Rational Acoustics

September 8th, 2011, 01:09 AM
Lately I have had the opportunity to mess around with Smaart in a gymnasium; however, when I try to collect subwoofer data using pink noise I get phase and magnitude traces that jump all over the place with poor coherence. My time frame is set to MTW with 4 sec averaging and 1/24 smoothing. I have tried putting the mic on the floor 3ft out, 20ft, 60ft out as well as in a stand. Nothing seems to work. I have tried using pink sync noise as well as music. I know my setup is correct because I never have a problem from 100Hz and up and always seem to get good data with strong coherence in the upper octaves. The gym is roughly 80' x 150' x 25' with the stage centered on the 80' wall. The subs are dual 18's on the floor centered on the wall roughly 35' apart. The PA is flown above the subs 20' in the air. PA is KF300e's / Subs are Renkus-Heinz CT6's

I have also run into some circumstances where the subs are placed inside a wall cavity and experience the same problem.

I always end up having to send a tone and flip the phase on the subs and align by ear...

I would really like to find away to collect good data so I can better align the phase traces as my processor is capable of providing all-pass filters to change the rate of phase shift in addition to delay.

Any insight into the murky waters of subwoofer alignment techniques would be appreciated. Thanks.

Smaart 7.0
Lexicon Lambda
Audix TM1

Dr. J
September 8th, 2011, 01:45 AM
Hey Jivey -- A couple of things here. Number one -- I am not even considered a seasoned tech or FOH guy although I do both. Take what I say with a grain of salt. I have done a ton of indoor measuring in horrible reverbant spaces & I always mess it up...:D

My advice is: If you don't have the tunings for the Subs -- you don't have much choice other than to take one of them outside away from all the walls. If Meyer sound drags there subwoofers out back for initial tunings (below 100Hz) then you are gonna have to be real good to get it right indoors.

I tune my subs outdoors in a field with the ground being the only surface to deal with using Post XO PEQ's. I get great coherence and the magnitude & phase trace is quite stable. The coherence (signal level) only needs to be so high before the data is considered trustworthy -- coherence that is -- that could make a difference for you inside because the volume is reduced a bunch. Just how loud do you have the pink noise inside? How low can you go before the coherence trace falls down from the top? Should be able to get pretty low.

Another thing I do is know my outdoor free field response so I have it to compare to an indoor measurement. You will see very quickly what 4 walls & ceiling & a floor did to your response. My experience in smaller places -- 1k on up doesn't get messed up too much. Below 1k and down the further you go --gets really mangled up.

Then the point that room modes occur I see a major boosted area at a particular frequency that will need to be tamed. The cancellations inside are more than likely false. Pull the mic back one foot -- then where there once was a cancellation is now a peak -- like a rolling tidal wave. Move the mic in -- another dip fills and another cancellation occurs up stream somewhere... it gives you a headache..

If you can take one sub outdoors -- do that first & take a pic of your traces then go back in and try to make sense of it while doing it inside. This helps me to stay on track a bit better.

Aligning will be difficult if you can't even get the trace to stabilize. I just use the lowest amount of volume I can before the coherence falls from +15. I measure from the spot the will allow me to get the best coverage area. Like in the Half of the left or right stack area. I do one side only. Some DSP's you copy to the other or redo...

I hate trying to align in a venue empty when in reality -- it should be filled

Now the big boys will show up and tell you how it is really done.:D

September 8th, 2011, 10:46 AM
Thanks Dr. J. for your response. I will go back and attempt with less level going through the system. I was attempting it with the level around 85dBa to get over any noise floor. I don't have the luxury of moving the subs outdoors.

What you are proposing makes sense for getting the tuning right, but I'm still curious how to get the alignment right.

Dr. J
September 8th, 2011, 11:46 AM
Just keep in mind Jivey that your coherence trace only need to reach the top (around +15) for it to be considered plenty loud enough. At that point -- there is no need to push volume.

When I do this outdoors -- I always work at low volume as to not disturb anyone but for a final look at the very end of my tuning -- I will raise the volume to see if something does in fact change. The only thing I think changes is in my subs. There seems to be a small narrow dip at 72hz at low volume that "pops" up out of the dip when pushed. Weird but anyways........

Maybe someone else will come along and give you some tips on your settings to help get a clearer picture on your screen and maybe tell you the best place to set the mic.

I have been told by the guys on here to set the mic in the space where most of the audience will be BUT since this is a gymnasium -- who knows.

I would probably place it in the Half of the Half. In other words -- split the PA down the middle (left or right side) then place the mic in one of those halfs. Keep trying -- you will get it.