, so I wondered if using 2 taking half the load each but being separated would distribute the bass better.
Also regarding stage speakers is the fall off in level, and the fall off in HF on L+R when the masking comes in for 1.85 or 1.66 noticably different, even with acoustic masking?

Posted by Gordon McLeod (Member # 33) on 11-09-2005, 05:41 PM:

There is some loss of HF even with tranperent making material but very little
I usually put the L and R just inside the 1.85:1 image edge

Posted by Alex Grueneberg (Member # 2652) on 11-10-2005, 06:16 AM:

Steve, you are correct. Sorry about that, bare with me I'm still learning about all the models. Ironically I thought I might have been incorrect and had some free time earlier today waiting for a print to arrive. I opened them up to check the back of the driver, sure enough 4645. I'll update with my room dimensions and amplifier specks tomorrow.

Posted by Amanda Mundin (Member # 3386) on 11-10-2005, 12:16 PM:

Thanks for the help, also can anyone confirm the distance between the front of the HF horn and front of the LF unit on a 4675C-8LF?

Posted by Gordon McLeod (Member # 33) on 11-11-2005, 11:22 AM:

I wouldn't worry about the "correct distance" if you use a variable time delay adjustment crossover like the Rane it is a non issue If it a non adjustable set it for the distance that the delay is fixed for to correct the phase at the crossover

Posted by Amanda Mundin (Member # 3386) on 11-11-2005, 12:40 PM:

I was thinking of putting the speakers in a Baffle Wall and assumed that the front of the LF driver should be flush with the Baffle front and just wondered how much further forward the HF driver protrudes, in terms of how far forward the screen sheet would need to be from the Baffle?

Posted by Gordon McLeod (Member # 33) on 11-12-2005, 02:12 AM:

the bass cabenet should be flush with the baffle wall and the horn placed where the phase alignment of the crossover is designed for