Uneven Bass Response

[jdcountryboy91]

Hello all,
I am having difficulty getting even bass response in our youth room. All the low end builds up at the back of the room where the sound board is but is all but lost in the audience seating. Here is a picture of the setup to help visualize:




I have tried different positioning of the sub on stage: upstage facing out, upstage facing the wall (backed off about a foot), downstage even with the main PA (on either side, and centered). I cannot seem to get even bass response in the room no matter the positioning. The down stage facing the wall setup has given me the best result but it's still not very even. Oh, and I am not referring to live instruments on stage, just tracks.

Specs: The main room is about 50'x20' with an A-frame ceiling peaking in the center at about 30'
Main PA: JBL JRX115i, JBL SRX718 powered by a Crown xs700 (bridged).

Any suggestions?

[cmchamp]

You will have LF build up in pressure zones against walls.
What are you using for processing, what's the crossover frequency?

[jdcountryboy91]

Thanks for the reply Cory. I am running the sub send via the mono out on our behringer eurodesk (unfortunate, I know). The crossover is set around 100hz. I figured there would be low end build up at the walls but it is almost non-existent in the audience. Even when it is pumping in the back.

[cw4u]

Shouldn't the sub be in line with the mains?

[cmchamp]

Derek, that's part of the problem.
jdcountryboy, are the mains crossed over at the same frequency?
It may be that the sub and mains are not properly time aligned so that energy from the mains and the sub meet at the same time at the listener's ear.
On another thought, how are you getting your pre-recorded music into the mixer?

[jdcountryboy91]

The mains are crossed over at 100hz as well. The main PA processing is a stereo 31 band graphic EQ with high pass and low pass filters on each channel. I am playing music via a simple 1/8" to stereo RCA cable going into one of the stereo channels.

If I had a way to time align the mains, I would and because of the stage setup it is not ideal to have the sub sitting in the same vertical plane as the mains. I have tried it there before and didn't notice much of an improvement.

[cmchamp]

Are you using just the HP/LP filters on the EQ as your crossover, or you have an actual crossover?
If you're using the EQ as your crossover, the slope most likely isn't steep enough, probably only 6dB/octave, depending on the make/model.
C.

[jdcountryboy91]

I'm using the crossover built into the GEQ: Behringer FBQ3102

[cmchamp]

Got it.
Try moving the crossover frequency down to around 80 and see what happens.

[Gracetech]

Is it uneven bass response or uneven mid and treble response?

Bass can get to just about everywhere. As long as your subs are together and not spaced to far apart your coverage will be pretty smooth through out the room. 120Hz and up can be alot more tricky to put where you want. Most of the time at the back of a room whats left of the mids and highs is a reverberant mess. The only thing that has any definition is the low frequencies. Why is this you ask? Low frequencies tend to go through walls instead of bouncing off of them and back into the room. In fact it would takes 3' thick concrete walls to prevent transmission of the lower frequencies.

Now in the case you have walls that are hollow you could have some sympathetic resonators. Only way to sort that out is to put some resistance to the resonator.

If you tilt your speakers up a bit it might help to seemingly smooth out the sound. Not being familiar with your room this is just a guess.

crt

[jdcountryboy91]

Thanks for the tips. I have the speakers aimed the way they are to avoid the back wall and the horrible reflections that would occur if they were aimed higher. When I get back home (currently at school) I will adjust the high pass on the mains and play around a little more with position of the sub.

[Gracetech]

I was going to say back your mains up and go a bit higher with them but judging from your picture that isn't possible.

Any possibility to diffuse or add a little absorption to the back wall? How large is your back wall?

crt