TMM driver array question

[Puppetnation]

Hi,
I'm gonna build a pair of tower TMM 2.5-way sealed boxes.

Some people suggest shared box volume for both midwoofers, some other says separate chambers are better for a 2.5-way system due to slight disturbance effects btw drivers...

Assuming I'll build cabinets with separate chambers, which arrangement options would you suggest, choosing between two close midwoofers (A) or two distant drivers (B)?

Commercial loudspeakers offer features both solutions, I'm wondering which one would be the best: I cannot build to test boxes.. please help me!

thanks
Puppetnation

[Salas]

There is no real distortion for a shared chamber. But if you have to do it, do B. A is prone to proximity and cavity resonances.
Commercial designs with chambers, normally explore staggered box tunings for better room integration.

[LineSource]

Personally, I would build a 3-way with a real midrange and a real woofer.

For 2.5 way I would put the two midbass speakers in separate sealed volumes, very close to eachother, and close to the sealed tweeter volume. Both midbass carry high frequencies which are directional, so the best radiation pattern requires that they be close to eachother.l

[peter_m]

Zaph's enclosure experiment

What designs are you considering?

[Puppetnation]

Originally posted by peter_m
Zaph's enclosure experiment

What designs are you considering?

I've read this Zaph article before posting (and I also consider him a real mind) but I also have listened to a pair of 2.5 towers made by an italian loudspeaker manufacturer named Opera Seconda which sound really great and are assembled exactly like my "B" drawing:
actually two identical enclosures divided by a simple straight MDF board, no dangerous traps nor corners inside.

I know close drivers would be better but I'm also considering this:

- lower woofer would play only low frequencies (up to 300 Hz) so placing it a little distant from each other would allow me to separate chambers with a simple straight MDF board without creating dangerous traps.

- close drivers would be ideal due to a more accurate emission source (and would also look better) but I must create dangerous corners, otherwise a common enclosure would leave me complete placement choice.

[sreten]

Hi,

There is no real need for the BSC woofer to be close mounted and
in fact in some designs it is spaced down to smooth the floor dip.

/sreten.

[Salas]

Originally posted by Puppetnation


I've read this Zaph article before posting (and I also consider him a real mind) but I also have listened to a pair of 2.5 towers made by an italian loudspeaker manufacturer named Opera Seconda which sound really great and are assembled exactly like my "B" drawing:
actually two identical enclosures divided by a simple straight MDF board, no dangerous traps nor corners inside.

I know close drivers would be better but I'm also considering this:

- lower woofer would play only low frequencies (up to 300 Hz) so placing it a little distant from each other would allow me to separate chambers with a simple straight MDF board without creating dangerous traps.

- close drivers would be ideal due to a more accurate emission source (and would also look better) but I must create dangerous corners, otherwise a common enclosure would leave me complete placement choice.

The separation board can be advantageous by stiffening the structure in a medium price commercial box that uses little or no other special means for stiffening. It also cuts the first vertical pipe resonance mode in half, shifting it at double the frequency and staggering it. Such speakers are normally slim and away from golden ratio dimensions. On the other hand there are absorption methods. Those are good side effects that can be utilized in a system, given the grade of approach.
In principle they have nothing to do with the main function of loading the bass drivers. The chambers could be planned for different volume with different Fs woofers in a 2.5 way so to explore staggered Fbox tunings and specialized drivers, suited better for bass and mids. Also a 45deg angled separation board could make the chambers non parallel. Not bad, nothing spectacular though.
As for getting point sources close, has nothing to do with a 2.5 way at baffle step wavelengths. To the contrary, in such a system we can explore Roy Alisson's boundary effects.

[peter_m]

Originally posted by salas
...in such a system we can explore Roy Alisson's boundary effects.

Vas ist das?

[Salas]

Roy Allison of AR fame.

From an Interview:

Lander: You hold a patent relating to this boundary-effects phenomenon. What does it cover?

Allison: The design of cabinets that get the woofer very close to one or more adjacent room surfaces. That changes the frequency range of the dip, because the closer the woofer is to a surface or to the point where surfaces intersect, the higher in frequency the dip occurs. In the case of a three-way system, it's possible to position the woofer so the dip is above its operating range, and to place the midrange driver far enough away from an intersection for the dip to occur below its range. In effect, that eliminates the problem. This approach really isn't feasible with two-way systems, because the woofer has to handle frequencies high enough to put the destructive reflections within its range. But you can build a cabinet that has the woofer very close to one surface—the best place is on top—and then position that cabinet so distances to the other nearby room surfaces are staggered. Doing that creates mild dips that are spaced along the frequency axis. They aren't able to add in the nonlinear manner that they would if the distances between the woofer and all adjacent room surfaces were equal.


Some comments by me:

The 'dip' he refers to is the floor bounce 120-200Hz centered cancellation effect in real rooms, not anechoic chambers. Roy, finds it awful as a coloration.
Lynn Olson considers it part of our everyday experience though, hence psychoacoustically not as bad as it measures.

*Roy made a 2 way speaker with the woofer flat on on top and put the back of the speaker against the wall.

**For a 3 way we can just keep the woofer down near the floor. In a 2.5 way the benefit is going to be less but worthwhile.

Pictured, is Allison model 1.

[Puppetnation]

OK,
thanks to all folks leaving a reply.

After a sort of mind-cleaning process I have made some final choices (I'll use separate volumes for each driver) but I still have some doubts to be solved:

- using two close woofers (instead of two distant units) would give my loudspeakers a nice look but... is this acoustically good?

I may use a smaller volume (so more dumping material) for the higher woofer and a larger volume (less damping material) for the lower one to fit separation board exactly between the two chambers (eg.1), resulting in a more controlled output in the upper woofer and a more free output in the lower one.

Is it a bad idea?

- as alternative I can build a box with 2 identical volumes inside (eg.2), simply placing a straight separation board which dividing cabinet height in half. Obviously this will result in a distant lower woofer.
Some people consider this solution the best for a 2.5 way loudspeaker.

I'm confused, commercial loudspeaker offers doesn't help me to choose.

What's your opinion?

thanks