Rational Acoustics

September 2nd, 2011, 10:51 AM
Rational Forum folks....could use some advice. Filling in for a vacationing Sound Tech on FOH. The venue is a local Veteran's hall / Legion, with an audience of about 100 to 150. Set up with the band last night but no chance to optimize their system or play with the EQ. Band played 2 tunes while I tried to dial in something workable. Have the opportunity to go back this afternoon (in a few hours) ..in advance of tonights gig and see if I can get things sounding better. Downloaded the SMAART demo and would like to give it a test run.

Speakers are a pair of internally crossed over EV 12's Mid Hi boxes on sticks and 1 x18" Sub. Dance floor is about 20x20 of hard Lino in front of the stage and then Carpeted seating area thru out. Set up in a corner stage. I've splayed the speakers to keep the sound off the walls and give even coverage thru out the venue. Overlap zone between the speakers falls in a service aisle.

I'm going to try to see If I can resolve any phase issues between the Sub and the Mid/Hi's but I only have an analog crossover and no way to set delay between the speakers. So, I'll do what I can. My concern is the Room EQ. Admittedly , I don't have the best ears. I am working on developing pitch/ frequency recognition...but that is not going to help me 4 hours from now (lol). I've read Bob McArthy's book cover to cover several times, exhausted the rational website...so the principles of dual FFT measurements are not foreign to me. But I could use a hand getting started.

I'm a bit confused as to the Ground Plane vs. Mic on a stand . I realize you have to consider ground bounce for the Mic on a stand. But their seems to be issues with certain aspects of the ground plane. I'm not going to have time today to source out some absorbtive acoustic / Auralex panels to kill the bounce so should I stick with a Ground plane? I can bring a 4x4 sheet of plywood along to sit on some tables farther back.

I only have 1 measurement Mic.McArthy's book advocates mutliple Mic measurements in a star pattern in the Isolation zone of the speaker. How can i achieve this with limited equipment.

I guess my question over all is..Can any advice be given on how to see room/ Speaker EQ trends with my limited set up? (I know...i know...I'd take a Rational class if there was one remotely near my area but their isn't. Travelling costs to some other part of the continent are too cost prohibitive for a part timer/ enthusiast / hobbyist like myself)

Thanks in advance...Jeff

Dr. J
September 2nd, 2011, 12:33 PM
Jeff -- Sounds like you are in a fix here. If I were you and was thrust into this situation knowing I had very limited time and tools -- I would either go with ground plane inside or drag a stack outside and measure it into open space.

I would prefer to take it outside simply because I am not very good at the complex decisions you have to make inside an empty venue with potentially high reflective surfaces. I have done plenty of this BUT when I "A/B" it with my outdoor FLAT preset (always my starting position) I always seem to like it better.

If you choose to do this inside -- go with ground plane. I would lay the system on its side (one stack) pointing into as much open space as you can inside and place the mic straight out in between the horn and mid woofer about 7-9 feet out. Place the mic straight down at the floor about an 1/8 -1/16 of an inch. This is very close to the floor. This eliminates the comb filter. Well, it moves it up into the upper frequencies where it isn't as much of an issue (8-10kHz).

Since you won't be able to align -- all you can do is smooth out the response with the graphic. If a dip exists over the crossover area -- don't get carried away with filling that hole because it may be an alignment issue and NOT an EQ issue. You can try EQ there BUT if it takes 8dB boost to fill a dip over a XO area -- then it most likely isn't an EQ issue but an alignment issue.

Go flat from 125Hz out to 8kHz and then roll off from there. Going the other way from 125Hz on down -- you may want a low end buildup there. For Rock music -- I have been told that a 10dB build up is a decent starting position. For an orchestra -- completely flat.

After you are done set it back up and play some music and make some more changes. You may want to push the system into feedback as well to identify trouble spots before the band hits the stage or before people get there.

Running Smaart inside a venue while the band is playing is definitely for the advanced. I have done it with a packed house priming the system with pink noise first and all the decisions I made slowly sucked the life back out of the system. I quickly went back to default and left it alone. I am missing something I haven't learned yet about inside interactions with smaart. The reflections & cancellations "trick" you into making bad decisions.

That is all I can say Jeff besides the fact that you are really late in the game at this point. :D

I have had to do what I just explained to you with someone's system about a year ago because they just didn't understand why it sounded so poorly. I offered to take mine in for a small fee BUT they wanted me to mix on their system. I explained that I would mix BUT I needed time with the system BEFORE we ever set foot inside the joint.

I tuned the system the best I could with a beat up crossover with NO delay and a graphic. It is a horrible way to go BUT hey -- it wasn't my call. At least I got to spend a couple of hours with it setting the gain structure which was way off and then going thru it setting the crossover up and response. Sounded waay better. A pretty high polished turd I guess!

Good luck! Do what I said above and you will get thru the gig for now. If they want you to continue to use that system -- got get a system controller at a minimum. It would help a bunch.

September 2nd, 2011, 12:35 PM
I need warp speed in 20 seconds or we're all dead! :eek: ;)

OK. Here's my short form answer - I hope others will chime in as well.

Take a few measurements in the "isolation zone" and compare them (or even average them together)- you are trying determine what are the common trends in the Magnitude Response in the listening area. The whole point of the ground plane measurement is to avoid the influence of the ground bounce in your measurement. The "star pattern" will effectively help randomize and minimize this as well. If you put pink noise through the system and walk & listen in the iso zone, you should be able to hear if there are any major consistency issues in the coverage (no golden ears required) - if you hear significant flanging as you walk, either you have multiple speakers interacting, or you have reflection problems.

Forget about trying to deal with the phase between sub and full range boxes. You don't have the controls to deal with the timing. I would simply find the freq where the two system cross over, then choose the polarity that gives the best addition.

You know, if you are in a serious time crunch, Rational does have a phone :)


September 2nd, 2011, 03:36 PM
Thanks Dr J and thanks Jamie. Your both right....warp speed and we're dead and coming way too late in the game...:eek:

You've affirmed to me what I thought I should do. I'm going to see what can be done in the time I've got. This is the first time I've run in to this..as I have always mixed on my own system. But I am starting to get some calls to drop in on other's gear so I am going to have to try to find a way to deal with this.