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[Mike Karseboom]

I have a venue that has a thin plywood front wall below the front stage lip but a dense and solid stem wall about 5' behind that.  The dense wall is causing significant cancellation of the two center clustered subs in the range of 45-55Hz, resulting in loss of spl in that range of frequencies throughout the room.


From a practical standpoint, the subs cannot be moved any closer or further from the dense stem wall.  They are on the floor and butt up to the thin front wall and that is pretty much where they have to be.


So I am wondering if I can use something cheap an easy like cinder blocks to build a 4' x 4' reflecting wall directly behind the subs.  This would be behind the thin wall so out of site.  The idea would be to force any reflections to be within about 3' of sub front face.  That would push any half wave cancellation up to around 90Hz which is mostly outside the subwoofer bandpass.


I can also get a sheet of "hardy backer" type material and could build some sort of wood frame if needed if that would reflect better.  Note that I am not so much thinking "absorption" or "bass trap". I am thinking "reflection" for low cost and possible free dB gain.


Is this  a feasible idea? Would stacked cinder blocks effectively reflect 35-80Hz?

[Len Zenith Jr]

For a wave to fully reflect, your obstacle has to be a wavelength long in all directions. That's about 22' @ 50hz. You're 4' x 4' proposed wall will reflect anything above 250 hz but not too much below that.

[Mike Karseboom]

OK so maybe no free +dB's but the main idea is to defeat the cancelling reflections.  Is there any merit to this idea?

[Tim McCulloch]

OK so maybe no free +dB's but the main idea is to defeat the cancelling reflections.  Is there any merit to this idea?

Len points out why.  Welcome to the world of unintended architectural consequences.

The fast thing would be to go back to L/R subs and see if that resonance cancels some of power alley and if you get more uniform coverage.

What other configurations have you tried?

BTW, what's up with all the power alley haters?  Not just you, Mike, but there's been a fair bit of traffic over the last couple of months that seek to minimize or eliminate power alley.  What I suspect is a trade publication article or online piece or U-Tubular video that has got folks to question their ways; that's not a bad thing in and of itself - what got my curiosity going is the way a simple dipole effect is seen as a defect rather that just one of a varied array of compromises we make.

[Mike Karseboom]

I can go with L/R stacks and will  try that next time I get a chance. 


I was going for center cluster partially for my generally held belief that the coverage on the dance floor in front of the stage would be more even and somewhat stronger just due to proximity.  The stage is about 30' wide.  But mostly I was after some presumed free dB's gained by center clustering the subs.  These are two single front loaded 18" subs in 40'w x 80' deep  fan shaped room that caps out at 320ppl and has a bunch tiered, padded theater seating.


As to the wall, I guess I was also under the impression that I would get some free dB's because the subs would go from half space loading to quarter space loading.


But this is predicated on the 4' x 4'  "wall" of dry stacked cinder blocks acting like a big enough and solid enough wall to do the actual reflection and not let the waves from 35-80Hz go through or around.  That is also, I think, a prerequisite for the main goal of defeating the 45-55Hz cancellations I am experiencing now. 


Note that  since I see the approximate 50Hz dip throughout the audience area I am pretty sure these are due to direct reflections from dense stem wall under the stage rather than some sort of room mode. I can even move  to the side and see the dip go down to 45Hz owing, I believe, to the longer path of reflection caused by being off axis from  the subs.


So really one key question is whether a 4' x 4' stack of cinder blocks qualifies as a reflective wall?

[Don T. Williams]

The wall may not be causing the problem.  The thin plywood wall might be acting as a membrane bass absorber  or there maybe something in the room that is acting as a bass trap.  Room acoustics get complex very quickly.  Moving the subs is a good start to finding the problem.

[dave briar]

--snip--
Is this  a feasible idea? Would stacked cinder blocks effectively reflect 35-80Hz?

This topic interests me as well as the venue I work at is considering expanding the indoor stage to extend over a pair of 18" subs and I'm worried that will cause the whole stage deck to resonate big time. The venue did similar a few years ago when they built an outdoor stage consisting of 30" high box trusses spaced 36" on center sitting on a concrete slab and running up/down stage decked with 2x6 tongue and grove pine. The four 18" subs installed under the front lip not surprisingly turned the entire stage deck into a big resonator  -- ever seen a tamborine walk sideways from 50 hz vibrations?  Enclosing the subs in a box of 3/4" MDF glued/screwed to 1/2" concrete board (typically used for backing tile showers) helped quite a bit but things are still far from optimal.  Building a sand-filled cinder block wall behind the subs as Mike proposes has been discussed hence my interest in his question -- if even only at the academic level. Yes, a cardioid sub arrangement might be the best solution for our needs but I do wonder about the physical options.

  ..db

[Luke Geis]

I am one such hater of the " power alley ", more so because I hate dead spots and seemingly uneven coverage. While the center clustered subs will essentially be a focused power alley, for the most part is distributes out rather well with fewer ares going totally dead. however as Ivan says often, " It depends "........ I will use whatever deployment technique will work the best within the space, or whichever deployment is appropriate for the event. This has meant putting the subs in weird places before.

[Jim McKeveny]

The dense wall is causing significant cancellation of the two center clustered subs ...

THAT is one large assumption. Any measurements to substantiate?

[Dennis Wiggins]

...I have a venue that has a thin plywood front wall below the front stage lip but a dense and solid stem wall about 5' behind that...

This big empty space with a floppy plywood facing is the cause of (my unscientific opinion) the LF problems.

How tall is the stage, and have you tried putting the subs up on the stage?

-Dennis

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