Dual opposing 2.5 speaker?

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Hi!


Im working on a 2.5 floorstander prototype using a pair of 8s and a horn tweeter.

The test box went too big due the dual woofers and the horn tweeter located in the front. The cabinet needed to be reinforced and I think such a tall box is prone to have low freq standing waves and vibration issues.

I know all this could be solved with more damping and bracing, but in the end, I think the major problem is that I dont like to look of such a tall cabinet

And now the question.
Do you think a dual opposing drivers in a push push config would work here? I have seen subwoofers using thing arrangement, but not in a 2.5 design.

I´ll have to increase the box depth to maintain the internal volume and to have enough space to put the front and back firing woofers. I think this opposing config would reduce cabinet vibrations too.

What I´m worried about is if I´ll have phase problems since the woofers wont be at the same distance to the listener now.

I want to know what do you think before start drawing. What other issues do you see for that configuration?

Thanks for you help!
 
Ewo, thanks for your response.


I know I will need space to place them but, how much? I like the midrange in this, so Im not convinced to buy an aditional unit to fire forward.


My idea is to use the front firing woofer for bass-midrange, and the rear to help in the low bass.
 
The top 8" must be directly below the tweeter for the best possible point source illusion, the best soundstage. A 2.5-way requires a smooth integration of the two 8" which a common front mounting performs best. The polar patterns of the two 8" should seamlessly blend together. Do you believe that you have exhausted the design and construction alternatives for the original stacked TMW with the two 8" midwoofers in front ?

Can use plywood instead of MDF? The extra strength requires less bracing.
Can you add a few 1" x 1" hardwood braces? Top-bottom, Left-right
 
mordikai, built a xover for testing purposes. woofers in parallel, 1mH coil and 60uf cap. No baffle step compensacion circuit or 2,5 xover yet cause I dont have big coils. Made some measurements anyway, and I get a hump in the 200-500Hz range, I think I could xover the .5 woofer at around 150-200Hz. I´ve been playing with Xsim to figure out what should work better, anyway, only measurements will tell.

LineSource, yeah, I´ll put the top 8" right below the tweeter as you say.

No problems on adding bracing or using wood, just wanted to reduce front baffle dimension and box height, and thought putting the lower 8" in the rear could work.

Even if I cross the rear that low, won´t the polar patterns blend well?
 
You don't need BSC in the crossover if using a 0.5 woofer as that does the job for you. I've found it best to crossover the 0.5 woofer between 30%-50% of the baffle step frequency (115/baffle width in metres) or it can be too much in the room.

I did a Peerless 850122 2.5 way in 2004 with a rear firing 0.5 woofer which was in line with the front woofer. Worked quite well but did need some space behind the enclosure. The crossover was quite low to keep most of the midrange out of the 0.5 woofer. Even experimented with different rear drivers as the 0.5 woofer in their own enclosure and over the years I built many 2.5 way speakers with asymmetrical woofers firing forward.

I tried this after reading some work by planet10 where he talks about the phase shift and issues hidden by shadow of the box. Read here:
Baffle Diffraction Step
 
rabbitz, thanks a lot for your response.

I think this is very similar to what I want to try. Could you tell me more about that project? So, what freq cutoff did you used? Why you say it needed more space behind the cabinet? too much lows?

My speakers will be placed in the room corners, so I think it´ll be the same if using front firing woofers. I´ll have to deal with rear and side walls anyway.
 
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There was nothing special about the project but I've added a pic for reference. As you can see they were too close to the wall and needed to be 1.25m into the room to work properly but I couldn't afford that distance. The 0.5 woofer was crossed over at 125Hz. The 0.5 woofer was in line with the front woofer.

The closer to room boundaries the less BSC you require and you can't control that with a 0.5 woofer as you get full BSC unless you attenuate the signal going to it. Place it in the corner and the effect will be worse as you will get massive gain at the bottom end and suck out higher up the range at around 0.3 x the wavelength to the boundary. See the attached Fig 1-3 which shows the variation with 1 boundary and compare to that shown with 3 boundaries. You only way you can overcome that is to have the drivers at least 1.25m into the room where the effects will occur well below 100Hz (ref Roy F. Allison).

You may not need the 0.5 woofer and the best advise is to have every dimension from the woofer to a boundary to be as different to one another as possible and not be a direct multiple as well. This will place the reflections at widely different frequencies and smooth out the low frequency output.

It's been said, "more bass is not necessarily better bass".
 

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Rabbitz,

Did you used a first order xover for the 0.5 woofer? I´ll need a huge coil if using 2nd order and crossing that low.

I think I´ll discard the rear woofer option due to room placement problems. can´t put the speakers far from the walls, so I think I´ll follow your advice and the graph you attached.

Thanks for your help
 
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