Don't think its been mentioned yet, but you can do a WTM (2.5 way) with the MTM layout. You're just flipping the drivers and using the same filtering scheme as with a typical 2.5 way. The result is basically the same and you can eliminate a bit of lobing. This will obviously reduce midrange output having only one driver reproducing the full mids. In return, as a compromise, it will require more tweeter padding.
A potential big issue for me with a tall 2.5 way floor stander is the 1/4 wave resonance, which also poses a challenge with vent placement and tuning (using ported enclosure). Separating the 2 woofer volumes tightens up the bass and reduces the primary 1/4 wave resonance. Splitting the LF output will almost always smooth out LF response.
The MTM layout is obviously susceptible to lobing issues. More often than not, this layout isn't approached correctly by the designer to avoid severe lobing. There are multiple opinions on what crossover frequencies and slopes generate the least detrimental lobing. D'appolito advocated 3rd order filters which don't sum flat and require some offset of -3 dB point to get reasonably flat-ish response. More often than not, the diffraction ripple will be a greater issue to deal with so the actual acoustic transition point will be chosen based on amplitude linearity than phase relationship (preferred).
The greatest Achilles heal of a typical 2.5 way system is the midbass response being less predictable. Most 2.5 ways I've come across sound "chesty" or thick in the lower mids. Its hard to predict this effect due to the varying reinforcement the acoustic space provides. This makes speaker placement more critical to obtain smoother LF and midbass response. Most 2.5 ways require the speaker to be placed further away from the corners and walls.
A potential big issue for me with a tall 2.5 way floor stander is the 1/4 wave resonance, which also poses a challenge with vent placement and tuning (using ported enclosure). Separating the 2 woofer volumes tightens up the bass and reduces the primary 1/4 wave resonance. Splitting the LF output will almost always smooth out LF response.
The MTM layout is obviously susceptible to lobing issues. More often than not, this layout isn't approached correctly by the designer to avoid severe lobing. There are multiple opinions on what crossover frequencies and slopes generate the least detrimental lobing. D'appolito advocated 3rd order filters which don't sum flat and require some offset of -3 dB point to get reasonably flat-ish response. More often than not, the diffraction ripple will be a greater issue to deal with so the actual acoustic transition point will be chosen based on amplitude linearity than phase relationship (preferred).
The greatest Achilles heal of a typical 2.5 way system is the midbass response being less predictable. Most 2.5 ways I've come across sound "chesty" or thick in the lower mids. Its hard to predict this effect due to the varying reinforcement the acoustic space provides. This makes speaker placement more critical to obtain smoother LF and midbass response. Most 2.5 ways require the speaker to be placed further away from the corners and walls.
Not necessarily. Putting a woofer in the middle already ups it and using 2 can potentially raise it further. When the woofers are mounted symmetrically there's no benefit from splitting the volume.
@AllenB From the perspective of net external acoustic room gain, you're right. My point was mainly from breaking up the internal cabinet height itself, which can reduce internal standing waves. Placing the port at the high pressure area of a 1/4 wave resonance usually produces more port output around that frequency. Using multiple ports instead of a single larger one can sometimes cancel out the external effects of high pressure nodes.
Yes, you can kill the resonance with a bunch of dampening material but that will kill the liveliness of the midbass. Yes, putting the woofer in the middle will mainly excite the 1/2 WL, but it forces a compromise in baffle layout. Not sure if I'd be willing to accept that design restriction.
We weren't really talking about one woofer, but two. It's a little easier that way.
I prefer the rear chamber to be clean and damped, and focus on what's coming out the front. I guess that's where we differ.
Really the fun of 2.5
Simple and straight forward.
2 woofers one box, your done.
tweet needs no chamber.
Since your using Passive radiator.
Rather pointless to overthink magical
double chambers that dont do much.
Since both are needed in same chamber for passive to work.
The .5 fills in losses below baffle step.
More woofer for bass reproduction, less bass EQ needed
in real life. For a little less distortion too at high listening levels.
More advanced designs have been done with separate
chambers for woofers. You have control over each drivers
Q and can tailor the response curve if needed.
By making one box larger or smaller.
Requires more advanced modeling, and often more suited
for sealed regardless.
Or in a 3 or more woofer setup with 2.5 the more wider bandwidth
driver doing topend
can be isolated. Even then the bass needed for baffle step
compensation can still be 2 more drivers sharing another chamber
Depends.
Simple and straight forward.
2 woofers one box, your done.
tweet needs no chamber.
Since your using Passive radiator.
Rather pointless to overthink magical
double chambers that dont do much.
Since both are needed in same chamber for passive to work.
The .5 fills in losses below baffle step.
More woofer for bass reproduction, less bass EQ needed
in real life. For a little less distortion too at high listening levels.
More advanced designs have been done with separate
chambers for woofers. You have control over each drivers
Q and can tailor the response curve if needed.
By making one box larger or smaller.
Requires more advanced modeling, and often more suited
for sealed regardless.
Or in a 3 or more woofer setup with 2.5 the more wider bandwidth
driver doing topend
can be isolated. Even then the bass needed for baffle step
compensation can still be 2 more drivers sharing another chamber
Depends.
Thanks everyone, I have the 270-PR passive radiator but I'm getting the impression that it will not work for what I'm trying to do (2.5 in a smaller enclosure) with my two Wavcor WF168WA02 drivers?
That's fine, I can use the PRs on another speaker.
I'm learning lots.
I'm started to think about a sub, I have a 12" driver (salvaged from a water damaged Thor IV sub) and I have two spare channels on the DAC...Thinking out loud, could I put the WF168WA02's in a much smaller enclosure then?
I'm a little bit worried about the speakers ending up being too big for my study.
That's fine, I can use the PRs on another speaker.
I'm learning lots.
I'm started to think about a sub, I have a 12" driver (salvaged from a water damaged Thor IV sub) and I have two spare channels on the DAC...Thinking out loud, could I put the WF168WA02's in a much smaller enclosure then?
I'm a little bit worried about the speakers ending up being too big for my study.
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Simulators are your best friend. You can check any driver against any combination a simulator is covering.
Even if your cabinet is of such low internal volume that it forms a frequency response hump around Fb,
that is driver Fs once placed in a cabinet, you can shape it right back as it should be with a HP filter of
some sort.
It is downright important that you feed the simulator with realistic T/S parameters, the ones you have measured
yourself, increasing the odds of a success.
Even if your cabinet is of such low internal volume that it forms a frequency response hump around Fb,
that is driver Fs once placed in a cabinet, you can shape it right back as it should be with a HP filter of
some sort.
It is downright important that you feed the simulator with realistic T/S parameters, the ones you have measured
yourself, increasing the odds of a success.
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@AllenB That makes sense to me. The reason I'm more careful about the midbass is its hard to get right on a ported box to begin with. Striking an acceptable balance between midbass quality and all the junk that can come out of the port is IMO tough, especially with tall cabinet designs. I usually try to kill anything from the back of rhe woofer but doing that vs minimal dampening becomes a gray area, forcing you to favor one over the other.
The larger the woofer and cabinet volume, the more of a challenge it becomes to get the best possible low level bass resolution. Thats why I've started to play with double chamber reflex boxes and 4th order vented bandpass. With 4th order BP, you can tune several attributes not possible with a common ported design. The drawback is being able to accommodate the necessary port length and surface area.
The larger the woofer and cabinet volume, the more of a challenge it becomes to get the best possible low level bass resolution. Thats why I've started to play with double chamber reflex boxes and 4th order vented bandpass. With 4th order BP, you can tune several attributes not possible with a common ported design. The drawback is being able to accommodate the necessary port length and surface area.