This is topic Room Tuning w/ Multiple Microphones in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Manny Knowles (Member # 1171) on 02-27-2002, 01:21 PM:
I have an RTA with a calibrated microphone, but I've never been happy with the resultant "sweet spot."Does anybody here use multiple microphones and, if so, do you go through a mixer or a multiplexer?
What's the ideal microphone spec for room tuning with multiple microphones?
Is it true that multiple mics don't have to be calibrated to the RTA?
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And, hey! Let's be careful out there.
~Manny.
Posted by Richard Fowler (Member # 893) on 02-27-2002, 01:59 PM:
Multiple miking through a multiplexer is preferred, especially in wide and stadium rooms as being built these days. Ultra-Stereo multiplexer with four mikes ( which are calibrated and numbered to work with the multiplexer box ) is around $1100.00 street price. I just inspected a complex we sold with CP650s where we had a dispute with the customer and we did not do the initial installation. Our crew is retuning the complex now because the local guy used a single mike and punched up "Auto EQ" which gave him very tubby sounding results since the software assumes you are using a multiplex set and severly overcorrected the rooms........we can't wait for this guy to go into his new business....selling fried chicken. 
....can not totally blame the guy since the cinema owner doesn't pay enough for him to maintain a basic test set for his market.
Richard Fowler
TVP-theatre & Video Products Inc. www.tvpmiami.com
TVP-Theatre & Video Products Inc. www.tvpmiami.com
Posted by Joe Beres (Member # 678) on 02-27-2002, 02:09 PM:
This reminded me to ask the following: Does anyone rent a multiplexer and the calibrated test mics? We installed a CP650 here, and don't have the equipment or the money to purchase a multiplexer or a tech to bring one. Between the software and my ears, we have it sounding pretty good, but I know we could do better. If any of you have any information, please let me know. Thanks.
Posted by Manny Knowles (Member # 1171) on 02-27-2002, 04:16 PM:
This raises another question:I was taught that the mics were calibrated to the RTA and not to the multiplexer.
Could someone clarify what is being calibrated?
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And, hey! Let's be careful out there.
~Manny.
Posted by Mark Gulbrandsen (Member # 72) on 02-27-2002, 04:32 PM:
In the case of the Ultra Stereo MUX the mics are not really calibrated at all. They are just high quality mics, In fact the same Countryman mics that THX uses I believe, they have a pretty flat response within reason. The preamps and muxing are done in the black box that hooks to your analyzer. Set all the mics to the same level and then go. Its still a very BIG compromise but at least a step in the right direction as far as taking the room into consideration.
In the case of the THX R-2 there is alot more going on and eq offsets are applied to each mic via the software operating system. The mics are calibrated by THX personell before they leave the Pixel Plant.
Mark @ NEW GTS
Posted by Will Kutler (Member # 770) on 02-27-2002, 05:24 PM:
I guess one good reason for multiple mikes would be to offset room modes from the theater architecture....a topic that I do not ever remember being discussed but was demonstrated to me by THX.Room Modes can really be something. I was in a brand new THX theater that was just being set to open. With test sound track running, stand in one spot and be blasted by sound, then move over a few inches and it was nice an quiet...all throughout the theater. This is something that architects try to minimize.
Posted by Manny Knowles (Member # 1171) on 02-27-2002, 05:52 PM:
Thanks for the info...The USL MMP-10 Multiplexer looks good. I'll investigate cost.By the way, I downloaded the user manual and it says: "Each microphone is calibrated with its multiplexer to insure utmost accuracy."
They are indeed Countryman lab grade test mics.
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And, hey! Let's be careful out there.
~Manny.
Posted by Paul G. Thompson (Member # 655) on 02-27-2002, 06:20 PM:
Multiple microphones are the way to go.
Posted by Richard Fowler (Member # 893) on 02-27-2002, 06:26 PM:
Countryman mikes come in different versions.....I had a client complaining he had lousy sound until I found that the local tech had replaced two of the mikes ( originals damaged )on his multiplexer with some that he found at a local music distributor.....they had peaking mid range response for "on the lapel" voice pickup.....
Richard Fowler
TVP-Theatre & Video Products Inc. www.tvpmiami.com
Posted by Gordon McLeod (Member # 33) on 02-27-2002, 07:04 PM:
We tested 2 different USL ples and mics against the R2 and they came with in 1/4 db of the same results across the room curve.
The plex box is calibrated against the mics in question according to USL
THe only thing I do is set the SPL output to match what my cel SPL meter reads
Posted by Steve Guttag (Member # 268) on 02-28-2002, 10:11 PM:
Gord,My tests do not back up yours. HF response using the USL plexer is totally dependant on the mic. There is no offset for frequency response. Furthermore, your analyzer can load the plexer and cause even more frequency anomolies, particualarly in the LF region.
The USL system is a good tool and is better than a single mic but it is a far cry from a calibrated system such as the R2 where not only frequency response it calibrated but the levels across the 4 mics are normalized.
For what its worth, the model number on the mics are not the same for the R2 as for the USL....
Steve
Posted by Don Sneed (Member # 955) on 03-01-2002, 04:17 PM:
Will, I use a R2 unit for my eq's, it sounds like the speakers were not aimed correctly, I use a sound pressure meter, pink noise, walk the aud. to get a overall response by sound pressure...I then have one of our techs to aim and tilt the speakers as needed, to give you an example, I just got finish doing a 10-plex All THX, the speakers were aimed as thje drawings called for, there was a 7-db difference from the front row seating to the rear, this is in a 700 seat aud. Sony 8.1 system, we reaimed the speakers position, speaker tilt we now have a 3db difference from front row to rear seating & side to side, now this took all day to get but we got a pretty good result by going the extra trouble...it sounds like the speaker you listen to was not properly aimed, tilted, & position....I know alot of techs do not go to this much length to get the speaker, freq. response, & coverage as they should....remember the "audience is listening"....
Posted by Steve Guttag (Member # 268) on 03-01-2002, 07:39 PM:
Don makes an excellent point. The set up of a system is so important before the EQ begins. Making sure the speakers are as close to the screen as possible and properly aimed is very important to the overall response. Then there is the room itself. Even a great speaker with excellent electronics will sound horrible if the auditorium is not made well (acoustically speaking). I'm having a bit of trouble with a customer that contracts us to merely wire and tune their sound systems. While the R2 may say all is well, the actual performance varys all over the place. In some theatres he has 50% fiberglass on the walls, some 100% some have fiberglass above the ceiling tiles and some places none. The speakers are installed by the screen guy and are "safely" put nearly a foot behind the screen. He also uses a large varitey of speakers (both new and used based on availability) so consistancy is virtually impossible.
One key thing about good sound systems is to get the room right first! It is a key part to a good sound system that is just overlooked once the wall drapes go up and the ceiling tiles are plopped into place.
Then, install what you have as well as you can.
Steve
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"Old projectionists never die, they just changeover!"